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INFS 222 OBJECT ORIENTED PROGRAMMING CHAPTER 2 C# PROGRAMMING

DEPARTMENT OF INFORMATION SYSTEMS, UNIVERSITY OF NIZWA, SULTANATE OF OMAN. 1

CHAPTER – 2

C# PROGRAMMING

1.1 INTRODUCTION TO VISUAL C# 2008 IDE

The Visual C# integrated development environment (IDE) is a collection of development

tools exposed through a common user interface. Some of the tools are shared with other Visual

Studio languages, and some, such as the C# compiler, are unique to Visual C#. The

documentation in this section provides an overview of how to use the most important Visual C#

tools as you work in the IDE in various phases of the development process.

VISUAL C# TOOLS

The following are the most important tools and windows in Visual C#. The windows for

most of these tools can be opened from the View menu.

The Code Editor, for writing source code.

The C# compiler, for converting C# source code into an executable program.

The Visual Studio debugger, for testing your program.

The Toolbox and Designer, for rapid development of user interfaces by using the mouse.

Solution Explorer, for viewing and managing project files and settings.

Project Designer, for configuring compiler options, deployment paths, resources, and

more.

Class View, for navigating through source code according to types, not files.



Properties Window, for configuring properties and events on controls in your user

interface.

Object Browser, for viewing the methods and classes available in dynamic link libraries

including .NET Framework assemblies and COM objects.

Document Explorer, for browsing and searching product documentation on a local

computer and on the Internet.

HOW THE IDE EXPOSES THE TOOLS

You interact with the tools through windows, menus, property pages, and wizards in the

IDE. The following represents the basic IDE:

You can quickly access any open tool windows or files by pressing CTRL + TAB. For more

information, see Navigating and Searching (Visual C#).

EDITOR AND WINDOWS FORM DESIGNER WINDOWS

The large main window is used by the Code Editor, the Windows Forms Designer, or the

Windows Presentation Foundation Designer. You can switch to Code view or Design view by

clicking Code or Designer on the View menu or by using the keyboard shortcuts F7 or SHIFT +

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F7. While in Design view, you can drag controls onto the window from the Toolbox, which you

can make visible by clicking the Toolbox tab on the left margin. For more information about the

Code Editor, see Editing Code (Visual C#). For more information about the Designers,

see Windows Forms Designer and Getting Started with the WPF Designer.

The Properties window in the lower-right is populated only in Design view. It enables

you to set properties and hook up events for user interface controls such as buttons, text boxes,

and so on. When you set this window to Auto Hide, it will collapse into the right margin

whenever you switch to Code View. For more information about the Properties window and the

Designer, see designing a User Interface (Visual C#).

SOLUTION EXPLORER AND PROJECT DESIGNER

The window in the upper-right is Solution Explorer, which shows all the files in your

project in a hierarchical tree view. When you use the Project menu to add new files to your

project, you will see them reflected in Solution Explorer. In addition to files, Solution

Explorer also displays your project settings, and references to external libraries required by your

application.

The Project Designer property pages are accessed by right-clicking the Properties node

in Solution Explorer, and then clicking Open. Use these pages to modify build options, security

requirements, deployment details, and many other project properties. For more information

about Solution Explorer and the Project Designer, see Creating a Project (Visual C#).

Compiler, Debugger, and Error List Windows

The C# compiler has no window because it is not an interactive tool, but you can set

compiler options in the Project Designer. When you click Build on the Build menu, the C#


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compiler is invoked by the IDE. If the build is successful, the status pane displays a Build

Succeeded message. If there were build errors, the Error List window appears below the

editor/designer window with a list of errors. Double-click an error to go to the problem line in

the source code. Press F1 to see Help documentation for the highlighted error.

The debugger has various windows that display values of variables and type information

as your application is running. You can use the Code Editor window while debugging to specify

a line at which to pause execution in the debugger, and to step through code one line at a time.

For more information, see Building and Debugging (Visual C#).




1.2 BASIC PROGRAMMING USING VISUAL C#

HELLO WORLD – YOUR FIRST PROGRAM (C# PROGRAMMING)

The following procedure creates a C# version of the traditional "Hello World!" program. The

program displays the string Hello World!

TO CREATE AND RUN A CONSOLE APPLICATION

1. Start Visual Studio.

2. On the menu bar, choose File, New, Project.

The New Project dialog box opens.

3. Expand Installed, expand Templates, expand Visual C#, and then choose Console

Application.

4. In the Name box, specify a name for your project, and then choose the OK button.

The new project appears in Solution Explorer.

5. If Program.cs isn't open in the Code Editor, open the shortcut menu

for Program.cs in Solution Explorer, and then choose View Code.

6. Replace the contents of Program.cs with the following code.



C# Program

1. // A Hello World! program in C#.

2. using System;

3. namespace HelloWorld

4. {

5. class Hello

6. {

7. static void Main()

8. {

9. Console.WriteLine("Hello World!");

10.

11. // Keep the console window open in debug mode.

12. Console.WriteLine("Press any key to exit.");

13. Console.ReadKey();

14. }

15. }

16. }

7. Choose the F5 key to run the project. A Command Prompt window appears that contains

the line Hello World!

Next, the important parts of this program are examined.



COMMENTS

The first line contains a comment. The characters // convert the rest of the line to a comment.

// A Hello World! program in C#.

You can also comment out a block of text by enclosing it between the /* and */ characters. This

is shown in the following example.

/* A "Hello World!" program in C#.

This program displays the string "Hello World!" on the screen. */

MAIN METHOD

A C# console application must contain a Main method, in which control starts and ends.

The Main method is where you create objects and execute other methods. The Main method is

a static (C# Reference) method that resides inside a class or a struct. In the previous "Hello

World!" example, it resides in a class named Hello. You can declare the Main method in one of

the following ways:

It can return void.

static void Main()

{

//...

}

It can also return an integer.

static int Main()

{

//...

return 0;

}

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With either of the return types, it can take arguments.

static void Main(string[] args)

{

//...

}

-or-

static int Main(string[] args)

{

//...

return 0;

}

The parameter of the Main method, args, is a string array that contains the command-line

arguments used to invoke the program. Unlike in C++, the array does not include the name of the

executable (exe) file.

For more information about how to use command-line arguments, see the examples in Main()

and Command-Line Arguments (C# Programming Guide) and How to: Create and Use

Assemblies Using the Command Line (C# and Visual Basic).

The call to ReadKey at the end of the Main method prevents the console window from

closing before you have a chance to read the output when you run your program in debug mode,

by pressing F5.

INPUT AND OUTPUT

C# programs generally use the input/output services provided by the run-time library of

the .NET Framework. The statement System.Console.WriteLine("Hello World!"); uses

the WriteLine method. This is one of the output methods of the Console class in the run-time

library. It displays its string parameter on the standard output stream followed by a new line.

Other Console methods are available for different input and output operations. If you include

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the using System; directive at the beginning of the program, you can directly use

the System classes and methods without fully qualifying them. For example, you can

call Console.WriteLine instead of System.Console.WriteLine:

using System;

Console.WriteLine("Hello World!");

For more information about input/output methods, see System.IO.

COMMAND-LINE COMPILATION AND EXECUTION

You can compile the "Hello World!" program by using the command line instead of the

Visual Studio Integrated Development Environment (IDE).

TO COMPILE AND RUN FROM A COMMAND PROMPT

1. Paste the code from the preceding procedure into any text editor, and then save the file as

a text file. Name the file Hello.cs. C# source code files use the extension.cs.

2. Perform one of the following steps to open a command-prompt window:

o In Windows 8, on the Start screen, search for Developer Command Prompt,

and then tap or choose Developer Command Prompt for VS2012.

A Developer Command Prompt window appears.

o In Windows 7, open the Start menu, expand the folder for the current version of

Visual Studio, open the shortcut menu for Visual Studio Tools, and then

choose Developer Command Prompt for VS2012.

A Developer Command Prompt window appears.

o Enable command-line builds from a standard Command Prompt window.

See How to: Set Environment Variables.

3. In the command-prompt window, navigate to the folder that contains your Hello.cs file.

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4. Enter the following command to compile Hello.cs.

csc Hello.cs

If your program has no compilation errors, an executable file that is named Hello.exe is

created.

5. In the command-prompt window, enter the following command to run the program:

Hello

C# BASICS

1. HOW TO WRITE ON SCREEN IN C#.

Console.WriteLine("I have been studying C# for 2 weeks!");

This will appear on screen, and will disappear immediately. So, we are writing another

statement.

Console.WriteLine("I have benn studying C# for 2 weeks!");

Console.ReadLine();

Now it will remain on screen, unless we press Enter key. Here we see that we can write letters,

numbers and special characters (#) on screen.

String: String is name of variable, and it could be anything letters, numbers and special

characters. We can use string for numbers also, but when we need any arithmetical operation, we

use integers, decimals etc. as variables.

2. STRING CONCATENATION:

We can add two strings, called string concatenation. For example, we are adding first

name and second name to get full name.

string a = "John ";

string b = "Smith";

string c = a + b;



Console.WriteLine(c);

Console.ReadLine();

3. ADDING TWO NUMBERS:

int a=10;

int b=12;

int c=a+b;


Console.ReadLine();

Here, we use integers as variables, because we want to add them (arithmetical operation).

Integers does NOT take fractional (decimal) values. If we want to perform arithmetical operation

of fractional values; we can take double as variables.

double a = 3.4;

double b = 5.2;

double c = a + b;


Console.ReadLine();

4. HOW C# TAKE INPUT

The program will take input from user, and will display it on screen

string name = Console.ReadLine();

Console.WriteLine(name);

Console.ReadLine();

Integer as input:

Input is always in string. So for integer, we need to convert it first.

int number = Convert.ToInt16(Console.ReadLine());

Console.WriteLine(number);

Console.ReadLine();



Remember: As the input is always in string, so for integers as input we need to convert it first.

5. TAKE TWO INPUTS (INTEGERS) FROM USER AND ADD THEM.

int number1 = Convert.ToInt16(Console.ReadLine());

int number2 = Convert.ToInt16(Console.ReadLine());

int number3 = number1 + number2;

Console.WriteLine(number3);

Console.ReadLine();

6. TAKE TWO INPUTS (STRING) FROM USER, AND ADD THEM.

string firstname = Console.ReadLine();

string lastname = Console.ReadLine();

string fullname = firstname + lastname;

Console.WriteLine(fullname);

Console.ReadLine();

As, input is always in string, so we did not need to convert it.

CONDITIONAL STATEMENTS IN C#:

The if, if / else, if / else if / else Statement

The switch Statement

THE IF STATEMENT:

Syntax:

if (condition)

{statement}



For example,

int a = Convert.ToInt16(Console.ReadLine());

if (a > 10)

{Console.WriteLine("The number is greater than 10");

Console.ReadLine();

THE IF / ELSE STATEMENT:

Syntax:

if (condition)

{statement}

else

{statement}

For example,


if (a > 10)

{Console.WriteLine("The number is greater than 10");}

else

{ Console.WriteLine("The number is 10 or less than 10");}

Console.ReadLine();



THE IF / ELSE IF / ELSE STATEMENT- (ALSO CALLED NESTED IF)

Syntax:

if (condition)

{statement}

else if (condition)

{statement}

else

{statement}

For example,


if (a > 10)

{Console.WriteLine("The number is greater than 10");}

else if (a == 10)

{Console.WriteLine("The number is 10");}

else

{ Console.WriteLine("The number is less than 10");}

Console.ReadLine();

NOTE: We write = two times.

THE SWITCH STATEMENT:

Syntax:

switch (integer a)

{

case 1:

statement

break;

case 2:

statement



break;

default:

statement

break;

}

NOTE: The default in switch statement is equivalent to else in if statement.

For example,

int week = Convert.ToInt16(Console.ReadLine());

switch (week)

{

case 1:

Console.WriteLine("Monday");

break;

Console.WriteLine("Tuesday");

break;

case 3:

Console.WriteLine("Wednesday");

break;

case 4:

Console.WriteLine("Thursday");

break;

case 5:

Console.WriteLine("Friday");

break;

case 6:

Console.WriteLine("Saturday");

break;



case 7:

Console.WriteLine("Sunday");

break;

default:

Console.WriteLine("NOT KNOWN");

break;

}

Console.ReadLine();

THE FOR LOOP IN C#

Syntax:

for (initial point; ending point; increament)

{

Statement(s)

}

For example, the following program will write counting from 1 to 20.

for (int i = 1; i < 21; i++)

{

Console.WriteLine(i);

}

Console.ReadLine();

Q. Write table of 2 using for loop in C#.

for (int i = 1; i < 11; i++)

{

int tab = 2 * i;

Console.WriteLine(tab);

}

Console.ReadLine();



Q. Write a program that print even numbers from 1 to 100.

for (int i = 1; i < 101; i++)

{

if (i % 2 == 0)

{

Console.WriteLine(i);

}

}

Console.ReadLine();

Q. Write a program that take input from user, and write table of that number.

Console.WriteLine("Enter a number:");

int num = Convert.ToInt16(Console.ReadLine());

for (int i = 1; i < 11; i++)

{

int tab = i * num;

Console.WriteLine(tab);

}

Console.ReadLine();

Q. Write a program in C sharp, to find the factorial of 5.

int fact = 1;

for (int i = 5; i > 0; i--)

{

fact=fact*i;

}

Console.WriteLine(fact);

Console.ReadLine();



Q. Write a program that take input from user, and find factorial of that number.

Console.WriteLine("Enter a number:");


Console.WriteLine("Its factorial is:");

int fact = 1;

for (int i = num; i > 0; i--)

{

fact=fact*i;

}

Console.WriteLine(fact);

Console.ReadLine();

THE WHILE LOOP

Syntax:

while (condition)

{

statement(s)

}

Q. Write a program in C# using while loop, that take a number from user and return cube of that

number. And the program ends when the input is 11.


int cube = num * num * num;

while (num != 11)

{

Console.WriteLine(cube);

break;

}

Console.ReadLine();



Q. Write a program that starts from 0, increase 1 by 1, and end before 10 using while loop in C

Sharp.

int number = 0;

while (number < 10)

{

Console.WriteLine(number);

number = number + 1;

}

Console.ReadLine();

THE DO - WHILE LOOP

Syntax:

do

{

statement(s)

}

while

{

statement(s)

}

Q. Write a program in C Sharp using do - while loop, that take a number, and increase it 2 by 2,

and ends before 30.


do

{

Console.WriteLine(num);

num = num + 2;

}



while (num < 30);

Console.ReadLine();

ARRAY IN C#

Syntax:

variable type [] variable name = new variable type [length]

Array of type integer with constant values

int[] myarray = new int[3] { 2, 5, 9 };

Console.WriteLine(myarray[0]);

Console.ReadLine();

NOTE: index 0 inside Console.WriteLine statement represents index of array, that is 2.

In the above myarray; index 0 = 2, index 1 = 5, index 2 = 9.

If we replace 0 by 1, the program will show 5, and for 2, the program will show 9.

Array of type string with constant values

string[] name = new string [3] { "Bilal", "Sohail", "Afzal" };

Console.WriteLine(name[0]);

Console.ReadLine();

Q. Write an array in C# of type integer that take 3 numbers as input (the program must close

after taking 3 inputs).

int[] myarray = new int[3];

myarray[0] = Convert.ToInt16(Console.ReadLine());



Console.WriteLine(myarray);

Console.ReadLine();



Q. Write an array in C Sharp of type string that take 3 strings (names) as input (the program must

ends after taking 3 inputs).

string[] name = new string[3];

name[0] = Console.ReadLine();



Console.WriteLine(name);

Console.ReadLine();

Q. Write an array in C# of type integer that take 10 numbers as input (Use forloop for

simplicity).


for (int i = 0; i < 10; i++)

{

myarray[i] = Convert.ToInt16(Console.ReadLine());

}

Console.WriteLine(myarray);

Console.ReadLine();

Q. Write a program in C Sharp that take 10 inputs from user, and show their sum.


for (int i = 0; i < 10; i++)

{


}

int a = 0;

for (int j = 0; j < 10; j++)

{

a = a + myarray[j];



}

Console.WriteLine(a);

Console.ReadLine();

Q. Write a program in C# that take 10 numbers, and show their average (input could be decimal

or fractional value also).

double[] myarray = new double[10];

for (int i = 0; i < 10; i++)

{


}

double a = 0;

double b = 0;

for (int j = 0; j < 10; j++)

{

a = a + myarray[j];

b = a / 10;

}

Console.WriteLine(b);

Console.ReadLine();

METHODS IN C#

In object oriented programming, we work with objects. Objects are the basic building

blocks of a program. An object consists of data and methods. Methods change the state of the

objects created. They are the dynamic part of the objects; data is the static part.



A method is a code block containing a series of statements. Methods must be declared

within a class or a structure. It is a good programming practice that methods do only one specific

task. Methods bring modularity to programs. Proper use of methods brings the following

advantages:

Reducing duplication of code

Decomposing complex problems into simpler pieces

Improving clarity of the code

Reuse of code

Information hiding

Basic characteristics of methods are:

Access level

Return value type

Method name

Method parameters

Parentheses

Block of statements

Access level of methods is controlled with access modifiers. They set the visibility of

methods. They determine who can call the method. Methods may return a value to the caller. In

case our method returns a value, we provide its data type. If not, we use the void keyword to

indicate that our method does not return values. Method parameters are surrounded by

parentheses and separated by commas. Empty parentheses indicate that the method requires no

parameters. The method block is surrounded with { } characters. The block contains one or more



statements that are executed, when the method is invoked. It is legal to have an empty method

block.

A method signature is a unique identification of a method for the C# compiler. The signature

consists of a method name and the type and kind (value, reference, or output) of each of its

formal parameters. Method signature does not include the return type.

Any legal character can be used in the name of a method. By convention, method names

begin with an uppercase letter. The method names are verbs or verbs followed by adjectives or

nouns. Each subsequent word starts with an uppercase character. The following are typical

names of methods in C#:

Execute

FindId

SetName

GetName

CheckIfValid

TestValidity



Simple example

We start with a simple example.

using System;

public class Base

{

public void ShowInfo()

{

Console.WriteLine("This is Base class");

}

}

public class SimpleMethod

{

static void Main()

{

Base bs = new Base();

bs.ShowInfo();

}

}

We have a ShowInfo() method that prints the name of its class.

public class Base

{

public void ShowInfo()

{


}

}



Each method must be defined inside a class or a struct. It must have a name. In our case

the name is ShowInfo. The keywords that precede the name of the method are access specifier

and the return type. Parentheses follow the name of the method. They may contain parameters of

the method. Our method does not take any parameters.

static void Main()

{

...

}

This is the Main() method. It is the entry point to each console or GUI application. It

must be declared static. We will see later why. The return type for a Main() method may be void

or int. The access specifier for the Main() method is omitted. In such a case a default one is used,

which is private. It is not recommended to use public access specifier for the Main() method. It is

not supposed to be called by any other methods in the assemblies. It is only the CLR that should

be able to call it when the application starts.

Base bs = new Base();

bs.ShowInfo();

We create an instance of the Base class. We call the ShowInfo() method upon the object.

We say that the method is an instance method, because it needs an instance to be called. The

method is called by specifying the object instance, followed by the member access operator —

the dot, followed by the method name.



METHOD PARAMETERS

A parameter is a value passed to the method. Methods can take one or more parameters.

If methods work with data, we must pass the data to the methods. We do it by specifying them

inside the parentheses. In the method definition, we must provide a name and type for each

parameter.

using System;

public class Addition

{

public int AddTwoValues(int x, int y)

{

return x + y;

}

public int AddThreeValues(int x, int y, int z)

{

return x + y + z;

}

}

public class MethodParameters

{

static void Main()

{

Addition a = new Addition();

int x = a.AddTwoValues(12, 13);

int y = a.AddThreeValues(12, 13, 14);



Console.WriteLine(x);

Console.WriteLine(y);

}

}

In the above example, we have two methods. One of them takes two parameters, the other

one takes three parameters.

public int AddTwoValues(int x, int y)

{

return x + y;

}

The AddTwoValues() method takes two parameters. These parameters have int type. The

method also returns an integer to the caller. We use the return keyword to return a value from the

method.

public int AddThreeValues(int x, int y, int z)

{

return x + y + z;

}

The AddThreeValues() is similar to the previous method. It takes three parameters.

int x = a.AddTwoValues(12, 13);

We call the AddTwoValues() method of the addition object. It takes two values. These

values are passed to the method. The method returns a value which is assigned to the x variable.



VARIABLE NUMBER OF ARGUMENTS

A method can take variable number of arguments. For this we use the params keyword.

No additional parameters are permitted after the params keyword. Only one params keyword is

permitted in a method declaration.

using System;

public class SumOfValues

{

static void Main()

{

Sum(1, 2, 3);

Sum(1, 2, 3, 4, 5);

}

static void Sum(params int[] list)

{

Console.WriteLine("There are {0} items", list.Length);

int sum = 0;

foreach (int i in list)

{

sum = sum + i;

}

Console.WriteLine("Their sum is {0}", sum);

}



}

We create a Sum() method which can take variable number of arguments. The method

will calculate the sum of values passed to the method.

Sum(1, 2, 3);

Sum(1, 2, 3, 4, 5);

We call the Sum() method twice. In one case, it takes 3 arguments, in the second case 5.

We call the same method.

static void Sum(params int[] list)

{

...

}

The Sum() method can take variable number of integer values. All values are added to the

list array.

Console.WriteLine("There are {0} items", list.Length);

We print the length of the list array.

int sum = 0;

foreach (int i in list)

{

sum = sum + i;

}

We compute the sum of the values in the list.

$ ./variableparams.exe

There are 3 items

Their sum is 6



There are 5 items

Their sum is 15

This is the output of the example.

ANONYMOUS METHODS

Anonymous methods are inline methods that do not have a name. Anonymous methods

reduce the coding overhead by eliminating the need to create a separate method. Without

anonymous methods developers often had to create a class just to call one method.

using System;

using System.Timers;

public class AnonymousMethod

{

static void Main()

{

Timer timer = new Timer();

timer.Elapsed += new ElapsedEventHandler(

delegate(object source, ElapsedEventArgs e)

{

Console.WriteLine("Event triggered at {0}", e.SignalTime);

}

);

timer.Interval = 2000;

timer.Enabled = true;

Console.ReadLine();

}

}



We create a timer object and every 2 seconds we call an anonymous method.

Timer timer = new Timer();

A Timer class generates recurring events in an application.

timer.Elapsed += new ElapsedEventHandler(

delegate(object source, ElapsedEventArgs e)

{

Console.WriteLine("Event triggered at {0}", e.SignalTime);

}

);

Here we plug the anonymous method to the Elapsed event. The delegate keyword is used

to denote an anonymous method.

Console.ReadLine();

At this moment, the program waits for an input from the user. The program ends when

we hit the Return key. Otherwise, the program would finish immediately before the events could

be generated.

PASSING ARGUMENTS BY VALUE, BY REFERENCE

C# supports two ways of passing arguments to methods: by value and by reference. The

default passing of arguments is by value. When we pass arguments by value, the method works

only with the copies of the values. This may lead to performance overheads when we work with

large amounts of data.

We use the ref keyword to pass a value by reference. When we pass values by reference,

the method receives a reference to the actual values. The original values are affected when

modified. This way of passing values is more time and space efficient. On the other hand, it is

more error prone.



Which way of passing arguments should we use? It depends on the situation. Say we

have a set of data, for example salaries of employees. If we want to compute some statistics of

the data, we do not need to modify them. We can pass by values. If we work with large amounts

of data and the speed of computation is critical, we pass by reference. If we want to modify the

data, e.g. do some reductions or raises to the salaries, we might pass by reference.

The following example shows how we pass arguments by values.

using System;

public class PassingByValues

{

static int a = 4;

static int b = 7;

static void Main()

{

Console.WriteLine("Outside Swap method");

Console.WriteLine("a is {0}", a);

Console.WriteLine("b is {0}", b);

Swap(a, b);




}

static void Swap(int a, int b)

{



int temp;

temp = a;

a = b;

b = temp;

Console.WriteLine("Inside Swap method");



}

}

The Swap() method swaps the numbers between the a and b variables. The original

variables are not affected.

static int a = 4;

static int b = 7;

At the beginning, these two variables are initiated. The variables must be declared static,

because they are used from static methods.

Swap(a, b);

We call the Swap() method. The method takes ‘a’ and ‘b’ variables as arguments.

temp = a;

a = b;

b = temp;

Inside the Swap() method, we change the values. Note that the a and b variables are

defined locally. They are valid only inside the Swap() method.

$ ./swapbyval.exe

Outside Swap method

a is 4

b is 7



Inside Swap method

a is 7

b is 4

Outside Swap method

a is 4

b is 7

The output shows that the original variables were not affected.

The next code example passes values to the method by reference. The original variables

are changed inside the Swap() method. Both the method definition and the method call must use

the ref keyword.

using System;

public class PassingByReference

{

static int a = 4;

static int b = 7;

static void Main()

{




Swap(ref a, ref b);




}



static void Swap(ref int a, ref int b)

{

int temp;

temp = a;

a = b;

b = temp;

Console.WriteLine("Inside Swap method");



}

}

In this example, calling the Swap() method will change the original values.

Swap(ref a, ref b);

We call the method with two arguments. They are preceded by the ref keyword to

indicate that we are passing arguments by reference.

static void Swap(ref int a, ref int b)

{

...

}

Also in the method declaration, we use the ref keyword to inform the compiler that we

accept references to the parameters and not the values.

$ ./swapbyref.exe

Outside Swap method

a is 4

b is 7



Inside Swap method

a is 7

b is 4

Outside Swap method

a is 7

b is 4

Here we see that the Swap() method really changed the values of the variables.

The out keyword is similar to the ref keyword. The difference is that when using the ref

keyword, the variable must be initialized before it is being passed. With the out keyword, it may

not be initialized. Both the method definition and the method call must use the out keyword.

using System;

public class OutKeyword

{

static void Main()

{

int val;

SetValue(out val);

Console.WriteLine(val);

}

static void SetValue(out int i)

{

i = 12;

}

}



An example shows the usage of the out keyword.

int val;

SetValue(out val);

The val variable is declared, but not initialized. We pass the variable to the SetValue()

method.

static void SetValue(out int i)

{

i = 12;

}

Inside the SetValue() method it is assigned a value which is later printed to the console.

METHOD OVERLOADING

Method overloading allows the creation of several methods with the same name which

differ from each other in the type of the input.

What is method overloading good for? The Qt4 library gives a nice example for the

usage. The QPainter class has three methods to draw a rectangle. Their name is drawRect() and

their parameters differ. One takes a reference to a floating point rectangle object, another takes a

reference to an integer rectangle object, and the last one takes four parameters: x, y, width,

height. If the C++ language, which is the language in which Qt is developed, didn't have method

overloading, the creators of the library would have to name the methods like drawRectRectF(),

drawRectRect(), drawRectXYWH(). The solution with method overloading is more elegant.

using System;

public class Sum

{

public int GetSum()

{



return 0;

}

public int GetSum(int x)

{

return x;

}

public int GetSum(int x, int y)

{

return x + y;

}

}

public class Overloading

{

static void Main()

{

Sum s = new Sum();

Console.WriteLine(s.GetSum());

Console.WriteLine(s.GetSum(20));

Console.WriteLine(s.GetSum(20, 30));

}

}



We have three methods called GetSum(). They differ in input parameters.

public int GetSum(int x)

{

return x;

}

This one takes one parameter.

Console.WriteLine(s.GetSum());

Console.WriteLine(s.GetSum(20));

Console.WriteLine(s.GetSum(20, 30));

We call all three methods.

$ ./overloading.exe

0

20

50

And this is what we get when we run the example.

RECURSION

Recursion, in mathematics and computer science, is a way of defining methods in which

the method being defined is applied within its own definition. In other words, a recursive method

calls itself to do its job. Recursion is a widely used approach to solve many programming tasks.

A typical example is the calculation of a factorial.

using System;

public class Recursion

{

static void Main()

{



Console.WriteLine(Factorial(6));

Console.WriteLine(Factorial(10));

}

static int Factorial(int n)

{

if (n == 0)

{

return 1;

} else

{

return n * Factorial(n-1);

}

}

}

In this code example, we calculate the factorial of two numbers.

return n * Factorial(n-1);

Inside the body of the factorial method, we call the factorial method with a modified argument.

The function calls itself.

$ ./recursion.exe

720

3628800

These are the results.



METHOD SCOPE

A variable declared inside a method has a method scope. The scope of a name is the

region of program text within which it is possible to refer to the entity declared by the name

without the qualification of the name. A variable which is declared inside a method has a method

scope. It is also called a local scope. The variable is valid only in this particular method.

using System;

public class Test

{

int x = 1;

public void exec1()

{

Console.WriteLine(this.x);


}

public void exec2()

{

int z = 5;


Console.WriteLine(z);

}

}

public class MethodScope

{

static void Main()

{

Test ts = new Test();



ts.exec1();

ts.exec2();

}

}

In the preceding example, we have the x variable defined outside the Show() method. The

variable has a class scope. It is valid everywhere inside the definition of the Test class, e.g.

between its curly brackets.

public void exec1()

{



}

The x variable, also called the x field, is an instance variable. And so it is accessible

through the this keyword. It is also valid inside the exec1() method and can be referred by its

bare name. Both statements refer to the same variable.

public void exec2()

{

int z = 5;


Console.WriteLine(z);

}

The x variable can be accessed also in the exec2() method. The z variable is defined in

the exec2() method. It has a method scope. It is valid only in this method.

$ ./methodscope.exe

1

1



1

5

This is the output of the methodscope.exe program.

A variable defined inside a method has a local/method scope. If a local variable has the

same name as an instance variable, it shadows the instance variable. The class variable is still

accessible inside the method by using the this keyword.

using System;

public class Test

{

int x = 1;

public void exec()

{

int x = 3;



}

}

public class Shadowing

{

static void Main()

{

Test ts = new Test();

ts.exec();

}

}



In the preceding example, we declare the x variable outside the exec() method and inside

the exec() method. Both variables have the same name, but they are not in conflict because they

live in different scopes.



The variables are accessed differently. The x variable defined inside the method, also

called the local variable, is simply accessed by its name. The instance variable can be referred by

using the this keyword.

$ ./shadowing.exe

1

3

This is the output of the shadowing.exe program.

STATIC METHODS

Static methods are called without an instance of the object. To call a static method, we

use the name of the class and the dot operator. Static methods can only work with static member

variables. Static methods are often used to represent data or calculations that do not change in

response to object state. An example is a math library which contains static methods for various

calculations. We use the static keyword to declare a static method. When no static modifier is

present, the method is said to be an instance method. We cannot use the this keyword in static

methods. It can be used in instance methods only.

The Main() method is an entry point to the C# console and GUI application. In C#, the

Main() method is required to be static. Before the application starts, no object is created yet. To

invoke non-static methods, we need to have an object instance. Static methods exist before a

class is instantiated so static is applied to the main entry point.



using System;

public class Basic

{

static int Id = 2321;

public static void ShowInfo()

{

Console.WriteLine("This is Basic class");

Console.WriteLine("The Id is: {0}", Id);

}

}

public class StaticMethod

{

static void Main()

{

Basic.ShowInfo();

}

}

In our code example, we define a static ShowInfo() method.

static int Id = 2321;

A static method can only work with static variables.

public static void ShowInfo()

{

Console.WriteLine("This is Basic class");

Console.WriteLine("The Id is: {0}", Id);

}

This is our static ShowInfo() method. It works with a static Id member.



Basic.ShowInfo();

To invoke a static method, we do not need an object instance. We call the method by

using the name of the class and the dot operator.

$ ./staticmethod.exe

This is Basic class

The Id is: 2321

This is the output of the example.

HIDING METHODS

When a derived class inherits from a base class, it can define methods that are already

present in the base class. We say that we hide the method of the class that we have derived from.

To explicitly inform the compiler about our intention to hide a method, we use the new keyword.

Without this keyword, the compiler issues a warning.

using System;

public class Base

{

public void Info()

{


}

}

public class Derived : Base

{

public new void Info()

{

base.Info();

Console.WriteLine("This is Derived class");



}

}

public class HidingMethods

{

static void Main()

{

Derived d = new Derived();

d.Info();

}

}

We have two classes. The Derived and the Base class. The Derived class inherits from

the Base class. Both have a method called Info().


{

...

}

The (:) character is used to inherit from a class.

public new void Info()

{

base.Info();


}

This is an implementation of the Info() method in the Derived class. We use the new

keyword to inform the compiler that we are hiding a method from the base class. Note that we

can still reach the original Info() method. With the help of the base keyword, we invoke the

Info() method of the Base class too.



$ ./hidingmethods.exe

This is Base class

This is Derived class

We have invoked both methods.

OVERRIDING METHODS

Now we will introduce two new keywords: the virtual keyword and the override

keyword. They are both method modifiers. They are used to implement polymorphic behaviour

of objects. The virtual keyword creates a virtual method. Virtual methods can be redefined in

derived classes. Later in the derived class we use the override keyword to redefine the method in

question. If the method in the derived class is preceded with the override keyword, objects of the

derived class will call that method rather than the base class method.

using System;

public class Base

{

public virtual void Info()

{


}

}


{

public override void Info()

{


}

}



public class CSharpApp

{

static void Main()

{

Base[] objs = {new Base(), new Derived(), new Base(),

new Base(), new Base(), new Derived() };

foreach (Base obj in objs)

{

obj.Info();

}

}

}

We create an array of the Base and Derived objects. We go through the array and invoke

the Info() method upon all of them.

public virtual void Info()

{


}

This is the virtual method of the Base class. It is expected to be overridden in the derived classes.

public override void Info()

{


}

We override the base Info() method in the Derived class. We use the override keyword.

Base[] objs = {new Base(), new Derived(), new Base(),

new Base(), new Base(), new Derived() };



Here we create an array of Base and Derived objects. Note that we used the Base type in

our array declaration. This is because a Derived class can be converted to the Base class because

it inherits from it. The opposite is not true. The only way to have both objects in one array is to

use a type which is top most in the inheritance hierarchy for all possible objects.

foreach (Base obj in objs)

{

obj.Info();

}

We traverse the array and call Info() on all objects in the array.

$ ./virtualmethods.exe

This is Base class


This is Base class

This is Base class

This is Base class


This is the output.

Now change the override keyword for new keyword. Compile the example again and run it.

$ ./virtualmethods.exe

This is Base class

This is Base class

This is Base class

This is Base class

This is Base class



This is Base class

This time we have a different output.

SEALED METHODS

A sealed method overrides an inherited virtual method with the same signature. A sealed

method shall also be marked with the override modifier. Use of the sealed modifier prevents a

derived class from further overriding the method. The word further is important. First, a method

must be virtual. It must be later overridden. And at this point, it can be sealed.

using System;

public class A

{

public virtual void F()

{

Console.WriteLine("A.F");

}

public virtual void G()

{

Console.WriteLine("A.G");

}

}

public class B : A

{

public override void F()

{

Console.WriteLine("B.F");

}



public sealed override void G()

{

Console.WriteLine("B.G");

}

}

public class C : B

{

public override void F()

{

Console.WriteLine("C.F");

}

/*public override void G()

{

Console.WriteLine("C.G");

}*/

}

public class SealedMethods

{

static void Main()

{

B b = new B();

b.F();

b.G();

C c = new C();

c.F();

c.G();

}

}



In the preceding example, we seal the method G() in class B.

public sealed override void G()

{

Console.WriteLine("B.G");

}

The method G() overrides a method with the same name in the ancestor of the B class. It

is also sealed to prevent from further overriding the method.

/*public override void G()

{

Console.WriteLine("C.G");

}*/

These lines are commented because otherwise the code example would not compile. The

Mono compiler would give the following error: sealedmethods.cs(36,26): error CS0239: `C.G()':

cannot override inherited member `B.G()' because it is sealed.

c.G();

This line prints "B.G()" to the console.

$ ./sealedmethods.exe

B.F

B.G

C.F

B.G

This is the output.

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