If you already know the C++ lanaguage, it wouldn't take you more than an hour to quickly go through the syntax of C#. C# is a language with the features of C++, programming style like Java and rapid application model of Basic. So if you are a little bit familiar with Jave program structure and the concept of packages, garbage collection of Java, it will definitely help you learn C# more quickly. So while discussing C# language constructs, i will presume, you know C++ language. This article dicusses the C# language constructs and features using code examples in a brief and quick way so just by having a glance at the code, you can understand the concepts.

Following topics of C# langauge:

Program Structure
Namespaces
Data Types
Variables
Operators and Expressions
Enumerations
Statements
Classes and Structs
Modifiers
Properties
Interfaces
Function Parameters
Arrays
Indexers
Boxing and Unboxing
Delegates
Inheritance and Polymorphism

Followings are not discussed:

Things which are common in C++ and C#.
Concepts like garbage collection, threading, file processing etc.
Data type conversions
Exception Handling
.Net library

Note: This article is not for C# gurus. There must be some other beginner's articles on C#, but this is yet another one.

Program Structure

Like C++, C# is case-sensitive. Semi colon ; is the statement separater. Unlike C++, there are no separate declaration (header) and implementation(cpp) files in C#. All code (class declaration and implementation) is placed in one file with extention cs.

Have a look at this Hello world program in C#.

using System;
namespace MyNameSpace
{
	class HelloWorld
	{
		static void Main(string[] args)
		{
			Console.WriteLine ("Hello World");
		}
	}
}

Everything in C# is packed into a class and classes in C# are packed into namespaces (just like files in a folder). Like C++, a main method is the entry point of your program. C++'s main function is called "main" whereas C#'s main function starts with capital M and is named as "Main".

No need to put a semi colon after a class block or struct definition. It was in C++, C# doesn't require that.

Namespace

Every class is packaged into a namespace.Namespaces are exactly the same concept as in C++, but in C# we use namespaces more frequently than in C++. You can access a class in a namespace using dot . qualifier. MyNameSpace is the namespace in hello world program above.

Now consider you want to access the HelloWorld class from some other class in some other namespace.

using System;
namespace AnotherNameSpace
{	
	class AnotherClass
	{
		public void Func()
		{
			Console.WriteLine ("Hello World");
		}
	}
}

Now from your HelloWorld Class you can access it as:

using System;
using AnotherNameSpace; // you will add this using statement
namespace MyNameSpace
{
	class HelloWorld
	{ 
		static void Main(string[] args)
		{ 
			AnotherClass obj = new AnotherClass();
			obj.Func();
		}
	}
 }

In .NET library, System is the top level namespace in which other namespaces exist. By dafault there exists a global namespace, so a class defined outside a namespaces goes directly into this global namespace and hence you can access this class without any qualifier.

you can also define nested namespaces.

Using

The #include directive is replaced with "using" keyword, which is followed by a namespace name. Just as "using System" as above. "System" is the base level namespace in which all other namespaces and classes are packed. The base class for all object is Object in the System namespace.

Variables

Variables in C# are almost the same as in C++ except these differences:

Variables in C# (unlike C++), always need to be initialized before you access them, otherwise you will get compile time error. Hence, it's impossible to access an uninitialized variable.
You can't access a �dangling� pointer in C#.
An expression that indexes an array beyond its bounds is also not accessible.
There are no global variables or functions in C# and the behavior of globals is acheived through static functions and static variables.

Data Types

All types of C# are derived from a base class object. There are two types of data types:

Basic/ Built-in types
User-defined types

Following is a table which lists built-in C# types:

Type

Bytes

Desc

byte 1 unsigned byte

sbyte 1 signed byte

short 2 signed short

ushort 2 unsigned short

int 4 signed integer

uint 4 unsigned integer

long 8 signed long

ulong 8 unsigned long

float 4 floating point number

double 8 double precision number

decimal 8 fixed precision number

string unicode string

char unicode char

bool true, false boolean

Note: type range in C# and C++ are different, example, long in C++ is 4 bytes, and in C# it is 8 bytes. Also the bool type and string types are different than those in C++. bool accepts only true and false and not any integer.

User defined types includes:

Classes
Structs
Interfaces

Memory allocation of the data types divides them into two types:

Value Types
Reference Types

Value Types

Values types are those data types which are allocated in stack. They include:

All basic or built-in types except strings
Structs
Enum types

Reference Type

Reference types are allocated on heap and are garbage collected when they are no longer being used. They are created using new operator, and there is no delete operator for these types unlike C++ where user has to explicitely delete the types created using delete operator. In C#, they are automatically collected by garbage collector.

Reference types include:

Classes
Interfaces
Collection types like Arrays
String

Enumeration

Enumerations in C# are exactly like C++. Defined through a keyword enum.

Example:

enum Weekdays
{
	Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday
}

Classes and Structs

Classes and Structs are same as in C++, except the difference of their memory allocation. Objects of classes are allocated in heap, and are created using new, where as structs are allocated in stack. Structs in C# are very light and fast datatypes. For heavy datatypes, you should create classes.

Examples:

struct Date
{
	int day;
	int month;
	int year;
}
		
class Date
{
	int day;
	int month;
	int year;
	string weekday; 
	string monthName;
	public int GetDay()
	{ 
		return day;
	}
	public int GetMonth() 
	{ 
		return month;
	}
	public int GetYear() 
	{ 
		return year;
	}
	public void SetDay(int Day) 
	{ 
		day = Day ;
	}
	public void SetMonth(int Month)
	{
		month = Month;
	}
	public void SetYear(int Year)
	{ 
		year = Year;
	}
	public bool IsLeapYear()
	{
		return (year/4 == 0);
	}
	public void SetDate (int day, int month, int year)
	{
	}
	...
}

Properties

If you familiar with object oriented way of C++, you must have an idea of properties. Properties in above example of Date class are day, month and year for which in C++, you write Get and Set methods. C# provides a more convinient, simple and staright forward way of accessing properties.

So above class can be written as:

using System;
class Date
{
	public int Day{
		get {
			return day; 
		}
		set {
			day = value; 
		}
	}
	int day;

	public int Month{
		get { 
			return month;
		}
		set {
			month = value; 
		}
	}
	int month;

	public int Year{
		get { 
			return year;
		}
		set {
			year = value;
		}
	}
	int year;

	public bool IsLeapYear(int year)
	{
		return year%4== 0 ? true: false; 
	}
	public void SetDate (int day, int month, int year)
	{
		this.day = day;
		this.month = month;
		this.year = year;
	}
}

Here is the way you will get and set these properties:
class User
{
   public static void Main()
   { 
        Date date = new Date(); 
        date.Day = 27; 
        date.Month = 6; 
        date.Year = 2003;
        Console.WriteLine("Date: {0}/{1}/{2}", date.Day, date.Month, date.Year);
    }
}

Modifiers

You must be aware of public, private and protected modifers that are commonly used in C++. I will here discuss some new modifiers introduced by C#.

readonly

readonly modifier is used only for the class data members. As the name indicates the readonly data members can only be read once they are written either directly initializing them or assigning values to them in constructor. The ifference between the readonly and const data members is that const requires you to initialize with the declaration, that is directly. See example code:

class MyClass
{
	const int constInt = 100; //directly
	readonly int myInt = 5; //directly
	readonly int myInt2;
	
	public MyClass()
	{
		myInt2 = 8;		//Indirectly
	}
	public Func()
	{
		myInt = 7; //Illegal
		Console.WriteLine(myInt2.ToString());
	}
	
}

sealed

sealed modifier with a class don't let you derive any class from it. So you use this sealed keyword for the classes which you don't want to be inherited from.

sealed class CanNotbeTheParent
{
	int a = 5;
}

unsafe

you can define an unsafe context in C# using unsafe modifier. In unsafe context, you can write an unsafe code, example: C++ pointers etc. See the following code:

public unsafe MyFunction( int * pInt, double* pDouble)
{
	int* pAnotherInt = new int;
	*pAnotherInt  = 10;
	pInt = pAnotherInt;
	...
	*pDouble = 8.9;	
}

Interfaces

If you have an idea of the COM, you will immediately know what i am talking about. An interface is the abstract base class containing only the function signatures whose implementaion is provided by the child class. In C#, you define such classes as interfaces using the interface keyword. .NET is based on such interfaces. In C#, where you can't use multiple class inheritance, which was previously allowed in C++, the essence of multiple inheritance is acheived through interfaces. That's your child class may implement multiple interfaces.

using System;
interface myDrawing
{
	int originx
	{
		get;
		set;
	}
	int originy
	{
		get;
		set;
	}
	void Draw(object shape);			
}

class Shape: myDrawing
{
	int OriX;
	int OriY;
	
	public int originx
	{
		get{
			return OriX;
		}
		set{
			OriX = value;
		}
	}
	public int originy
	{
		get{
			return OriY;
		}
		set{
			OriY = value;
		}
	}
	public void Draw(object shape)
	{
		... // do something 
	}
	
	// class's own method
	public void MoveShape(int newX, int newY)
	{
	.....
	}		
	
}

Arrays

Arrays in C# are much better than C++. Arrays are allocated in heap and thus are reference type. You can't access an out of bound element in an array. So C# prevents you from that type of bugs. Also some helper functions to iterate array elements are provided. foreach is the statement for such iteration. The difference between the syntax of C++ and C# array is:

the square brackets are placed after the type and not after the variable name
you create element locations using new operator.

C# supports single dimensional, multi dimensional, and jagged array (array of array).

Examples:

// single-dimensional array of int 
	int[] array = new int[10]; 
	for (int i = 0; i < array.Length; i++) 
	array[i] = i; 
	
	// 2-dimensional array of int 
	int[,] array2 = new int[5,10];
	array2[1,2] = 5;	
	
	// 3-dimensional array of int 
	int[,,] array3 = new int[5,10,5];
	array3[0,2,4] = 9;
		
	// Jagged array - array of array of int 
	int[][] arrayOfarray = = new int[2];
	arrayOfarray[0] = new int[4];
	arrayOfarray[0] = new int[] {1,2,15};

Indexers

Indexer is used to write a method to access an element from a collection using straight way of using [], like an array. All you need is to specify the index to access an instance or element. Syntax of and Indexer is same as that of class properties, except they take the input parameter, that is the index of the element.

Example:

Note: CollectionBase is the library class used for making collections. List is the protected member of CollectionBase which stores the collection list.

class Shapes: CollectionBase 
{ 
	public void add(Shape shp)
	{ 
		List.Add(shp);
	}

	//indexer
	public Shape this[int index]
	{ 
		get { 
			return (Shape) List[index];
		} 
		set {
			List[index] = value ;
		 }
	 }
}

Boxing/Unboxing

The idea of boxing is new in C#. As mentioned above, all data types, built-in or user defined are derived from a base class object in the System namespace. So the packing of basic or primitive type into an object is called boxing, whereas the reverse of this known as unboxing.

Example:

class Test
{
   static void Main() 
   {
      int myInt = 12;
      object obj = myInt ;      // boxing
      int myInt2 = (int) obj;   // unboxing
   }
}

Example shows both boxing and unboxing. An int value can be converted to object and back again to int. When a variable of a value type needs to be converted to a reference type, an object box is allocated to hold the value, and the value is copied into the box. Unboxing is just the opposite. When an object box is cast back to its original value type, the value is copied out of the box and into the appropriate storage location.

Function Parameters

Parameters in C# are of three types:

By-Value/In parameters
By-Reference/In-Out Parameters
Out Parameters

If you have an idea of COM interface and it's parameters types, you will easily understand the C# parameter types.

By-Value/In parameters

The concept of value parameters is same as in C++. The value of the passed value is copied into a location and is passed to the function.

Example:

SetDay(5);
...
void SetDay(int day) 
{ 
	....
}

By-Reference/In-Out Parameters

The reference parameters in C++ are passed either through pointers or reference operator &. In C# reference parameters are less error prone. Reference parameters are also called In-Out parameters because you pass a reference address of the location, so you pass an input value and get an output value from that function.

You can not pass an uninitialized reference parameter into a function. C# uses a keyword ref for the reference parameters.

Example:

	int a= 5;
	FunctionA(a);
	Console.WriteLine(a); // prints 20

	void FunctionA(ref int Val)
	{
		int x= Val; 
		Val = x* 4;	
	}

Out Parameter

Out parameter is the parameter which only returns value from the function. The input value is not required. C# uses a keyword out for the out parameters

Example:

	int Val;
	GetNodeValue(Val);

	bool GetNodeValue(out int Val)
	{
		Val = value;
		return true; 
	}

Variable number of parameters and Arrays

Arrays in C# when are passed through a keyword params. An array type parameter should always be the right most argument of the function. Only one parameter can be of array type. You can pass any number of elements as an argument of type of that array. You can better understand it from example

Example:

	void Func(params int[] array)
	{
		Console.WriteLine("number of elements {0}", array.Length);
	}

	Func(); // prints 0
	Func(5); // prints 1
	Func(7,9); // prints 2
	Func(new int[] {3,8,10}); // prints 3
	int[] array = new int[8] {1,3,4,5,5,6,7,5};
	Func(array); // prints 8

Operators and Expressions

Operators are exactly the same as of C++ and thus the expression also. However some new and useful operators are also added. Some of them are discussed here.

is operator

is operator is used to check whether the operand types are equal or convertable. The is operator is aprticularly useful in the polymorphism scenarios. is operator takes two operands and the result is a boolean. See the example:

function(object param)
{ 
	if(param is ClassA)
		//do something 
	else if(param is MyStruct)
		//do something 		
	}
}

as operator

as operator checks if the type of the operands are convertable or equal (as is done by is operator) and if it is, the result is a converted or boxed object (if the operand can be boxed into the target type, see boxing/unboxing). If the objects are not convertable or boxable, the return is a null. Have a look at the example below to better understand the concept.

Shape shp = new Shape(); 
Vehicle veh = shp as Vehicle; // result is null, types are not convertable

Circle cir = new Circle(); 
Shape shp = cir; 
Circle cir2 = shp as Circle;  //will be converted

object[] objects = new object[2];
objects[0] = "Aisha";
object[1] = new Shape();

string str;
for(int i=0; i&< objects.Length; i++)
{
	str = objects[i] as string;
	if(str == null)
		Console.WriteLine("can not be converted");
	else
		Console.WriteLine("{0}",str);
}

Output:
Aisha
can not be converted

Statements

Statements in C# are just like in C++ except some additions of new statements and modifications in some statements.

Followings are new statements:

foreach

For iteration of collections like arrays etc.

Example:

	foreach (string s in array) 
		Console.WriteLine(s);

lock

Used in threads for locking a block of code making it a critical section.

checked/unchecked

The statements are for overflow checking in numeric operations.

Example:

	int x = Int32.MaxValue;

	x++; // Overflow

	checked 
	{ 
		x++;  // Exception 
	}
	unchecked
	{
		x++; // Overflow}
	}

Following statements are modified:

Switch

Switch statement is modified in C#.

Now after executing a case statement, program flow can not jump to next case which was previously allowed in C++. Example:

int var = 100;
switch (var) 
{ 
case 100: Console.WriteLine("<Value is 100>"); // No break here
case 200: Console.WriteLine("<Value is 200>"); break; 
} 
Output in C++: 
<Value is 100><Value is 200>
In C# you get compile time error: 
error CS0163: Control cannot fall through from one case 
label ('case 100:') to another

However you can do this similarly you do it in C++:

switch (var)
{
	case 100: 
	case 200: Console.WriteLine("100 or 200"); 
				break; 
}

You can also use constant variables for case values:

Example:

const string WeekEnd = "Sunday";
const string WeekDay1 = "Monday";

....

string WeekDay = Console.ReadLine();
switch (WeekDay ) 
{ 
case WeekEnd: Console.WriteLine("It's weekend!!"); break; 
case WeekDay1: Console.WriteLine("It's Monday"); break;

Delegates

Delegate let us store function references into a variable. In C++, this is like using and storing function pointer for which we usually use typedef.

Delegates are declared using a keyword delegate. Have a look at this example, and you will understand what delegates are:

Example:

delegate int Operation(int val1, int val2);
public int Add(int val1, int val2) 
{ 
	return val1 + val2; 
}
public int Subtract (int val1, int val2) 
{ 
	return val1- val2;
}

public void Perform()
{
	Operation Oper;
	Console.WriteLine("Enter + or - ");
	string optor = Console.ReadLine();
	Console.WriteLine("Enter 2 operands");
			
	string opnd1 = Console.ReadLine();
	string opnd2 = Console.ReadLine();
			
	int val1 = Convert.ToInt32 (opnd1);			
	int val2 = Convert.ToInt32 (opnd2);
			
	if (optor == "+")
		Oper = new Operation(Add);
	else
		Oper = new Operation(Subtract);
		
	Console.WriteLine(" Result = {0}", Oper(val1, val2));
}

Inheritance and Polymorphism

Only single inheritance is allowed in C#. Mutiple inheritance can be acheived using interfaces.

Example:

class Parent{
}

class Child : Parent

Virtual Functions

Virtual functions to implement the concept of polymorphism are same in C#, except you use the override keyword with the virtual function implementaion in the child class. The parent class uses the same virtual keyword. Every class which overrides the virtual method will use override keyword.

class Shape
{
	public virtual void Draw()
	{
		Console.WriteLine("Shape.Draw")	;
	}
}

class Rectangle : Shape

{
	public override void Draw()
	{
		Console.WriteLine("Rectangle.Draw");
	}			
}

class Square : Rectangle
{
	public override void Draw()
	{
		Console.WriteLine("Square.Draw");
	}
}
class MainClass
{
	static void Main(string[] args)
	{
		Shape[] shp = new Shape[3];
		Rectangle rect = new Rectangle();
		
		shp[0] = new Shape();
		shp[1] = rect;
		shp[2] = new Square();
			        
		shp[0].Draw();
		shp[1].Draw();
		shp[2].Draw();
	}
}
Output:
Shape.Draw
Rectangle.Draw
Square.Draw

Hiding parent functions using "new"

You can define in a child class a new version of a function, hiding the one which is in base class. A keyword new is used to define a new version. Consider the example below, which is a modified version of above example and note the output this time, when i replace the keyword override with a keyword new in Rectangle class.

class Shape
{
	public virtual void Draw()
	{
		Console.WriteLine("Shape.Draw")	;
	}
}

class Rectangle : Shape
{
	public new void Draw()
	{
		Console.WriteLine("Rectangle.Draw");
	}			
}
class Square : Rectangle
{
	//wouldn't let u override it here
	public new void Draw() 
	{
		Console.WriteLine("Square.Draw");
	}
}
class MainClass
{
	static void Main(string[] args)
	{
		Console.WriteLine("Using Polymorphism:");
		Shape[] shp = new Shape[3];
		Rectangle rect = new Rectangle();
			
		shp[0] = new Shape();
		shp[1] = rect;
		shp[2] = new Square();
			            
		shp[0].Draw();
		shp[1].Draw();
		shp[2].Draw();
			
		Console.WriteLine("Using without Polymorphism:");
		rect.Draw();			
		Square sqr = new Square();
		sqr.Draw();
	}
}	
	
Output:
Using Polymorphism
Shape.Draw
Shape.Draw
Shape.Draw
Using without Polymorphism:
Rectangle.Draw
Square.Draw

See when the polymorphism doesn't take the Rectangle class's Draw method as a polymorphic form of the Shape's Draw method, instead it considers it a different method. So in order to avoid the naming conflict between parent and child, we have used new modifier.

Note: you can not use in the same class the two version of a method, one with new modifier and other with override or virtual. Like in above example i can not add another method named Draw in Rectangle class which is a virtual or override method. Also in the Square class i can't override the virtual Draw method of Shape class.

Calling base class members

If the child class has the data members with same name as that of base class, in order to avoid naming conflicts, base class data members and functions are accessed using a keyword base. See in examples how the base class constructors are called and how the data members are used.

public Child(int val) :base(val)
{
	myVar = 5;
	base.myVar;
}

OR

public Child(int val)
{
	base(val);
	myVar = 5 ;
	base.myVar;
}

Future Additions:

This article is just a quick overwiew of the C# language so that you can just become familiar with the langauge features.
Although I have tried to discuss almost all the major concepts in C# in a brief and comprehensive way with code examples yet I i think there is lot much to be added and discussed. In future, I would like to add more commands and concepts not yet discussed including events etc.
I would also like to write for beginners the Windows programming using C#.

References:

Our most commonly known MSDN
Inside C# by Tom Archer
A Programmer's Introduction to C# by Eric Gunnerson
Beginning C# by Karli Watson
Programming C# (O'Reilly)

About the Author

Introduction

Program Structure

Namespace

Using

Variables

Data Types

Value Types

Reference Type

Enumeration

Classes and Structs

Properties

Modifiers

readonly

sealed

unsafe

Interfaces

Arrays

Indexers

Boxing/Unboxing

Function Parameters

By-Value/In parameters

By-Reference/In-Out Parameters

Out Parameter

Variable number of parameters and Arrays

Operators and Expressions

is operator

as operator

Statements

foreach

lock

checked/unchecked

Switch

Delegates

Inheritance and Polymorphism

Virtual Functions

Hiding parent functions using "new"

Calling base class members

Future Additions:

References: