public virtual void DrawWindow( )

using System;

public class DrawingObject
{
public virtual void Draw()
{
Console.WriteLine("I'm just a generic drawing object.");
}
}
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.");
}
}
public class DrawDemo
{
public static int Main(string[] args)
{
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;
}
}

Output:
I'm a Line.
I'm a Circle.
I'm a Square.
I'm just a generic drawing object.

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.

public class ChildClass : ParentClass

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 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.

((Parent)child).print();

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]);
}
}
}

this [argument list]
{
get
{
// Get codes goes here
}
set
{
// Set codes goes here
}
}

myInd[0]: empty
myInd[1]: empty
myInd[2]: empty
myInd[3]: Another Value
myInd[4]: empty
myInd[5]: Any Value
myInd[6]: empty
myInd[7]: empty
myInd[8]: empty
myInd[9]: Some Value

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"]);
}
}

myInd[0]: no value
myInd[1]: no value
myInd[2]: no value
myInd[3]: Another Value
myInd[4]: no value
myInd[5]: Any Value
myInd[6]: no value
myInd[7]: no value
myInd[8]: no value
myInd[9]: Some Value

Number of "no value" entries: 7

public object this[int param1, ..., int paramN]
{
get
{
// process and return some class data
}
set
{
// process and assign some class data
}
}

using System;

class IndexExample
{
string Message;

public static void Main()
{
IndexExample obj=new IndexExample("Welcome");

/* This will access the String variable Message
using array like notation
*/
for(int i=0;i < obj.Length;i++)
{
Console.WriteLine(obj[i]);
}
obj[obj.Length-1]="e to C#";

Console.WriteLine(obj.Message);

}

public IndexExample(string s)
{
Message=s;
}

public string this[int i]
{
get
{
if(i >= 0 && i < Message.Length)
{
return Message.Substring(i,1);
}
else
{
return "";
}
}
set
{
if(i >= 0 && i < Message.Length)
{
Message=Message.Substring(0,i) + value + Message.Substring(i+1);
}
}
}

public int Length
{
get
{
if(Message!=null)
{
return Message.Length;
}
else
return 0;
}
}
}