c # Console applications tutirial

The C# Station Tutorial
by Joe Mayo created 8/20/00, updated 9/24/01, 3/6/03, 8/16/03, 1/16/05, 4/30/07, 2/21/08, 3/12/08, 4/29/08, 7/6/08, 8/16/08, 1/12/09
Lesson 1: Getting Started with C#
This lesson will get you started with C# by introducing a few very simple programs. Here are the objectives of this lesson:
Understand the basic structure of a C# program.
Obtain a basic familiarization of what a "Namespace" is.
Obtain a basic understanding of what a Class is.
Learn what a Main method does.
Learn how to obtain command-line input.
Learn about console input/output (I/O).
A Simple C# Program
There are basic elements that all C# executable programs have and that's what we'll concentrate on for this first lesson, starting off with a simple C# program. After reviewing the code in Listing 1-1, I'll explain the basic concepts that will follow for all C# programs we will write throughout this tutorial. Please see Listing 1-1 to view this first program.
Warning: C# is case-sensitive.
Listing 1-1. A Simple Welcome Program: Welcome.cs
// Namespace Declarationusing System;// Program start classclass WelcomeCSS{ // Main begins program execution. static void Main() { // Write to console Console.WriteLine("Welcome to the C# Station Tutorial!"); }}
The program in Listing 1-1 has 4 primary elements, a namespace declaration, a class, a Main method, and a program statement. It can be compiled with the following command line: csc.exe Welcome.cs
This produces a file named Welcome.exe, which can then be executed. Other programs can be compiled similarly by substituting their file name instead of Welcome.cs. For more help about command line options, type "csc -help" on the command line. The file name and the class name can be totally different.
Note for VS.NET Users: The screen will run and close quickly when launching this program from Visual Studio .NET. To prevent this, add the following code as the last line in the Main method:
// keep screen from going away// when run from VS.NETConsole.ReadLine();
Note: The command-line is a window that allows you to run commands and programs by typing the text in manually. It is often refered to as the DOS prompt, which was the operating system people used years ago, before Windows. The .NET Framework SDK, which is free, uses mostly command line tools. Therefore, I wrote this tutorial so that anyone would be able to use it. Do a search through Windows Explorer for "csc.exe", which is the C# compiler. When you know its location, add that location to your Windows path. If you can't figure out how to add something to your path, get a friend to help you. With all the different versions of Windows available, I don't have the time in this tutorial, which is about C# language programming, to show you how to use your operating system. Then open the command window by going to the Windows Start menu, selecting Run, and typing cmd.exe.
The first thing you should be aware of is that C# is case-sensitive. The word "Main" is not the same as its lower case spelling, "main". They are different identifiers. If you are coming from a language that is not case sensitive, this will trip you up several times until you become accustomed to it.
The namespace declaration, using System;, indicates that you are referencing the System namespace. Namespaces contain groups of code that can be called upon by C# programs. With the using System; declaration, you are telling your program that it can reference the code in the System namespace without pre-pending the word System to every reference. I'll discuss this in more detail in Lesson 06: Namespaces, which is dedicated specifically to namespaces.
The class declaration, class WelcomeCSS, contains the data and method definitions that your program uses to execute. A class is one of a few different types of elements your program can use to describe objects, such as structs, interfaces , delegates, and enums, which will be discussed in more detail in Lesson 12: Structs, Lesson 13: Interfaces, Lesson 14: Delegates, and Lesson 17: Enums, respectively. This particular class has no data, but it does have one method. This method defines the behavior of this class (or what it is capable of doing). I'll discuss classes more in Lesson 07: Introduction to Classes. We'll be covering a lot of information about classes throughout this tutorial.
The one method within the WelcomeCSS class tells what this class will do when executed. The method name, Main, is reserved for the starting point of a program. Main is often called the "entry point" and if you ever receive a compiler error message saying that it can't find the entry point, it means that you tried to compile an executable program without a Main method.
A static modifier precedes the word Main, meaning that this method works in this specific class only, rather than an instance of the class. This is necessary, because when a program begins, no object instances exist. I'll tell you more about classes, objects, and instances in Lesson 07: Introduction to Classes.
Every method must have a return type. In this case it is void, which means that Main does not return a value. Every method also has a parameter list following its name with zero or more parameters between parenthesis. For simplicity, we did not add parameters to Main. Later in this lesson you'll see what type of parameter the Main method can have. You'll learn more about methods in Lesson 05: Methods.
The Main method specifies its behavior with the Console.WriteLine(...) statement. Console is a class in the System namespace. WriteLine(...) is a method in the Console class. We use the ".", dot, operator to separate subordinate program elements. Note that we could also write this statement as System.Console.WriteLine(...). This follows the pattern "namespace.class.method" as a fully qualified statement. Had we left out the using System declaration at the top of the program, it would have been mandatory for us to use the fully qualified form System.Console.WriteLine(...). This statement is what causes the string, "Welcome to the C# Station Tutorial!" to print on the console screen.
Observe that comments are marked with "//". These are single line comments, meaning that they are valid until the end-of-line. If you wish to span multiple lines with a comment, begin with "/*" and end with "*/". Everything in between is part of the comment. Comments are ignored when your program compiles. They are there to document what your program does in plain English (or the native language you speak with every day).
All statements end with a ";", semi-colon. Classes and methods begin with "{", left curly brace, and end with a "}", right curly brace. Any statements within and including "{" and "}" define a block. Blocks define scope (or lifetime and visibility) of program elements.
Accepting Command-Line Input
In the previous example, you simply ran the program and it produced output. However, many programs are written to accept command-line input. This makes it easier to write automated scripts that can invoke your program and pass information to it. If you look at many of the programs, including Windows OS utilities, that you use everyday; most of them have some type of command-line interface. For example, if you type Notepad.exe MyFile.txt (assuming the file exists), then the Notepad program will open your MyFile.txt file so you can begin editing it. You can make your programs accept command-line input also, as shown in Listing 1-2, which shows a program that accepts a name from the command line and writes it to the console.
Note: When running the NamedWelcome.exe application in Listing 1-2, you must supply a command-line argument. For example, type the name of the program, followed by your name: NamedWelcome YourName. This is the purpose of Listing 1-2 - to show you how to handle command-line input. Therefore, you must provide an argument on the command-line for the program to work. If you are running Visual Studio, right-click on the project in Solution Explorer, select Properties, click the Debug tab, locate Start Options, and type YourName into Command line arguments. If you forget to to enter YourName on the command-line or enter it into the project properties, as I just explained, you will receive an exception that says "Index was outside the bounds of the array." To keep the program simple and concentrate only on the subject of handling command-line input, I didn't add exception handling. Besides, I haven't taught you how to add exception handling to your program yet - but I will. In Lesson 15: Introduction to Exception Handling, you'll learn more about exceptions and how to handle them properly.
Listing 1-2. Getting Command-Line Input: NamedWelcome.cs
// Namespace Declarationusing System;// Program start classclass NamedWelcome{ // Main begins program execution. static void Main(string[] args) { // Write to console Console.WriteLine("Hello, {0}!", args[0]); Console.WriteLine("Welcome to the C# Station Tutorial!"); }}
In Listing 1-2, you'll notice an entry in the Main method's parameter list. The parameter name is args, which you'll use to refer to the parameter later in your program. The string[] expression defines the type of parameter that args is. The string type holds characters. These characters could form a single word, or multiple words. The "[]", square brackets denote an Array, which is like a list. Therefore, the type of the args parameter, is a list of words from the command-line. Anytime you add string[] args to the parameter list of the Main method, the C# compiler emits code that parses command-line arguments and loads the command-line arguments into args. By reading args, you have access to all arguments, minus the application name, that were typed on the command-line.
You'll also notice an additional Console.WriteLine(...) statement within the Main method. The argument list within this statement is different than before. It has a formatted string with a "{0}" parameter embedded in it. The first parameter in a formatted string begins at number 0, the second is 1, and so on. The "{0}" parameter means that the next argument following the end quote will determine what goes in that position. Hold that thought, and now we'll look at the next argument following the end quote.
The args[0] argument refers to the first string in the args array. The first element of an Array is number 0, the second is number 1, and so on. For example, if I typed NamedWelcome Joe on the command-line, the value of args[0] would be "Joe". This is a little tricky because you know that you typed NamedWelcome.exe on the command-line, but C# doesn't include the executable application name in the args list - only the first parameter after the executable application.
Returning to the embedded "{0}" parameter in the formatted string: Since args[0] is the first argument, after the formatted string, of the Console.WriteLine() statement, its value will be placed into the first embedded parameter of the formatted string. When this command is executed, the value of args[0], which is "Joe" will replace "{0}" in the formatted string. Upon execution of the command-line with "NamedWelcome Joe", the output will be as follows: Hello, Joe!
Welcome to the C# Station Tutorial!
Interacting via the Command-Line
Besides command-line input, another way to provide input to a program is via the Console. Typically, it works like this: You prompt the user for some input, they type something in and press the Enter key, and you read their input and take some action. Listing 1-3 shows how to obtain interactive input from the user.
Listing 1-3. Getting Interactive Input: InteractiveWelcome.cs
// Namespace Declarationusing System;// Program start classclass InteractiveWelcome{ // Main begins program execution. public static void Main() { // Write to console/get input Console.Write("What is your name?: "); Console.Write("Hello, {0}! ", Console.ReadLine()); Console.WriteLine("Welcome to the C# Station Tutorial!"); }}
In Listing 1-3, the Main method doesn't have any parameters -- mostly because it isn't necessary this time. Notice also that I prefixed the Main method declaration with the public keyword. The public keyword means that any class outside of this one can access that class member. For Main, it doesn't matter because your code would never call Main, but as you go through this tutorial, you'll see how you can create classes with members that must be public so they can be used. The default access is private, which means that only members inside of the same class can access it. Keywords such as public and private are referred to as access modifiers. Lesson 19 discusses access modifiers in more depth.
There are three statements inside of Main and the first two are different from the third. They are Console.Write(...) instead of Console.WriteLine(...). The difference is that the Console.Write(...) statement writes to the console and stops on the same line, but the Console.WriteLine(...) goes to the next line after writing to the console.
The first statement simply writes "What is your name?: " to the console.
The second statement doesn't write anything until its arguments are properly evaluated. The first argument after the formatted string is Console.ReadLine(). This causes the program to wait for user input at the console. After the user types input, their name in this case, they must press the Enter key. The return value from this method replaces the "{0}" parameter of the formatted string and is written to the console. This line could have also been written like this:
string name = Console.ReadLine(); Console.Write("Hello, {0}! ", name);
The last statement writes to the console as described earlier. Upon execution of the command-line with "InteractiveWelcome", the output will be as follows:
>What is your Name? [Enter Key]>Hello, ! Welcome to the C# Station Tutorial!
Summary
Now you know the basic structure of a C# program. using statements let you reference a namespace and allow code to have shorter and more readable notation. The Main method is the entry point to start a C# program. You can capture command-line input when an application is run by reading items from a string[] (string array) parameter to your Main method. Interactive I/O can be performed with the ReadLine, Write and WriteLine methods of the Console class.
This is just the beginning, the first of many lessons. I invite you back to take Lesson 2: Expressions, Types, and Variables.
Your feedback and constructive contributions are welcome. Please feel free to contact me for feedback or comments you may have about this lesson.
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The C# Station Tutorial
by Joe Mayo,created 8/27/00, updated 10/6/01, 3/12/03, 1/22/05, 2/21/08, 4/29/08, 8/16/08, 10/11/08, and 1/12/09
Lesson 2: Operators, Types, and Variables
This lesson introduces C# operators, types, and variables. Its goal is to meet the following objectives:
Understand what a variable is.
Familiarization with C# built-in types.
Get an introduction to C# operators.
Learn how to use Arrays.
Variables and Types
"Variables" are simply storage locations for data. You can place data into them and retrieve their contents as part of a C# expression. The interpretation of the data in a variable is controlled through "Types".
C# is a "Strongly Typed" language. Thus all operations on variables are performed with consideration of what the variable's "Type" is. There are rules that define what operations are legal in order to maintain the integrity of the data you put in a variable.
The C# simple types consist of the Boolean type and three numeric types - Integrals, Floating Point, Decimal, and String. The term "Integrals", which is defined in the C# Programming Language Specification, refers to the classification of types that include sbyte, byte, short, ushort, int, uint, long, ulong, and char. More details are available in the Integral Types section later in this lesson. The term "Floating Point" refers to the float and double types, which are discussed, along with the decimal type, in more detail in the Floating Point and Decimal Types section later in this lesson. The string type represents a string of characters and is discussed in The String Type section, later in this lesson. The next section introduces the boolean type.
The Boolean Type
Boolean types are declared using the keyword, bool. They have two values: true or false. In other languages, such as C and C++, boolean conditions can be satisfied where 0 means false and anything else means true. However, in C# the only values that satisfy a boolean condition is true and false, which are official keywords. Listing 2-1 shows one of many ways that boolean types can be used in a program.
Listing 2-1. Displaying Boolean Values: Boolean.cs
using System;class Booleans{ public static void Main() { bool content = true; bool noContent = false; Console.WriteLine("It is {0} that C# Station provides C# programming language content.", content); Console.WriteLine("The statement above is not {0}.", noContent); }}
In Listing 2-1, the boolean values are written to the console as a part of a sentence. The only legal values for the bool type are either true or false, as shown by the assignment of true to content and false to noContent. When run, this program produces the following output: It is True that C# Station provides C# programming language content.
The statement above is not False.
Integral Types
In C#, an integral is a category of types. For anyone confused because the word Integral sounds like a mathematical term, from the perspective of C# programming, these are actually defined as Integral types in the C# programming language specification. They are whole numbers, either signed or unsigned, and the char type. The char type is a Unicode character, as defined by the Unicode Standard. For more information, visit The Unicode Home Page. table 2-1 shows the integral types, their size, and range.
Table 2-1. The Size and Range of C# Integral Types
Type
Size (in bits)
Range
sbyte
8
-128 to 127
byte
8
0 to 255
short
16
-32768 to 32767
ushort
16
0 to 65535
int
32
-2147483648 to 2147483647
uint
32
0 to 4294967295
long
64
-9223372036854775808 to 9223372036854775807
ulong
64
0 to 18446744073709551615
char
16
0 to 65535
Integral types are well suited for those operations involving whole number calculations. The char type is the exception, representing a single Unicode character. As you can see from the table above, you have a wide range of options to choose from, depending on your requirements.
Floating Point and Decimal Types
A C# floating point type is either a float or double. They are used any time you need to represent a real number, as defined by IEEE 754. For more information on IEEE 754, visit the IEEE Web Site. Decimal types should be used when representing financial or money values. table 2-2 shows the floating point and decimal types, their size, precision, and range.
Table 2-2. The Floating Point and Decimal Types with Size, precision, and Range
Type
Size (in bits)
precision
Range
float
32
7 digits
1.5 x 10-45 to 3.4 x 1038
double
64
15-16 digits
5.0 x 10-324 to 1.7 x 10308
decimal
128
28-29 decimal places
1.0 x 10-28 to 7.9 x 1028
Floating point types are used when you need to perform operations requiring fractional representations. However, for financial calculations, the decimal type is the best choice because you can avoid rounding errors.
The string Type
A string is a sequence of text characters. You typically create a string with a string literal, enclosed in quotes: "This is an example of a string." You've seen strings being used in Lesson 1, where we used the Console.WriteLine method to send output to the console.
Some characters aren't printable, but you still need to use them in strings. Therefore, C# has a special syntax where characters can be escaped to represent non-printable characters. For example, it is common to use newlines in text, which is represented by the '\n' char. The backslash, '\', represents the escape. When preceded by the escape character, the 'n' is no longer interpreted as an alphabetical character, but now represents a newline.
You may be now wondering how you could represent a backslash character in your code. We have to escape that too by typing two backslashes, as in '\\'. table 2-3 shows a list of common escape sequences.
Table 2-3. C# Character Escape Sequences
Escape Sequence
Meaning
\'
Single Quote
\"
Double Quote
\\
Backslash
\0
Null, not the same as the C# null value
\a
Bell
\b
Backspace
\f
form Feed
\n
Newline
\r
Carriage Return
\t
Horizontal Tab
\v
Vertical Tab
Another useful feature of C# strings is the verbatim literal, which is a string with a @ symbol prefix, as in @"Some string". Verbatim literals make escape sequences translate as normal characters to enhance readability. To appreciate the value of verbatim literals, consider a path statement such as "c:\\topdir\\subdir\\subdir\\myapp.exe". As you can see, the backslashes are escaped, causing the string to be less readable. You can improve the string with a verbatim literal, like this: @"c:\topdir\subdir\subdir\myapp.exe".
That is fine, but now you have the problem where quoting text is not as easy. In that case, you would specify double double quotes. For example, the string "copy \"c:\\source file name with spaces.txt\" c:\\newfilename.txt" would be written as the verbatim literal @"copy ""c:\source file name with spaces.txt"" c:\newfilename.txt".
C# Operators
Results are computed by building expressions. These expressions are built by combining variables and operators together into statements. The following table describes the allowable operators, their precedence, and associativity.
Table 2-4. Operators with their precedence and Associativity
Category (by precedence)
Operator(s)
Associativity
Primary
x.y f(x) a[x] x++ x-- new typeof default checked unchecked delegate
left
Unary
+ - ! ~ ++x --x (T)x
left
Multiplicative
* / %
left
Additive
+ -
left
Shift
<< >>
left
Relational
< > <= >= is as
left
Equality
== !=
right
Logical AND
&
left
Logical XOR
^
left
Logical OR
|
left
Conditional AND
&&
left
Conditional OR
||
left
Null Coalescing
??
left
Ternary
?:
right
Assignment
= *= /= %= += -= <<= >>= &= ^= |= =>
right
Left associativity means that operations are evaluated from left to right. Right associativity mean all operations occur from right to left, such as assignment operators where everything to the right is evaluated before the result is placed into the variable on the left.
Most operators are either unary or binary. Unary operators form expressions on a single variable, but binary operators form expressions with two variables. Listing 2-2 demonstrates how unary operators are used.
Listing 2-2. Unary Operators: Unary.cs
using System;class Unary{ public static void Main() { int unary = 0; int preIncrement; int preDecrement; int postIncrement; int postDecrement; int positive; int negative; sbyte bitNot; bool logNot; preIncrement = ++unary; Console.WriteLine("pre-Increment: {0}", preIncrement); preDecrement = --unary; Console.WriteLine("pre-Decrement: {0}", preDecrement); postDecrement = unary--; Console.WriteLine("Post-Decrement: {0}", postDecrement); postIncrement = unary++; Console.WriteLine("Post-Increment: {0}", postIncrement); Console.WriteLine("Final Value of Unary: {0}", unary); positive = -postIncrement; Console.WriteLine("Positive: {0}", positive); negative = +postIncrement; Console.WriteLine("Negative: {0}", negative); bitNot = 0; bitNot = (sbyte)(~bitNot); Console.WriteLine("Bitwise Not: {0}", bitNot); logNot = false; logNot = !logNot; Console.WriteLine("Logical Not: {0}", logNot); }}
When evaluating expressions, post-increment (x++) and post-decrement (x--) operators return their current value and then apply the operators. However, when using pre-increment (++x) and pre-decrement (--x) operators, the operator is applied to the variable prior to returning the final value.
In Listing 2-2, the unary variable is initialized to zero. When the pre-increment (++x) operator is used, unary is incremented to 1 and the value 1 is assigned to the preIncrement variable. The pre-decrement (--x) operator turns unary back to a 0 and then assigns the value to the preDecrement variable.
When the post-decrement (x--) operator is used, the value of unary, 0, is placed into the postDecrement variable and then unary is decremented to -1. Next the post-increment (x++) operator moves the current value of unary, -1, to the postIncrement variable and then increments unary to 0.
The variable bitNot is initialized to 0 and the bitwise not (~) operator is applied. The bitwise not (~) operator flips the bits in the variable. In this case, the binary representation of 0, "00000000", was transformed into -1, "11111111".
While the (~) operator works by flipping bits, the logical negation operator (!) is a logical operator that works on bool values, changing true to false or false to true. In the case of the logNot variable in Listing 2-2, the value is initialized to false, and the next line applies the logical negation operator, (!), which returns true and reassigns the new value, true, to logNot. Essentially, it is toggling the value of the bool variable, logNot.
The setting of positive is a little tricky. At the time that it is set, the postIncrement variable is equal to -1. Applying the minus (-) operator to a negative number results in a positive number, meaning that postitive will equal 1, instead of -1. The minus operator (-), which is not the same as the pre-decrement operator (--), doesn't change the value of postInc - it just applies a sign negation. The plus operator (+) doesn't affect the value of a number, assigning negative with the same value as postIncrement, -1.
Notice the expression (sbyte)(~bitNot). Any operation performed on types sbyte, byte, short, or ushort return int values. To assign the result into the bitNot variable we had to use a cast, (Type), operator, where Type is the type you wish to convert to (in this case - sbyte). The cast operator is shown as the Unary operator, (T)x, in table 2-4. Cast operators must be performed explicity when you go from a larger type to a smaller type because of the potential for lost data. Generally speaking, assigning a smaller type to a larger type is no problem, since the larger type has room to hold the entire value. Also be aware of the dangers of casting between signed and unsigned types. You want to be sure to preserve the integrity of your data. Many basic programming texts contain good descriptions of bit representations of variables and the dangers of explicit casting.
Here's the output from the Listing 2-2: pre-Increment: 1
pre-Decrement 0
Post-Decrement: 0
Post-Increment: -1
Final Value of Unary: 0
Positive: 1
Negative: -1
Bitwise Not: -1
Logical Not: true
In addition to unary operators, C# has binary operators that form expressions of two variables. Listing 2-3 shows how to use the binary operators.
Listing 2-3. Binary Operators: Binary.cs
using System;class Binary{ public static void Main() { int x, y, result; float floatresult; x = 7; y = 5; result = x+y; Console.WriteLine("x+y: {0}", result); result = x-y; Console.WriteLine("x-y: {0}", result); result = x*y; Console.WriteLine("x*y: {0}", result); result = x/y; Console.WriteLine("x/y: {0}", result); floatresult = (float)x/(float)y; Console.WriteLine("x/y: {0}", floatresult); result = x%y; Console.WriteLine("x%y: {0}", result); result += x; Console.WriteLine("result+=x: {0}", result); }}
And here's the output: x+y: 12
x-y: 2
x*y: 35
x/y: 1
x/y: 1.4
x%y: 2
result+=x: 9
Listing 2-3 shows several examples of binary operators. As you might expect, the results of addition (+), subtraction (-), multiplication (*), and division (/) produce the expected mathematical results.
The floatresult variable is a floating point type. We explicitly cast the integer variables x and y to calculate a floating point value.
There is also an example of the remainder(%) operator. It performs a division operation on two values and returns the remainder.
The last statement shows another form of the assignment with operation (+=) operator. Any time you use the assignment with operation operator, it is the same as applying the binary operator to both the left hand and right hand sides of the operator and putting the results into the left hand side. The example could have been written as result = result + x; and returned the same value.
The Array Type
Another data type is the Array, which can be thought of as a container that has a list of storage locations for a specified type. When declaring an Array, specify the type, name, dimensions, and size.
Listing 2-4. Array Operations: Array.cs
using System;class Array{ public static void Main() { int[] myInts = { 5, 10, 15 }; bool[][] myBools = new bool[2][]; myBools[0] = new bool[2]; myBools[1] = new bool[1]; double[,] myDoubles = new double[2, 2]; string[] myStrings = new string[3]; Console.WriteLine("myInts[0]: {0}, myInts[1]: {1}, myInts[2]: {2}", myInts[0], myInts[1], myInts[2]); myBools[0][0] = true; myBools[0][1] = false; myBools[1][0] = true; Console.WriteLine("myBools[0][0]: {0}, myBools[1][0]: {1}", myBools[0][0], myBools[1][0]); myDoubles[0, 0] = 3.147; myDoubles[0, 1] = 7.157; myDoubles[1, 1] = 2.117; myDoubles[1, 0] = 56.00138917; Console.WriteLine("myDoubles[0, 0]: {0}, myDoubles[1, 0]: {1}", myDoubles[0, 0], myDoubles[1, 0]); myStrings[0] = "Joe"; myStrings[1] = "Matt"; myStrings[2] = "Robert"; Console.WriteLine("myStrings[0]: {0}, myStrings[1]: {1}, myStrings[2]: {2}", myStrings[0], myStrings[1], myStrings[2]); }}
And here's the output: myInts[0]: 5, myInts[1]: 10, myInts[2]: 15
myBools[0][0]: true, myBools[1][0]: true
myDoubles[0, 0]: 3.147, myDoubles[1, 0]: 56.00138917
myStrings[0]: Joe, myStrings[1]: Matt, myStrings[2]: Robert
Listing 2-4 shows different implementations of Arrays. The first example is the myInts Array, which is a single-dimension array. It is initialized at declaration time with explicit values.
Next is a jagged array, myBools. It is essentially an array of arrays. We needed to use the new operator to instantiate the size of the primary array and then use the new operator again for each sub-array.
The third example is a two dimensional array, myDoubles. Arrays can be multi-dimensional, with each dimension separated by a comma. It must also be instantiated with the new operator.
One of the differences between jagged arrays, myBools[][], and multi-dimension arrays, myDoubles[,], is that a multi-dimension array will allocate memory for every element of each dimension, whereas a jagged array will only allocate memory for the size of each array in each dimension that you define. Most of the time, you'll be using multi-dimension arrays, if you need multiple dimensions, and will only use jagged arrays in very special circumstances when you are able to save significant memory by explicitly specifying the sizes of the arrays in each dimension.
Finally, we have the single-dimensional array of string types, myStrings.
In each case, you can see that array elements are accessed by identifying the integer index for the item you wish to refer to. Arrays sizes can be any int type value. Their indexes begin at 0.
Summary
A variable is an identifier with a type that holds a value of that type. Simple types include the integrals, floating points, decimal, and bool. C# has several mathematical and logical operators that participate in forming expressions. C# also offers the single dimension, multi-dimension and jagged array types.
In this lesson you learned how to write simple statements and code a program that works linearly from start to finish. However, this is not as useful as it can be because you need to be able to make decisions and execute different blocks of code depending on different conditions. I invite you to return for Lesson 3: Control Statements - Selection, where you can learn how to branch your logic for more powerful decision making.
Your feedback and constructive contributions are welcome. Please feel free to contact me for feedback or comments you may have about this lesson.
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Copyright © 2000-2009 C# Station, All Rights Reserved
LESSON 03
The C# Station Tutorial
by Joe Mayo, 9/2/00, updated 10/6/01, 3/12/03, 1/11/04, 1/25/05, 2/21/08, 4/29/08, and 1/12/09
Lesson 3: Control Statements - Selection
In the last couple of lessons, every program you saw contained a limited amount of sequential steps and then stopped. There were no decisions you could make with the input and the only constraint was to follow straight through to the end. The information in this lesson will help you branch into separate logical sequences based on decisions you make. More specifically, the goals of this lesson are as follows:
Learn the if statements.
Learn the switch statement.
Learn how break is used in switch statements.
Understand proper use of the goto statement.
The if Statement
An if statement allows you to take different paths of logic, depending on a given condition. When the condition evaluates to a boolean true, a block of code for that true condition will execute. You have the option of a single if statement, multiple else if statements, and an optional else statement. Listing 3-1 shows how each of these types of if statements work.
Listing 3-1. forms of the if statement: IfSelection.cs
using System;class IfSelect{ public static void Main() { string myInput; int myInt;
Console.Write("Please enter a number: "); myInput = Console.ReadLine(); myInt = Int32.Parse(myInput);
// Single Decision and Action with braces if (myInt > 0) { Console.WriteLine("Your number {0} is greater than zero.", myInt); }
// Single Decision and Action without brackets if (myInt < 0) Console.WriteLine("Your number {0} is less than zero.", myInt);
// Either/Or Decision if (myInt != 0) { Console.WriteLine("Your number {0} is not equal to zero.", myInt); } else { Console.WriteLine("Your number {0} is equal to zero.", myInt); }
// Multiple Case Decision if (myInt < myint ="=""> 0 && myInt <= 10) { Console.WriteLine("Your number {0} is in the range from 1 to 10.", myInt); } else if (myInt > 10 && myInt <= 20) { Console.WriteLine("Your number {0} is in the range from 11 to 20.", myInt); } else if (myInt > 20 && myInt <= 30) { Console.WriteLine("Your number {0} is in the range from 21 to 30.", myInt); } else { Console.WriteLine("Your number {0} is greater than 30.", myInt); } }}
The statements in Listing 3-1 use the same input variable, myInt as a part of their evaluations. This is another way of obtaining interactive input from the user. Here's the pertinent code:
Console.Write("Please enter a number: "); myInput = Console.ReadLine(); myInt = Int32.Parse(myInput);
We first print the line "Please enter a number: " to the console. The Console.ReadLine() statement causes the program to wait for input from the user, who types a number and then presses Enter. This number is returned in the form of a string into the myInput variable, which is a string type. Since we must evaluate the user's input in the form of an int, myInput must be converted. This is done with the command Int32.Parse(myInput). (Int32 and similar types will be covered in another lesson on advanced types) The result is placed into the myInt variable, which is an int type.
Now that we have a variable in the type we wanted, we will evaluate it with if statements. The first statement is of the form if (boolean expression) { statements }, as shown below:
// Single Decision and Action with braces if (myInt > 0) { Console.WriteLine("Your number {0} is greater than zero.", myInt); }
You must begin with the keyword if. Next is the boolean expression between parenthesis. This boolean expression must evaluate to a true or false value. In this case, we are checking the user's input to see if it is greater than (>) 0. If this expression evaluates to true, we execute the statements within the curly braces. (We refer to the structure with curly braces as a "block") There could be one or more statements within this block. If the boolean expression evaluates to false, we ignore the statements inside the block and continue program execution with the next statement after the block.
Note: In other languages, such as C and C++, conditions can be evaluated where a result of 0 is false and any other number is true. In C#, the condition must evaluate to a boolean value of either true or false. If you need to simulate a numeric condition with C#, you can do so by writing it as (myInt != 0), which means that the expression evaluate to true if myInt is not 0.
The second if statement is much like the first, except it does not have a block, as shown here:
// Single Decision and Action without braces if (myInt < 0) Console.WriteLine("Your number {0} is less than zero.", myInt);
If its boolean expression evaluates to true, the first statement after the boolean expression will be executed. When the boolean expression evaluates to false, the first statement after the boolean expression will be skipped and the next program statement will be executed. This form of if statement is adequate when you only have a single statement to execute. If you want to execute two or more statements when the boolean expression evaluates to true, you must enclose them in a block.
Most of the time, you'll want to make an either/or kind of decision. This is called an if/else statement. The third if statement in Listing 3-1 presents this idea, as shown below:
// Either/Or Decision if (myInt != 0) { Console.WriteLine("Your number {0} is not equal to zero.", myInt); } else { Console.WriteLine("Your number {0} is equal to zero.", myInt); }
When the boolean expression evaluates to true, the statement(s) in the block immediately following the if statement are executed. However, when the boolean expression evaluates to false, the statements in the block following the else keyword are executed.
When you have multiple expressions to evaluate, you can use the if/else if/else form of the if statement. We show this form in the fourth if statement of Listing 3-1, and repeated below:
// Multiple Case Decision if (myInt < myint ="=""> 0 && myInt <= 10) { Console.WriteLine("Your number {0} is in the range from 1 to 10.", myInt); } else if (myInt > 10 && myInt <= 20) { Console.WriteLine("Your number {0} is in the range from 11 to 20.", myInt); } else if (myInt > 20 && myInt <= 30) { Console.WriteLine("Your number {0} is in the range from 21 to 30.", myInt); } else { Console.WriteLine("Your number {0} is greater than 30.", myInt); }
This example begins with the if keyword, again executing the following block if the boolean expression evaluates to true. However, this time you can evaluate multiple subsequent conditions with the else if keyword combination. the else if statement also takes a boolean expression, just like the if statement. The rules are the same, when the boolean expression for the else if statement evaluates to true, the block immediately following the boolean expression is executed. When none of the other if or else if boolean expressions evaluate to true, the block following the else keyword will be executed. Only one section of an if/else if/else statement will be executed.
One difference in the last statement from the others is the boolean expressions. The boolean expression, (myInt < 0 || myInt == 0), contains the conditional OR (||) operator. In both the regular OR (|) operator and the conditional OR (||) operator, the boolean expression will evaluate to true if either of the two sub-expressions on either side of the operator evaluate to true. The primary difference between the two OR forms are that the regular OR operator will evaluate both sub-expressions every time. However, the conditional OR will evaluate the second sub-expression only if the first sub-expression evaluates to false.
The boolean expression, (myInt > 0 && myInt <= 10), contains the conditional AND operator. Both the regular AND (&) operator and the conditional AND (&&) operator will return true when both of the sub-expressions on either side of the operator evaluate to true. The difference between the two is that the regular AND operator will evaluate both expressions every time. However, the conditional AND operator will evaluate the second sub-expression only when the first sub-expression evaluates to true.
The conditional operators (&& and ||) are commonly called short-circuit operators because they do not always evaluate the entire expression. Thus, they are also used to produce more efficient code by ignoring unnecessary logic.
The switch Statement
Another form of selection statement is the switch statement, which executes a set of logic depending on the value of a given parameter. The types of the values a switch statement operates on can be booleans, enums, integral types, and strings. Lesson 2: Operators, Types, and Variables discussed the bool type, integral types and strings and Lesson 17: Enums will teach you what an enum type is. Listing 3-2 shows how to use the switch statement with both int and string types.
Listing 3-2. Switch Statements: SwitchSelection.cs
using System;class SwitchSelect{ public static void Main() { string myInput; int myInt; begin: Console.Write("Please enter a number between 1 and 3: "); myInput = Console.ReadLine(); myInt = Int32.Parse(myInput); // switch with integer type switch (myInt) { case 1: Console.WriteLine("Your number is {0}.", myInt); break; case 2: Console.WriteLine("Your number is {0}.", myInt); break; case 3: Console.WriteLine("Your number is {0}.", myInt); break; default: Console.WriteLine("Your number {0} is not between 1 and 3.", myInt); break; } decide: Console.Write("Type \"continue\" to go on or \"quit\" to stop: "); myInput = Console.ReadLine(); // switch with string type switch (myInput) { case "continue": goto begin; case "quit": Console.WriteLine("Bye."); break; default: Console.WriteLine("Your input {0} is incorrect.", myInput); goto decide; } }}
Note: Listing 3-2 will throw an exception if you enter any value other than an int. i.e. the letter 'a' would be an error. You can visit Lesson 15: Introduction to Exception Handling to learn more about how to anticipate and handle these type of problems.
Listing 3-2 shows a couple of switch statements. The switch statement begins with the switch keyword followed by the switch expression. In the first switch statement in listing 3-2, the switch expression evaluates to an int type, as follows:
// switch with integer type switch (myInt) { case 1: Console.WriteLine("Your number is {0}.", myInt); break; case 2: Console.WriteLine("Your number is {0}.", myInt); break; case 3: Console.WriteLine("Your number is {0}.", myInt); break; default: Console.WriteLine("Your number {0} is not between 1 and 3.", myInt); break; }
The switch block follows the switch expression, where one or more choices are evaluated for a possible match with the switch expression. Each choice is labeled with the case keyword, followed by an example that is of the same type as the switch expression and followed by a colon (:). In the example we have case 1:, case 2:, and case 3:. When the result evaluated in the switch expression matches one of these choices, the statements immediately following the matching choice are executed, up to and including a branching statement, which could be either a break, continue, goto , return, or throw statement. table 3-1 summarizes the branching statements.
Table 3-1. C# Branching Statements
Branching statement
Description
break
Leaves the switch block
continue
Leaves the switch block, skips remaining logic in enclosing loop, and goes back to loop condition to determine if loop should be executed again from the beginning. Works only if switch statement is in a loop as described in Lesson 04: Control Statements - Loops.
goto
Leaves the switch block and jumps directly to a label of the form ":"
return
Leaves the current method. Methods are described in more detail in Lesson 05: Methods.
throw
Throws an exception, as discussed in Lesson 15: Introduction to Exception Handling.
You may also include a default choice following all other choices. If none of the other choices match, then the default choice is taken and its statements are executed. Although use of the default label is optional, I highly recommend that you always include it. This will help catch unforeseen circumstances and make your programs more reliable.
Each case label must end with a branching statement, as described in table 3-1, which is normally the break statement. The break statement will cause the program to exit the switch statement and begin execution with the next statement after the switch block. There are two exceptions to this: adjacent case statements with no code in between or using a goto statement. Here's an example that shows how to combine case statements:
switch (myInt) { case 1: case 2: case 3: Console.WriteLine("Your number is {0}.", myInt); break; default: Console.WriteLine("Your number {0} is not between 1 and 3.", myInt); break; }
By placing case statements together, with no code in-between, you create a single case for multiple values. A case without any code will automatically fall through to the next case. The example above shows how the three cases for myInt equal to 1, 2, or 3, where case 1 and case 2 will fall through and execute code for case 3.
A case statement can only be an exact match and you can't use logical conditions. If you need to use logical conditions, you can use an if/else if/else statement.
Another way to control the flow of logic in a switch statement is by using the goto statement. You can either jump to another case statement, or jump out of the switch statement. The second switch statement in Listing 3-2 shows the use of the goto statement, as shown below:
// switch with string type switch (myInput) { case "continue": goto begin; case "quit": Console.WriteLine("Bye."); break; default: Console.WriteLine("Your input {0} is incorrect.", myInput); goto decide; }
Note: in the current example, "continue", is a case of the switch statement -- not the keyword.
The goto statement causes program execution to jump to the label following the goto keyword. During execution, if the user types in "continue", the switch statement matches this input (a string type) with the case "continue": label and executes the "goto begin:" instruction. The program will then leave the switch statement and start executing the first program statement following the begin: label. This is effectively a loop, allowing you to execute the same code multiple times. The loop will end when the user types the string "quit". This will be evaluated with the case "quit": choice, which will print "Bye." to the console, break out of the switch statement and end the program.
Warning: You should not create loops like this. It is *bad* programming style. The only reason it is here is because I wanted to show you the syntax of the goto statement. Instead, use one of the structured looping statements, described in Lesson 04: Control Statements - Loops.
When neither the "continue" nor "quit" strings are entered, the "default:" case will be entered. It will print an error message to the console and then execute the goto decide: command. This will cause program execution to jump to the first statement following the decide: label, which will ask the user if they want to continue or quit. This is effectively another loop.
Clearly, the goto statement is powerful and can, under controlled circumstances, be useful. However, I must caution you strongly on its use. The goto statement has great potential for misuse. You could possibly create a very difficult program to debug and maintain. Imagine the spaghetti code that could be created by random goto statements throughout a program. In the next lesson, I'll show you a better way to create loops in your program.
Summary
The if statement can be written in multiple ways to implement different branches of logic. The switch statement allows a choice among a set of bool, enum, integral, or string types. You use break, continue, goto, return, or throw statements to leave a case statement. Be sure to avoid the goto statement in your code unless you have an extremely good reason for using it.
In addition to branching based on a condition, it is useful to be able to execute a block of statements multiple times. A goto statement is not proper or adequate for such logic. Therefore, I invite you to return for Lesson 4: Control Statements - Loops. This will be a continuation of the same topic.
Your feedback and constructive contributions are welcome. Please feel free to contact me for feedback or comments you may have about this lesson.
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Copyright © 2000-2009 C# Station, All Rights Reserved
Lesson 04
The C# Station Tutorial
by Joe Mayo, 9/7/00, updated 3/12/03, 2/20/05, 2/21/05, 1/12/09
Lesson 4: Control Statements - Loops
In the last lesson, you learned how to create a simple loop by using the goto statement. I advised you that this is not the best way to perform loops in C#. The information in this lesson will teach you the proper way to execute iterative logic with the various C# looping statements. Its goal is to meet the following objectives:
Learn the while loop.
Learn the do loop.
Learn the for loop.
Learn the foreach loop.
Complete your knowledge of the break statement.
Teach you how to use the continue statement.
The while Loop
A while loop will check a condition and then continues to execute a block of code as long as the condition evaluates to a boolean value of true. Its syntax is as follows: while () { }. The statements can be any valid C# statements. The boolean expression is evaluated before any code in the following block has executed. When the boolean expression evaluates to true, the statements will execute. Once the statements have executed, control returns to the beginning of the while loop to check the boolean expression again.
When the boolean expression evaluates to false, the while loop statements are skipped and execution begins after the closing brace of that block of code. Before entering the loop, ensure that variables evaluated in the loop condition are set to an initial state. During execution, make sure you update variables associated with the boolean expression so that the loop will end when you want it to. Listing 4-1 shows how to implement a while loop.
Listing 4-1. The While Loop: WhileLoop.cs
using System;class WhileLoop{ public static void Main() { int myInt = 0; while (myInt < 10) { Console.Write("{0} ", myInt); myInt++; } Console.WriteLine(); }}
Listing 4-1 shows a simple while loop. It begins with the keyword while, followed by a boolean expression. All control statements use boolean expressions as their condition for entering/continuing the loop. This means that the expression must evaluate to either a true or false value. In this case we are checking the myInt variable to see if it is less than (<) 10. Since myInt was initialized to 0, the boolean expression will return true the first time it is evaluated. When the boolean expression evaluates to true, the block immediately following the boolean expression will be executed.
Within the while block we print the number and a space to the console. Then we increment (++) myInt to the next integer. Once the statements in the while block have executed, the boolean expression is evaluated again. This sequence will continue until the boolean expression evaluates to false. Once the boolean expression is evaluated as false, program control will jump to the first statement following the while block. In this case, we will write the numbers 0 through 9 to the console, exit the while block, and print a new line to the console.
The do Loop
A do loop is similar to the while loop, except that it checks its condition at the end of the loop. This means that the do loop is guaranteed to execute at least one time. On the other hand, a while loop evaluates its boolean expression at the beginning and there is generally no guarantee that the statements inside the loop will be executed, unless you program the code to explicitly do so. One reason you may want to use a do loop instead of a while loop is to present a message or menu such as the one in Listing 4-2 and then retrieve input from a user.
Listing 4-2. The Do Loop: DoLoop.cs
using System;class DoLoop{ public static void Main() { string myChoice; do { // Print A Menu Console.WriteLine("My Address Book\n"); Console.WriteLine("A - Add New Address"); Console.WriteLine("D - Delete Address"); Console.WriteLine("M - Modify Address"); Console.WriteLine("V - View Addresses"); Console.WriteLine("Q - Quit\n"); Console.WriteLine("Choice (A,D,M,V,or Q): "); // Retrieve the user's choice myChoice = Console.ReadLine(); // Make a decision based on the user's choice switch(myChoice) { case "A": case "a": Console.WriteLine("You wish to add an address."); break; case "D": case "d": Console.WriteLine("You wish to delete an address."); break; case "M": case "m": Console.WriteLine("You wish to modify an address."); break; case "V": case "v": Console.WriteLine("You wish to view the address list."); break; case "Q": case "q": Console.WriteLine("Bye."); break; default: Console.WriteLine("{0} is not a valid choice", myChoice); break; } // Pause to allow the user to see the results Console.Write("press Enter key to continue..."); Console.ReadLine(); Console.WriteLine(); } while (myChoice != "Q" && myChoice != "q"); // Keep going until the user wants to quit }}
Listing 4-2 shows a do loop in action. The syntax of the do loop is do { } while ();. The statements can be any valid C# programming statements you like. The boolean expression is the same as all others we've encountered so far. It returns either true or false.
In the Main method, we declare the variable myChoice of type string. Then we print a series of statement to the console. This is a menu of choices for the user. We must get input from the user, which is in the form of a Console.ReadLine method which returns the user's value into the myChoice variable. We must take the user's input and process it. A very efficient way to do this is with a switch statement. Notice that we've placed matching upper and lower case letters together to obtain the same functionality. This is the only legal way to have automatic fall through between cases. If you were to place any statements between two cases, you would not be able to fall through. Another point is that we used the default: case, which is a very good habit for the reasons stated in Lesson 3: Control Statements - Selection.
The for Loop
A for loop works like a while loop, except that the syntax of the for loop includes initialization and condition modification. for loops are appropriate when you know exactly how many times you want to perform the statements within the loop. The contents within the for loop parentheses hold three sections separated by semicolons (; ; ) { }.
The initializer list is a comma separated list of expressions. These expressions are evaluated only once during the lifetime of the for loop. This is a one-time operation, before loop execution. This section is commonly used to initialize an integer to be used as a counter.
Once the initializer list has been evaluated, the for loop gives control to its second section, the boolean expression. There is only one boolean expression, but it can be as complicated as you like as long as the result evaluates to true or false. The boolean expression is commonly used to verify the status of a counter variable.
When the boolean expression evaluates to true, the statements within the curly braces of the for loop are executed. After executing for loop statements, control moves to the top of loop and executes the iterator list, which is normally used to increment or decrement a counter. The iterator list can contain a comma separated list of statements, but is generally only one statement. Listing 4-3 shows how to implement a for loop. The purpose of the program is to print only odd numbers less than 10.
Listing 4-3. The For Loop: ForLoop.cs
using System;class ForLoop{ public static void Main() { for (int i=0; i < 20; i++) { if (i == 10) break; if (i % 2 == 0) continue; Console.Write("{0} ", i); } Console.WriteLine(); }}
Normally, for loop statements execute from the opening curly brace to the closing curly brace without interruption. However, in Listing 4-3, we've made a couple exceptions. There are a couple if statements disrupting the flow of control within the for block.
The first if statement checks to see if i is equal to 10. Now you see another use of the break statement. Its behavior is similar to the selection statements, as discussed in Lesson 3: Control Statements - Selection. It simply breaks out of the loop at that point and transfers control to the first statement following the end of the for block.
The second if statement uses the remainder operator to see if i is a multiple of 2. This will evaluate to true when i is divided by 2 with a remainder equal to zero, (0). When true, the continue statement is executed, causing control to skip over the remaining statements in the loop and transfer back to the iterator list. By arranging the statements within a block properly, you can conditionally execute them based upon whatever condition you need.
When program control reaches either a continue statement or end of block, it transfers to the third section within the for loop parentheses, the iterator list. This is a comma separated list of actions that are executed after the statements in the for block have been executed. Listing 4-3 is a typical action, incrementing the counter. Once this is complete, control transfers to the boolean expression for evaluation.
Similar to the while loop, a for loop will continue as long as the boolean expression is true. When the boolean expression becomes false, control is transferred to the first statement following the for block.
For this tutorial, I chose to implement break and continue statements in Listing 4-3 only. However, they may be used in any of the loop statements.
The foreach Loop
A foreach loop is used to iterate through the items in a list. It operates on arrays or collections such as ArrayList, which can be found in the System.Collections namespace. The syntax of a foreach loop is foreach ( in ) { }. The type is the type of item contained in the list. For example, if the type of the list was int[] then the type would be int.
The iteration variable is an identifier that you choose, which could be anything but should be meaningful. For example, if the list contained an array of people's ages, then a meaningful name for item name would be age.
The in keyword is required.
As mentioned earlier, the list could be either an array or a collection. You learned about arrays in Lesson 02: Operators, Types, and Variables. You can also iterate over C# generic collections also, described in Lesson 20: Introduction to Generic Collections.
While iterating through the items of a list with a foreach loop, the list is read-only. This means that you can't modify the iteration variable within a foreach loop. There is a subtlety here; Later, you'll learn how to create custom types, called class and struct, that can contain multiple fields. You can change the fields of the class or struct, but not the iteration variable for the class or struct itself in a foreach loop.
On each iteration through a foreach loop the list is queried for a new value. As long as the list can return a value, this value will be put into the read-only iteration variable, causing the statements in the foreach block to be executed. When the collection has been fully traversed, control will transfer to the first executable statement following the end of the foreach block. Listing 4-4 demonstrates how to use a foreach loop.
Listing 4-4. The ForEach Loop: ForEachLoop.cs
using System;class ForEachLoop{ public static void Main() { string[] names = {"Cheryl", "Joe", "Matt", "Robert"}; foreach (string person in names) { Console.WriteLine("{0} ", person); } }}
In Listing 4-4, the first thing we've done inside the Main method is declare and initialize the names array with 4 strings. This is the list used in the foreachloop.
In the foreach loop, we've used a string variable, person, as the item name, to hold each element of the names array. As long as there are names in the array that have not been returned, the Console.WriteLine method will print each value of the person variable to the screen.
Summary
Loops allow you to execute a block of statements repeatedly. C# offers several statements to construct loops with, including the while, do, for, and foreach loops. while loops execute a block of statements as long as an expression is true, do loops execute a block of statements at least once and then keep going as long as a condition is true, for loops execute a block of statements a specified amount of times, and foreach loops execute a block of statements for each item in a collection. Normally a block of statements will execute from beginning to end. However, the normal flow of a loop can be changed with the break and continue statements.
So far, the only method you've seen in this tutorial is the Main method, which is the entry point of a C# application. However, you are probably wanting to write larger programs to test your new knowledge. This requires breaking up the code into methods to keep it organized and logical. For this, I invite you to return for Lesson 5: Introduction to Methods, where you can learn new techniques of organizing your code.
Your feedback and constructive contributions are welcome. Please feel free to contact me for feedback or comments you may have about this lesson.
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Copyright © 2000-2009 C# Station, All Rights Reserved
Lesson 05
The C# Station Tutorial
by Joe Mayo, 9/25/00, updated 3/12/03, 2/21/08, 1/12/09
Lesson 5: Methods
In previous lessons of this tutorial, all of our functionality for each program resided in the Main() method. While this was adequate for the simple programs we used to learn earlier concepts, there is a better way to organize your program, using methods. A method helps you separate your code into modules that perform a given task. The objectives of this lesson are as follows:
Understand the structure of a method.
Know the difference between static and instance methods.
Learn to instantiate objects.
Learn how to call methods of an instantiated object.
Understand the 4 types of parameters.
Learn how to use the this reference.
Method Structure
Methods are extremely useful because they allow you to separate your logic into different units. You can pass information to methods, have it perform one or more statements, and retrieve a return value. The capability to pass parameters and return values is optional and depends on what you want the method to do. Here's a description of the syntax required for creating a method: attributes modifiers return-type method-name(parameters )
{
statements
}
We defer discussion of attributes and modifiers to a later lesson. The return-type can be any C# type. It can be assigned to a variable for use later in the program. The method name is a unique identifier for what you wish to call a method. To promote understanding of your code, a method name should be meaningful and associated with the task the method performs. Parameters allow you to pass information to and from a method. They are surrounded by parenthesis. Statements within the curly braces carry out the functionality of the method.
Listing 5-1. One Simple Method: OneMethod.cs
using System;class OneMethod{ public static void Main() { string myChoice; OneMethod om = new OneMethod(); do { myChoice = om.getChoice(); // Make a decision based on the user's choice switch(myChoice) { case "A": case "a": Console.WriteLine("You wish to add an address."); break; case "D": case "d": Console.WriteLine("You wish to delete an address."); break; case "M": case "m": Console.WriteLine("You wish to modify an address."); break; case "V": case "v": Console.WriteLine("You wish to view the address list."); break; case "Q": case "q": Console.WriteLine("Bye."); break; default: Console.WriteLine("{0} is not a valid choice", myChoice); break; } // Pause to allow the user to see the results Console.WriteLine(); Console.Write("press Enter key to continue..."); Console.ReadLine(); Console.WriteLine(); } while (myChoice != "Q" && myChoice != "q"); // Keep going until the user wants to quit } string getChoice() { string myChoice; // Print A Menu Console.WriteLine("My Address Book\n"); Console.WriteLine("A - Add New Address"); Console.WriteLine("D - Delete Address"); Console.WriteLine("M - Modify Address"); Console.WriteLine("V - View Addresses"); Console.WriteLine("Q - Quit\n"); Console.Write("Choice (A,D,M,V,or Q): "); // Retrieve the user's choice myChoice = Console.ReadLine(); Console.WriteLine(); return myChoice; }}
The program in Listing 5-1 is similar to the DoLoop program from Lesson 4, except for one difference. Instead of printing the menu and accepting input in the Main() method, this functionality has been moved to a new method called getChoice(). The return type is a string. This string is used in the switch statement in Main(). The method name "getChoice" describes what happens when it is invoked. Since the parentheses are empty, no information will be transferred to the getChoice() method.
Within the method block we first declare the variable myChoice. Although this is the same name and type as the myChoice variable in Main(), they are both unique variables. They are local variables and they are visible only in the block they are declared. In other words, the myChoice in getChoice() knows nothing about the existence of the myChoice in Main(), and vice versa.
The getChoice() method prints a menu to the console and gets the user's input. The return statement sends the data from the myChoice variable back to the caller, Main(), of getChoice(). Notice that the type returned by the return statement must be the same as the return-type in the function declaration. In this case it is a string.
In the Main() method we must instantiate a new OneMethod object before we can use getChoice(). This is because of the way getChoice() is declared. Since we did not specify a static modifier, as for Main(), getChoice() becomes an instance method. The difference between instance methods and static methods is that multiple instances of a class can be created (or instantiated) and each instance has its own separate getChoice() method. However, when a method is static, there are no instances of that method, and you can invoke only that one definition of the static method.
So, as stated, getChoice() is not static and therefore, we must instantiate a new object to use it. This is done with the declaration OneMethod om = new OneMethod(). On the left hand side of the declaration is the object reference om which is of type OneMethod. The distinction of om being a reference is important. It is not an object itself, but it is a variable that can refer (or point ) to an object of type OneMethod. On the right hand side of the declaration is an assignment of a new OneMethod object to the reference om. The keyword new is a C# operator that creates a new instance of an object on the heap. What is happening here is that a new OneMethod instance is being created on the heap and then being assigned to the om reference. Now that we have an instance of the OneMethod class referenced by om, we can manipulate that instance through the om reference.
Methods, fields, and other class members can be accessed, identified, or manipulated through the "." (dot) operator. Since we want to call getChoice(), we do so by using the dot operator through the om reference: om.getChoice(). The program then executes the statements in the getChoice() block and returns. To capture the value getChoice() returns, we use the "=" (assignment) operator. The returned string is placed into Main()'s local myChoice variable. From there, the rest of the program executes as expected, using concepts from earlier lessons.
Listing 5-2. Method Parameters: MethodParams.cs
using System;class Address{ public string name; public string address;}class MethodParams{ public static void Main() { string myChoice; MethodParams mp = new MethodParams(); do { // show menu and get input from user myChoice = mp.getChoice(); // Make a decision based on the user's choice mp.makeDecision(myChoice); // Pause to allow the user to see the results Console.Write("press Enter key to continue..."); Console.ReadLine(); Console.WriteLine(); } while (myChoice != "Q" && myChoice != "q"); // Keep going until the user wants to quit } // show menu and get user's choice string getChoice() { string myChoice; // Print A Menu Console.WriteLine("My Address Book\n"); Console.WriteLine("A - Add New Address"); Console.WriteLine("D - Delete Address"); Console.WriteLine("M - Modify Address"); Console.WriteLine("V - View Addresses"); Console.WriteLine("Q - Quit\n"); Console.WriteLine("Choice (A,D,M,V,or Q): "); // Retrieve the user's choice myChoice = Console.ReadLine(); return myChoice; } // make decision void makeDecision(string myChoice) { Address addr = new Address(); switch(myChoice) { case "A": case "a": addr.name = "Joe"; addr.address = "C# Station"; this.addAddress(ref addr); break; case "D": case "d": addr.name = "Robert"; this.deleteAddress(addr.name); break; case "M": case "m": addr.name = "Matt"; this.modifyAddress(out addr); Console.WriteLine("Name is now {0}.", addr.name); break; case "V": case "v": this.viewAddresses("Cheryl", "Joe", "Matt", "Robert"); break; case "Q": case "q": Console.WriteLine("Bye."); break; default: Console.WriteLine("{0} is not a valid choice", myChoice); break; } } // insert an address void addAddress(ref Address addr) { Console.WriteLine("Name: {0}, Address: {1} added.", addr.name, addr.address); } // remove an address void deleteAddress(string name) { Console.WriteLine("You wish to delete {0}'s address.", name); } // change an address void modifyAddress(out Address addr) { //Console.WriteLine("Name: {0}.", addr.name); // causes error! addr = new Address(); addr.name = "Joe"; addr.address = "C# Station"; } // show addresses void viewAddresses(params string[] names) { foreach (string name in names) { Console.WriteLine("Name: {0}", name); } }}
Listing 5-2 is a modification of Listing 5-1, modularizing the program and adding more implementation to show parameter passing. There are 4 kinds of parameters a C# method can handle: out, ref, params, and value. To help illustrate usage of parameters, we created an Address class with two string fields.
In Main() we call getChoice() to get the user's input and put that string in the myChoice variable. Then we use myChoice as an argument to makeDecision(). In the declaration of makeDecision() you'll notice its one parameter is declared as a string with the name myChoice. Again, this is a new myChoice, separate from the caller's argument and local only to this method. Since makeDecision()'s myChoice parameter does not have any other modifiers, it is considered a value parameter. The actual value of the argument is copied on the stack. Variables given by value parameters are local and any changes to that local variable do not affect the value of the variable used in the caller's argument.
The switch statement in makeDecision() calls a method for each case. These method calls are different from the ones we used in Main(). Instead of using the mp reference, they use the this keyword. this is a reference to the current object. We know the current object has been instantiated because makeDecision() is not a static method. Therefore, we can use the this reference to call methods within the same instance.
The addAddress() method takes a ref parameter. This means that a reference to the parameter is copied to the method. This reference still refers to the same object on the heap as the original reference used in the caller's argument. This means any changes to the local reference's object also changes the caller reference's object. The code can't change the reference, but it can make changes to the object being referenced. You can think of this as a way to have an input/output parameter.
As you know, methods have return values, but sometimes you'll want to return more than one value from a method. An out parameter allows you to return additional values from a method.
modifyAddress() has an out parameter. out parameters are only passed back to the calling function. Because of definite assignment rules, you cannot use this variable until it has a valid value assigned. The first line in modifyAddress() is commented on purpose to illustrate this point. Uncomment it and compile to see what happens. Once assigned and the program returns, the value of the out parameter will be copied into the caller's argument variable. You must assign a value to an out parameter before your method returns.
A very useful addition to the C# language is the params parameter, which lets you define a method that can accept a variable number of arguments. The params parameter must be a single dimension or jagged array. When calling viewAddresses(), we pass in four string arguments. The number of arguments is variable and will be converted to a string[] automatically. In viewAddresses() we use a foreach loop to print each of these strings. Instead of the list of string arguments, the input could have also been a string array. The params parameter is considered an input only parameter and any changes affect the local copy only.
In summary, you understand the structure of a method. The four types of paramters are value, ref, out, and params. When you wish to use an instance method, you must instantiate its object as opposed to static methods that can be called any time. The this reference refers to its containing object and may be used to refer to its containing object's members, including methods.
I invite you to return for Lesson 6: Namespaces.
Your feedback and constructive contributions are welcome. Please feel free to contact me for feedback or comments you may have about this lesson.
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Copyright © 2000-2009 C# Station, All Rights Reserved
Laeeson 06
The C# Station Tutorial
by Joe Mayo, 10/15/00, updated 11/11/01, 3/12/03, 2/21/08, 1/12/09
Lesson 6: Namespaces
This lesson introduces you to C# Namespaces. Our objectives are as follows:
Understand what Namespace is.
Learn how to implement the using directive.
Learn to use alias directive.
Understand what are namespace members.
In Lesson 1, you saw the using System; directive in the SimpleHello program. This directive allowed you to use members of the System namespace. Because of the narrow focus of that lesson, we needed to delay explanation until now. When you've completed this lesson you will understand the using directive and more.
Namespaces are C# program elements designed to help you organize your programs. They also provide assistance in avoiding name clashes between two sets of code. Implementing Namespaces in your own code is a good habit because it is likely to save you from problems later when you want to reuse some of your code. For example, if you created a class named Console, you would need to put it in your own namespace to ensure that there wasn't any confusion about when the System.Console class should be used or when your class should be used. Generally, it would be a bad idea to create a class named Console, but in many cases your classes will be named the same as classes in either the .NET Framework Class Library or a third party library and namespaces help you avoid the problems that identical class names would cause.
Namespaces don't correspond to file or directory names. If naming directories and files to correspond to namespaces helps you organize your code, then you may do so, but it is not required.
Listing 6-1. The C# Station Namespace: NamespaceCSS.cs
// Namespace Declarationusing System;// The C# Station Namespacenamespace csharp_station { // Program start class class NamespaceCSS { // Main begins program execution. public static void Main() { // Write to console Console.WriteLine("This is the new C# Station Namespace."); } }}
Listing 6-1 shows how to create a namespace. We declare the new namespace by putting the word namespace in front of csharp_station. Curly braces surround the members inside the csharp_station namespace.
Listing 6-2. Nested Namespace 1: NestedNamespace1.cs
// Namespace Declarationusing System;// The C# Station Tutorial Namespacenamespace csharp_station { namespace tutorial { // Program start class class NamespaceCSS { // Main begins program execution. public static void Main() { // Write to console Console.WriteLine("This is the new C# Station Tutorial Namespace."); } } }}
Namespaces allow you to create a system to organize your code. A good way to organize your namespaces is via a hierarchical system. You put the more general names at the top of the hierarchy and get more specific as you go down. This hierarchical system can be represented by nested namespaces. Listing 6-2 shows how to create a nested namespace. By placing code in different sub-namespaces, you can keep your code organized.
Listing 6-3. Nested Namespace 2: NestedNamespace2.cs
// Namespace Declarationusing System;// The C# Station Tutorial Namespacenamespace csharp_station.tutorial { // Program start class class NamespaceCSS { // Main begins program execution. public static void Main() { // Write to console Console.WriteLine("This is the new C# Station Tutorial Namespace."); } }}
Listing 6-3 shows another way of writing nested namespaces. It specifies the nested namespace with the dot operator between csharp_station and tutorial. The result is exactly the same as Listing 6-2. However, Listing 6-3 is easier to write.
Listing 6-4. Calling Namespace Members: NamespaceCall.cs
// Namespace Declarationusing System;namespace csharp_station { // nested namespace namespace tutorial { class myExample1 { public static void myPrint1() { Console.WriteLine("First Example of calling another namespace member."); } } } // Program start class class NamespaceCalling { // Main begins program execution. public static void Main() { // Write to console tutorial.myExample1.myPrint1(); tutorial.myExample2.myPrint2(); } }}// same namespace as nested namespace abovenamespace csharp_station.tutorial { class myExample2 { public static void myPrint2() { Console.WriteLine("Second Example of calling another namespace member."); } }}
Listing 6-4 provides an example of how to call namespace members with fully qualified names. A fully qualified name contains every language element from the namespace name down to the method call. At the top of the listing there is a nested namespace tutorial within the csharp-station namespace with class myExample1 and method myPrint1. Main() calls this method with the fully qualified name of tutorial.myExample1.myPrint(). Since Main() and the tutorial namespace are located in the same namespace, using csharp_station in the fully qualified name is unnecessary.
At the bottom of Listing 6-4 is an addition to the csharp_station.tutorial namespace. The classes myExample1 and myExample2 both belong to the same namespace. Additionally, they could be written in separate files and still belong to the same namespace. In Main(), the myPrint2() method is called with the fully qualified name tutorial.myExample2.myPrint2(). Although the class myExample2 is outside the bounding braces of where the method myPrint2 is called, the namespace csharp_station does not need to be a part of the fully qualified name. This is because both classes belong to the same namespace, csharp_station.
Notice that I used different names for the two classes myExample1 and myExample2. This was necessary because every namespace member of the same type must have a unique name. Remember, they are both in the same namespace and you wouldn't want any ambiguity about which class to use. The methods myPrint1() and myPrint2() have different names only because it would make the lesson a little easier to follow. They could have had the same name with no effect, because their classes are different, thus avoiding any ambiguity.
Listing 6-5. The using Directive: UsingDirective.cs
// Namespace Declarationusing System;using csharp_station.tutorial;// Program start classclass UsingDirective { // Main begins program execution. public static void Main() { // Call namespace member myExample.myPrint(); }}// C# Station Tutorial Namespacenamespace csharp_station.tutorial { class myExample { public static void myPrint() { Console.WriteLine("Example of using a using directive."); } }}
If you would like to call methods without typing their fully qualified name, you can implement the using directive. In Listing 6-5, we show two using directives. The first, using System, is the same using directive you have seen in every program in this tutorial. It allows you to type the method names of members of the System namespace without typing the word System every time. In myPrint(), Console is a class member of the System namespace with the method WriteLine(). Its fully qualified name is System.Console.WriteLine(...).
Similarly, the using directive using csharp_station.tutorial allows us to call members of the csharp_station.tutorial namespace without typing the fully qualified name. This is why we can type myExample.myPrint(). Without the using directive, we would have to type csharp_station.tutorial.myExample.myPrint() every time we wanted to call that method.
Listing 6-6. The Alias Directive: AliasDirective.cs
// Namespace Declarationusing System;using csTut = csharp_station.tutorial.myExample; // alias// Program start classclass AliasDirective { // Main begins program execution. public static void Main() { // Call namespace member csTut.myPrint(); myPrint(); } // Potentially ambiguous method. static void myPrint() { Console.WriteLine("Not a member of csharp_station.tutorial.myExample."); }}// C# Station Tutorial Namespacenamespace csharp_station.tutorial { class myExample { public static void myPrint() { Console.WriteLine("This is a member of csharp_station.tutorial.myExample."); } }}
Sometimes you may encounter a long namespace and wish to have it shorter. This could improve readability and still avoid name clashes with similarly named methods. Listing 6-6 shows how to create an alias with the alias directive using csTut = csharp_station.tutorial.myExample. Now the expression csTut can be used anywhere, in this file, in place of csharp_station.tutorial.myExample. We use it in Main().
Also in Main() is a call to the myPrint() method of the AliasDirective class. This is the same name as the myPrint() method in the myExample class . The reason both of these methods can be called in the same method call is because the myPrint() method in the myExample class is qualified with the csTut alias. This lets the compiler know exactly which method is to be executed. Had we mistakenly omitted csTut from the method call, the compiler would have set up the myPrint() method of the AliasDirective class to run twice.
So far, all we've shown in our namespaces are classes. However, namespaces can hold other types as follows:
Classes
Structures
Interfaces
Enumerations
Delegates
Future chapters we will cover what these types are in more detail.
In summary, you know what a namespace is and you can declare your own namespaces. If you don't want to type a fully qualified name, you know how to implement the using directive. When you want to shorten a long namespace declaration, you can use the alias directive. Also, you have been introduced to some of the other namespace members in addition to the class type.
I invite you to return for Lesson 7: Introduction to Classes.
Your feedback and constructive contributions are welcome. Please feel free to contact me for feedback or comments you may have about this lesson.
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Copyright © 2000-2009 C# Station, All Rights Reserved
Lesson 07
The C# Station Tutorial
by Joe Mayo, 10/29/00, updated 11/13/01, 3/12/03, 2/21/08, 1/12/09
Lesson 7: Introduction to Classes
This lesson introduces you to C# Classes. Our objectives are as follows:
Implement Constructors.
Know the difference between instance and static members.
Understand Destructors.
Familiarization with Class Members.
Since the beginning of this tutorial, you have been using classes. By now, you should have a sense of what a class is for and how to specify one. This lesson will build upon what you already know and introduce the various class members.
Classes are declared by using the keyword class followed by the class name and a set of class members surrounded by curly braces. Every class has a constructor, which is called automatically any time an instance of a class is created. The purpose of constructors is to initialize class members when an instance of the class is created. Constructors do not have return values and always have the same name as the class. Listing 7-1 is an example of a class.
Listing 7-1. Example C# Classes: Classes.cs
// Namespace Declarationusing System;// helper classclass OutputClass { string myString; // Constructor public OutputClass(string inputString) { myString = inputString; } // Instance Method public void printString() { Console.WriteLine("{0}", myString); } // Destructor ~OutputClass() { // Some resource cleanup routines }}// Program start classclass ExampleClass { // Main begins program execution. public static void Main() { // Instance of OutputClass OutputClass outCl = new OutputClass("This is printed by the output class."); // Call Output class' method outCl.printString(); }}
Listing 7-1 shows two classes. The top class, OutputClass, has a constructor, instance method, and a destructor. It also had a field named myString. Notice how the OutputClass constructor is used to initialize data members of the class. In this case, the OutputClass constructor accepts a string argument, inputString. This string is copied to the class field myString.
Constructors are not mandatory, as indicated by the implementation of ExampleClass. In this case, a default constructor is provided. A default constructor is simply a constructor with no arguments. However, a constructor with no arguments is not always useful. To make default constructors more useful, you can implement them with initializers. Here is an example:
public OutputClass() : this("Default Constructor String") { }
Imagine this constructor was included in class OutputClass from Listing 7-1. This default constructor is followed by an initializer. The colon, ":", marks the beginning of the initializer, followed by the this keyword. The this keyword refers to this particular object. It effectively makes a call to the constructor of the same object it is defined in. After the this keyword is a parameter list with a string. The action taken by the initializer above is to invoke the OutputClass constructor that takes a string type as an argument. The initializer helps you to ensure your class fields are initialized when a class is instantiated.
The example above illustrates how a class can have multiple constructors. The specific constructor called depends on the number of parameters and the type of each parameter.
In C#, there are two types of class members, instance and static. Instance class members belong to a specific occurrence of a class. Every time you declare an object of a certain class, you create a new instance of that class. The ExampleClass Main() method creates an instance of the OutputClass named outCl. You can create multiple instances of OutputClass with different names. Each of these instances are separate and stand alone. For example, if you create two OutputClass instances as follows:
OutputClass oc1 = new OutputClass("OutputClass1"); OutputClass oc2 = new OutputClass("OutputClass2");
You create two separate instances of OutputClass with separate myString fields and separate printString() methods. On the other hand, if a class member is static, you can access it simply by using the syntax .. The instance names are oc1 and oc2.
Suppose OutputClass had the following static method:
public static void staticPrinter() { Console.WriteLine("There is only one of me."); }
Then you could call that function from Main() like this: OutputClass.staticPrinter();
You must call static class members through their class name and not their instance name. This means that you don't need to instantiate a class to use its static members. There is only ever one copy of a static class member. A good use of static members is when there is a function to be performed and no intermediate state is required, such as math calculations. Matter of fact, the .NET Frameworks Base Class Library includes a Math class that makes extensive use of static members.
Another type of constructor is the static constructor. Use static constructor to initialize static fields in a class. You declare a static constructor by using the keyword static just in front of the constructor name. A static constructor is called before an instance of a class is created, before a static member is called, and before the static constructor of a derived class (covered in a later chapter). They are called only once.
OutputClass also has a destructor. Destructors look just like constructors, except they have a tilde, "~", in front of them. They don't take any parameters and do not return a value. Destructors are places where you could put code to release any resources your class was holding during its lifetime. They are normally called when the C# garbage collector decides to clean your object from memory.
Note: You've probably noticed the use of the public modifier (an access modifier), meaning that a class member can be accessed from other classes. When used on a class, it means that the class can be accessed by DLLs outside of the Assembly (which is commonly a *.exe or *.dll file). Lesson 19: Encapsulation discusses access modifiers in more depth.
So far, the only class members you've seen are Fields, Methods, Constructors, and Destructors. Here is a complete list of the types of members you can have in your classes:
Constructors
Destructors
Fields
Methods
Properties
Indexers
Delegates
Events
Nested Classes
Those items not covered in this lesson will be covered in later lessons.
In summary, you can declare instance and static constructors. You know how to initialize class fields. When there is no need to instantiate an object, you can create static class members. You can also declare destructors for cleaning up resources.
I invite you to return for Lesson 8: Class Inheritance.
Your feedback and constructive contributions are welcome. Please feel free to contact me for feedback or comments you may have about this lesson.
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Copyright © 2000-2009 C# Station, All Rights Reserved
Lesson 08
The C# Station Tutorial
by Joe Mayo, 12/9/00, updated 12/8/01, 3/12/03, 2/21/08, 1/12/09
Lesson 8: Class Inheritance
This lesson teaches about C# Inheritance. Our objectives are as follows:
Implement Base Classes.
Implement Derived Classes.
Initialize Base Classes from Derived Classes.
Learn How to Call Base Class Members.
Learn How to Hide Base Class Members.
Inheritance is one of the primary concepts of object-oriented programming. It allows you to reuse existing code. Through effective employment of reuse, you can save time in your programming.
Listing 8-1. Inheritance: BaseClass.cs
using System;public class ParentClass{ public ParentClass() { Console.WriteLine("Parent Constructor."); } public void print() { Console.WriteLine("I'm a Parent Class."); }}public class ChildClass : ParentClass{ public ChildClass() { Console.WriteLine("Child Constructor."); } public static void Main() { ChildClass child = new ChildClass(); child.print(); }}
Output: Parent Constructor.
Child Constructor.
I'm a Parent Class.
Listing 8-1 shows two classes. The top class is named ParentClass and the main class is called ChildClass. What we want to do is create a child class, using existing code from ParentClass.
First we must declare our intention to use ParentClass as the base class of ChildClass. This is accomplished through the ChildClass declaration public class ChildClass : ParentClass. The base class is specified by adding a colon, ":", after the derived class identifier and then specifying the base class name.
Note: C# supports single class inheritance only. Therefore, you can specify only one base class to inherit from. However, it does allow multiple interface inheritance, a subject covered in a later lesson.
ChildClass has exactly the same capabilities as ParentClass. Because of this, you can also say ChildClass "is" a ParentClass. This is shown in the Main() method of ChildClass when the print() method is called. ChildClass does not have its own print() method, so it uses the ParentClass print() method. You can see the results in the 3rd line of output.
Base classes are automatically instantiated before derived classes. Notice the output from Listing 8-1. The ParentClass constructor executed before the ChildClass constructor.
Listing 8-2. Derived Class Communicating with Base Class: BaseTalk.cs
using System;

public class Parent
{
string parentString;
public Parent()
{
Console.WriteLine("Parent Constructor.");
}
public Parent(string myString)
{
parentString = myString;
Console.WriteLine(parentString);
}
public void print()
{
Console.WriteLine("I'm a Parent Class.");
}
}

public class Child : Parent
{
public Child() : base("From Derived")
{
Console.WriteLine("Child Constructor.");
}
public new void print()
{
base.print();
Console.WriteLine("I'm a Child Class.");
}
public static void Main()
{
Child child = new Child();
child.print();
((Parent)child).print();
}
}
Output: From Derived
Child Constructor.
I'm a Parent Class.
I'm a Child Class.
I'm a Parent Class.
Derived classes can communicate with base classes during instantiation. Listing 8-2 shows how this is done at the child constructor declaration. The colon, ":", and keyword base call the base class constructor with the matching parameter list. If the code had not appended base("From Derived") to the Derived constructor, the code would have automatically called Parent(). The first line of output shows the base class constructor being called with the string "From Derived".
Sometimes you may want to create your own implementation of a method that exists in a base class. The Child class does this by declaring its own print() method. The Child print() method hides the Parent print() method. The effect is the Parent print() method will not be called, unless we do something special to make sure it is called.
Inside the Child print() method, we explicitly call the Parent print() method. This is done by prefixing the method name with "base.". Using the base keyword, you can access any of a base class public or protected class members. The output from the Child print() method is on output lines 3 and 4.
Another way to access base class members is through an explicit cast. This is done in the last statement of the Child class Main() method. Remember that a derived class is a specialization of its base class. This fact allows us to perform a cast on the derived class, making it an instance of its base class. The last line of output from Listing 8-2 shows the Parent print() method was indeed executed.
Notice the new modifier on the Child class print() method. This enables this method to hide the Parent class print() method and explicitly states your intention that you don't want polymorphism to occur. Without the new modifier, the compiler will produce a warning to draw your attention to this. See the next lesson for a detailed discussion of polymorphism.
In summary, you know how to create a derived/base class relationship. You can control instantiation of your base class and call its methods either implicitly or explicitly. You also understand that a derived class is a specialization of its base class.
I invite you to return for Lesson 9: Polymorphism.
Your feedback and constructive contributions are welcome. Please feel free to contact me for feedback or comments you may have about this lesson.
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Copyright © 2000-2009 C# Station, All Rights Reserved
Lesson 09
The C# Station Tutorial
by Joe Mayo, 01/15/01, updated 3/12/03, 2/21/08, 1/12/09
Lesson 9: Polymorphism
This lesson teaches about Polymorphism in C#. Our objectives are as follows:
Learn What Polymorphism Is.
Implement a Virtual Method.
Override a Virtual Method.
Use Polymorphism in a Program.
Another primary concept of object-oriented programming is Polymorphism. It allows you to invoke derived class methods through a base class reference during run-time. This is handy when you need to assign a group of objects to an array and then invoke each of their methods. They won't necessarily have to be the same object type. However, if they're related by inheritance, you can add them to the array as the inherited type. Then if they all share the same method name, that method of each object can be invoked. This lesson will show you how to accomplish this.
Listing 9-1. A Base Class With a Virtual Method: DrawingObject.cs
using System;public class DrawingObject{ public virtual void Draw() { Console.WriteLine("I'm just a generic drawing object."); }}
Listing 9-1 shows the DrawingObject class. This will be the base class for other objects to inherit from. It has a single method named Draw(). The Draw() method has a virtual modifier. The virtual modifier indicates to derived classes that they can override this method. The Draw() method of the DrawingObject class performs a single action of printing the statement, "I'm just a generic drawing object.", to the console.
Listing 9-2. Derived Classes With Override Methods: Line.cs, Circle.cs, and Square.cs
using System;public class Line : DrawingObject{ public override void Draw() { Console.WriteLine("I'm a Line."); }}public class Circle : DrawingObject{ public override void Draw() { Console.WriteLine("I'm a Circle."); }}public class Square : DrawingObject{ public override void Draw() { Console.WriteLine("I'm a Square."); }}
Listing 9-2 shows three classes. These classes inherit the DrawingObject class. Each class has a Draw() method and each Draw() method has an override modifier. The override modifier allows a method to override the virtual method of its base class at run-time. The override will happen only if the class is referenced through a base class reference. Overriding methods must have the same signature, name and parameters, as the virtual base class method it is overriding.
Listing 9-3. Program Implementing Polymorphism: DrawDemo.cs
using System;public class DrawDemo{ public static int Main( ) { DrawingObject[] dObj = new DrawingObject[4]; dObj[0] = new Line(); dObj[1] = new Circle(); dObj[2] = new Square(); dObj[3] = new DrawingObject(); foreach (DrawingObject drawObj in dObj) { drawObj.Draw(); } return 0; }}
Listing 9-3 shows a program that uses the classes defined in Listing 9-1 and Listing 9-2. This program implements polymorphism. In the Main() method of the DrawDemo class, there is an array being created. The type of object in this array is the DrawingObject class. The array is named dObj and is being initialized to hold four objects of type DrawingObject.
Next the dObj array is initialized. Because of their inheritance relationship with the DrawingObject class, the Line, Circle, and Square classes can be assigned to the dObj array. Without this capability, you would have to create an array for each type. Inheritance allows derived objects to act like their base class, which saves work.
After the array is initialized, there is a foreach loop that looks at each element of the array. Within the foreach loop the Draw() method is invoked on each element of the dObj array. Because of polymorphism, the run-time type of each object is invoked. The type of the reference object from the dObj array is a DrawingObject. However, that doesn't matter because the derived classes override the virtual Draw() method of the DrawingObject class. This makes the overriden Draw() methods of the derived classes execute when the Draw() method is called using the DrawingObject base class reference from the dObj array. Here's what the output looks like:
Output: I'm a Line. I'm a Circle. I'm a Square. I'm just a generic drawing object.
The override Draw() method of each derived class executes as shown in the DrawDemo program. The last line is from the virtual Draw() method of the DrawingObject class. This is because the actual run-time type of the fourth array element was a DrawingObject object.
The code in this lesson can be compiled with the following command line: csc DrawDemo.cs DrawingObject.cs Circle.cs Line.cs Square.cs
It will create the file DrawDemo.exe, which defaulted to the name of the first file on the command line.
Summary
You should now have a basic understanding of polymorphism. You know how to define a virtual method. You can implement a derived class method that overrides a virtual method. This relationship between virtual methods and the derived class methods that override them enables polymorphism. This lesson showed how to use this relationship between classes to implement polymorphism in a program.
I invite you to return for Lesson 10: Properties.
Your feedback and constructive contributions are welcome. Please feel free to contact me for feedback or comments you may have about this lesson.
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Copyright © 2000-2009 C# Station, All Rights Reserved
Lesson 10
The C# Station Tutorial
by Joe Mayo created 02/10/01, updated 3/12/03, 2/22/08, 4/29/08, and 9/19/08
Lesson 10: Properties
This lesson teaches C# Properties. Our objectives are as follows:
Understand What Properties Are For.
Implement a Property.
Create a Read-Only Property.
Create a Write-Only Property.
Create an auto-implemented property.
Overview of Properties
Properties provide the opportunity to protect a field in a class by reading and writing to it through the property. In other languages, this is often accomplished by programs implementing specialized getter and setter methods. C# properties enable this type of protection while also letting you access the property just like it was a field.
Another benefit of properties over fields is that you can change their internal implementation over time. With a public field, the underlying data type must always be the same because calling code depends on the field being the same. However, with a property, you can change the implementation. For example, if a customer has an ID that is originally stored as an int, you might have a requirements change that made you perform a validation to ensure that calling code could never set the ID to a negative value. If it was a field, you would never be able to do this, but a property allows you to make such a change without breaking code. Now, lets see how to use properties.
Traditional Encapsultation Without Properties
Languages that don't have properties will use methods (functions or procedures) for encapsultation. The idea is to manage the values inside of the object, state, avoiding corruption and misuse by calling code. Listing 10-1 demonstrates how this traditional method works, encapsulating Customer information via accessor methods.
Listing 10-1. An Example of Traditional Class Field Accessusing System; public class Customer{ private int m_id = -1; public int GetID() { return m_id; } public void SetID(int id) { m_id = id; } private string m_name = string.Empty; public string GetName() { return m_name; } public void SetName(string name) { m_name = name; }} public class CustomerManagerWithAccessorMethods{ public static void Main() { Customer cust = new Customer(); cust.SetID(1); cust.SetName("Amelio Rosales"); Console.WriteLine( "ID: {0}, Name: {1}", cust.GetID(), cust.GetName()); Console.ReadKey(); }}
Listing 10-1 shows the traditional method of accessing class fields. The Customer class has four properties, two for each private field that the class encapsulates: m_id and m_name. As you can see, SetID and SetName assign a new values and GetID and GetName return values.
Observe how Main calls the SetXxx methods, which sets m_id to 1 and m_name to "Amelio Rosales" in the Customer instance, cust. The call to Console.WriteLine demonstrates how to read m_id and m_name from cust, via GetID and GetName method calls, respectively.
This is such a common pattern, that C# has embraced it in the form of a language feature called properties, which you'll see in the next section.
Encapsulating Type State with Properties
The practice of accessing field data via methods was good because it supported the object-oriented concept of encapsulation. For example, if the type of m_id or m_name changed from an int type to byte, calling code would still work. Now the same thing can be accomplished in a much smoother fashion with properties, as shown in Listing 10-2.
Listing 10-2. Accessing Class Fields With Propertiesusing System; public class Customer{ private int m_id = -1; public int ID { get { return m_id; } set { m_id = value; } } private string m_name = string.Empty; public string Name { get { return m_name; } set { m_name = value; } }} public class CustomerManagerWithProperties{ public static void Main() { Customer cust = new Customer(); cust.ID = 1; cust.Name = "Amelio Rosales"; Console.WriteLine( "ID: {0}, Name: {1}", cust.ID, cust.Name); Console.ReadKey(); }}
Listing 10-2 shows how to create and use a property. The Customer class has the ID and Name property implementations. There are also private fields named m_id and m_name; which ID and Name, respectively, encapsulate. Each property has two accessors, get and set. The accessor returns the value of a field. The set accessor sets the value of a field with the contents of value, which is the value being assigned by calling code. The value shown in the accessor is a C# reserved word.
When setting a property, just assign a value to the property as if it were a field. The CustomerManagerWithProperties class uses the ID and Name properties in the Customer class. The first line of Main instantiates a Customer object named cust. Next the value of the m_id and m_name fields of cust are set by using the ID and Name properties.
To read from a property, use the property as if it were a field. Console.WriteLine prints the value of the m_id and m_name fields of cust. It does this by calling the ID and Name properties of cust.
This was a read/write property, but you can also create read-only properties, which you'll learn about next.
Creating Read-Only Properties
Properties can be made read-only. This is accomplished by having only a get accessor in the property implementation. Listing 10-3 demonstrates how you can create a read-only property.
Listing 10-3. Read-Only Propertiesusing System; public class Customer{ private int m_id = -1; private string m_name = string.Empty; public Customer(int id, string name) { m_id = id; m_name = name; } public int ID { get { return m_id; } } public string Name { get { return m_name; } }} public class ReadOnlyCustomerManager{ public static void Main() { Customer cust = new Customer(1, "Amelio Rosales"); Console.WriteLine( "ID: {0}, Name: {1}", cust.ID, cust.Name); Console.ReadKey(); }}
The Customer class in Listing 10-3 has two read-only properties, ID and Name. You can tell that each property is read-only because they only have get accessors. At some time, values for the m_id and m_name must be assigned, which is the role of the constructor in this example.
The Main method of the ReadOnlyCustomerManager class instantiates a new Customer object named cust. The instantiation of cust uses the constructor of Customer class, which takes int and string type parameters. In this case, the values are 1 and "Amelio Rosales". This initializes the m_id and m_name fields of cust.
Since the ID and Name properties of the Customer class are read-only, there is no other way to set the value of the m_id and m_name fields. If you inserted cust.ID = 7 into the listing, the program would not compile, because ID is read-only; the same goes for Name. When the ID and Name properties are used in Console.WriteLine, they work fine. This is because these are read operations which only invoke the get accessor of the ID and Name properties.
One question you might have now is "If a property can be read-only, can it also be write-only?" The answer is yes, and explained in the next section.
Creating a Write-Only Property
You can assign values to, but not read from, a write-only property. A write-only property only has a set accessor. Listing 10-4 shows you how to create and use write-only properties.
Listing 10-4. Write-Only Propertiesusing System; public class Customer{ private int m_id = -1; public int ID { set { m_id = value; } } private string m_name = string.Empty; public string Name { set { m_name = value; } } public void DisplayCustomerData() { Console.WriteLine("ID: {0}, Name: {1}", m_id, m_name); }} public class WriteOnlyCustomerManager{ public static void Main() { Customer cust = new Customer(); cust.ID = 1; cust.Name = "Amelio Rosales"; cust.DisplayCustomerData(); Console.ReadKey(); }}
This time, the get accessor is removed from the ID and Name properties of the Customer class, shown in Listing 10-1. The set accessors have been added, assigning value to the backing store fields, m_id and m_name.
The Main method of the WriteOnlyCustomerManager class instantiates the Customer class with a default constructor. Then it uses the ID and Name properties of cust to set the m_id and m_name fields of cust to 1 and "Amelio Rosales", respectively. This invokes the set accessor of ID and Name properties from the cust instance.
When you have a lot of properties in a class or struct, there can also be a lot of code associated with those properties. In the next section, you'll see how to write properties with less code.
Creating Auto-Implemented Properties
The patterns you see here, where a property encapsulates a property with get and set accessors, without any other logic is common. It is more code than we should have to write for such a common scenario. That's why C# 3.0 introduced a new syntax for a property, called an auto-implemented property, which allows you to create properties without get and set accessor implementations. Listing 10-5 shows how to add auto-implemented properties to a class.
Listing 10-5. Auto-Impemented Propertiesusing System; public class Customer{ public int ID { get; set; } public string Name { get; set; }} public class AutoImplementedCustomerManager{ static void Main() { Customer cust = new Customer(); cust.ID = 1; cust.Name = "Amelio Rosales"; Console.WriteLine( "ID: {0}, Name: {1}", cust.ID, cust.Name); Console.ReadKey(); }}
Notice how the get and set accessors in Listing 10-5 do not have implementations. In an auto-implemented property, the C# compiler creates the backing store field behind the scenes, giving the same logic that exists with traditional properties, but saving you from having to use all of the syntax of the traditional property. As you can see in the Main method, the usage of an auto-implemented property is exactly the same as traditional properties, which you learned about in previous sections.
Summary
You now know what properties are for and how they're used. Traditional techniques of encapsulation have relied on separate methods. Properties allow you to access objects state with field-like syntax. Properties can be made read-only or write-only. You also learned how to write properties with less code by using auto-implemented properties.
I invite you to return for Lesson 11: Indexers.
Your feedback and constructive contributions are welcome. Please feel free to contact me for feedback or comments you may have about this lesson.
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Copyright © 2000-2009 C# Station, All Rights Reserved
Lesson 11
The C# Station Tutorial
by Joe Mayo, 04/23/01, updated 3/12/03, 2/21/08, 1/12/09
Lesson 11: Indexers
This lesson teaches C# Indexers. Our objectives are as follows:
Understand What Indexers Are For.
Implement an Indexer.
Overload Indexers.
Understand How to Implement Multi-Parameter Indexers.
Indexers are real easy. They allow your class to be used just like an array. On the inside of a class, you manage a collection of values any way you want. These objects could be a finite set of class members, another array, or some complex data structure. Regardless of the internal implementation of the class, its data can be obtained consistently through the use of indexers. Here's an example.
Listing 11-1. An Example of An Indexer: IntIndexer.cs
using System;///

/// A simple indexer example.///

class IntIndexer{ private string[] myData; public IntIndexer(int size) { myData = new string[size]; for (int i=0; i < size; i++) { myData[i] = "empty"; } } public string this[int pos] { get { return myData[pos]; } set { myData[pos] = value; } } static void Main(string[] args) { int size = 10; IntIndexer myInd = new IntIndexer(size); myInd[9] = "Some Value"; myInd[3] = "Another Value"; myInd[5] = "Any Value"; Console.WriteLine("\nIndexer Output\n"); for (int i=0; i < size; i++) { Console.WriteLine("myInd[{0}]: {1}", i, myInd[i]); } }}
Listing 11-1 shows how to implement an Indexer. The IntIndexer class has a string array named myData. This is a private array that external users can't see. This array is initialized in the constructor, which accepts an int size parameter, instantiates the myData array, and then fills each element with the word "empty".
The next class member is the Indexer, which is identified by the this keyword and square brackets, this[int pos]. It accepts a single position parameter, pos. As you may have already guessed, the implementation of an Indexer is the same as a Property. It has get and set accessors that are used exactly like those in a Property. This indexer returns a string, as indicated by the string return value in the Indexer declaration.
The Main() method simply instantiates a new IntIndexer object, adds some values, and prints the results. Here's the output: Indexer Output myInd[0]: emptymyInd[1]: emptymyInd[2]: emptymyInd[3]: Another ValuemyInd[4]: emptymyInd[5]: Any ValuemyInd[6]: emptymyInd[7]: emptymyInd[8]: emptymyInd[9]: Some Value
Using an integer is a common means of accessing arrays in many languages, but the C# Indexer goes beyond this. Indexers can be declared with multiple parameters and each parameter may be a different type. Additional parameters are separated by commas, the same as a method parameter list. Valid parameter types for Indexers include integers, enums, and strings. Additionally, Indexers can be overloaded. In listing 11-2, we modify the previous program to accept overloaded Indexers that accept different types.
Listing 11-2. Overloaded Indexers: OvrIndexer.cs
using System;///

/// Implements overloaded indexers.///

class OvrIndexer{ private string[] myData; private int arrSize; public OvrIndexer(int size) { arrSize = size; myData = new string[size]; for (int i=0; i < size; i++) { myData[i] = "empty"; } } public string this[int pos] { get { return myData[pos]; } set { myData[pos] = value; } } public string this[string data] { get { int count = 0; for (int i=0; i < arrSize; i++) { if (myData[i] == data) { count++; } } return count.ToString(); } set { for (int i=0; i < arrSize; i++) { if (myData[i] == data) { myData[i] = value; } } } } static void Main(string[] args) { int size = 10; OvrIndexer myInd = new OvrIndexer(size); myInd[9] = "Some Value"; myInd[3] = "Another Value"; myInd[5] = "Any Value"; myInd["empty"] = "no value"; Console.WriteLine("\nIndexer Output\n"); for (int i=0; i < size; i++) { Console.WriteLine("myInd[{0}]: {1}", i, myInd[i]); } Console.WriteLine("\nNumber of \"no value\" entries: {0}", myInd["no value"]); }}
Listing 11-2 shows how to overload Indexers. The first Indexer, with the int parameter, pos, is the same as in Listing 11-1, but there is a new Indexer that takes a string parameter. The get accessor of the new indexer returns a string representation of the number of items that match the parameter value, data. The set accessor changes each entry in the array that matches the data parameter to the value that is assigned to the Indexer.
The behavior of the overloaded Indexer that takes a string parameter is demonstrated in the Main() method of Listing 11-2. It invokes the set accessor, which assigns the value of "no value" to every member of the myInd class that has the value of "empty". It uses the following command: myInd["empty"] = "no value";. After each entry of the myInd class is printed, a final entry is printed to the console, indicating the number of entries with the "no value" string. This happens by invoking the get accessor with the following code: myInd["no value"]. Here's the output: Indexer Output myInd[0]: no valuemyInd[1]: no valuemyInd[2]: no valuemyInd[3]: Another ValuemyInd[4]: no valuemyInd[5]: Any ValuemyInd[6]: no valuemyInd[7]: no valuemyInd[8]: no valuemyInd[9]: Some Value Number of "no value" entries: 7
The reason both Indexers in Listing 11-2 can coexist in the same class is because they have different signatures. An Indexer signature is specified by the number and type of parameters in an Indexers parameter list. The class will be smart enough to figure out which Indexer to invoke, based on the number and type of arguments in the Indexer call. An indexer with multiple parameters would be implemented something like this:
public object this[int param1, ..., int paramN] { get { // process and return some class data } set { // process and assign some class data } }
Summary
You now know what Indexers are for and how they're used. You can create an Indexer to access class members similar to arrays. Overloaded and multi-parameter Indexers were also covered.
I invite you to return for Lesson 12: Structs.
Your feedback and constructive contributions are welcome. Please feel free to contact me for feedback or comments you may have about this lesson.
Feedback
I like this site and want to support it!
Copyright © 2000-2009 C# Station, All Rights Reserved