System.IDisposable

It only has one method
Provides a form of deterministic destruction

public interface IDisposable 
{ 
   void Dispose(); 
} 

The Dispose method makes a call to GC.SuppressFinalize
Setting an object to null does not trigger the Dispose() method to be called
As a rule of thumb, when you use an IDisposable object you should declare and instantiate it in a using statement
This calls the Dispose method on the object in the right way.

using Statement

This provides a convenient syntax that ensures the correct use of IDisposable objects.

using (StreamReader myReader = new StreamReader("C:\temp\myfile.txt") 
{ 
} 

The StreamReader is an example of a managed type that accesses unmanaged resources.
Any unmanaged resources or class libraries that contain unmanaged resources must implement the IDisposable interface.

This is equivalent to:

try { 
    StreamReader myReader = new StreamReader("C:\temp\myfile.txt"); 
} 
finally 
{ 
   (IDisposable)myReader.Dispose(); 
} 

The using block ensures that the Dispose method is called even if an exception occurs.
It is possible to declare multiple instances of a type in a using statement.

using (StreamReader myReader1 = new StreamReader("C:\temp\myfile1.txt"), 
        StreamReader myReader2 = new StreamReader("C:\temp\myfile2.txt") 
{ 
}