using Statement

Defines a scope, outside of which an object or objects will be disposed.

using (Font font1 = new Font("Arial", 10.0f))
{
}

C#, through the .NET Framework common language runtime (CLR), automatically releases the memory used to store objects that are no longer required. The release of memory is non-deterministic; memory is released whenever the CLR decides to perform garbage collection. However, it is usually best to release limited resources such as file handles and network connections as quickly as possible.

The using statement allows the programmer to specify when objects that use resources should release them. The object provided to the using statement must implement the IDisposable interface. This interface provides the Dispose method, which should release the object's resources.

A using statement can be exited either when the end of the using statement is reached or if an exception is thrown and control leaves the statement block before the end of the statement.

Multiple objects can be used in with a using statement, but they must be declared inside the using statement, like this:

using (Font font3 = new Font("Arial", 10.0f), font4 = new Font("Arial", 10.0f))
{
// Use font3 and font4.
}


Example:
The following sample shows how a user-defined class can implement its own Dispose behavior. Note that your type must inherit from IDisposable.

using System;

class C : IDisposable
{
public void UseLimitedResource()
{
Console.WriteLine("Using limited resource...");
}

void IDisposable.Dispose()
{
Console.WriteLine("Disposing limited resource.");
}
}

class Program
{
static void Main()
{
using (C c = new C())
{
c.UseLimitedResource();
}
Console.WriteLine("Now outside using statement.");
Console.ReadLine();
}
}


The new code would looking something like this:

using (SqlConnection cn = new SqlConnection(connectionString))
{
using (SqlCommand cm = new SqlCommand(commandString, cn))
{
cn.Open();
cm.ExecuteNonQuery();
}
}

This is essentially equivalent to the following, although my guess is that C# will internally generate two try / finally blocks (one for the SqlConnection and one for the SqlCommand), but you get the idea:

SqlConnection cn = null;
SqlCommand cm = null;

try
{
cn = new SqlConnection(connectionString);
cm = new SqlCommand(commandString, cn);
cn.Open();
cm.ExecuteNonQuery();
}
finally
{
if (null != cm);
cm.Dispose();
if (null != cn)
cn.Dispose();
}


Reference :http://msdn.microsoft.com/en-us/library/yh598w02(VS.80).aspx