Using & Dispose

Using Statement : Provides a convenient syntax that ensures the correct use of IDisposable objects.
IDisposable Interface : The primary use of this interface is to release unmanaged resources. The garbage collector automatically releases the memory allocated to a managed object when that object is no longer used. However, it is not possible to predict when garbage collection will occur. Furthermore, the garbage collector has no knowledge of unmanaged resources such as window handles, or open files and streams. Use the Dispose method of this interface to explicitly release unmanaged resources in conjunction with the garbage collector. The consumer of an object can call this method when the object is no longer needed.
When you use an IDisposable object, you should declare and instantiate it in a using statement. The using statement calls the Dispose method on the object in the correct way, and (when you use it as shown earlier) it also causes the object itself to go out of scope as soon as Dispose is called. Within the using block, the object is read-only and cannot be modified or reassigned.

The using statement ensures that Dispose is called even if an exception occurs while you are calling methods on the object. You can achieve the same result by putting the object inside a try block and then calling Dispose in a finally block; in fact, this is how the using statement is translated by the compiler.

Classes with finalizers require a minimum of two garbage collection cycles to be reclaimed. This prolongs the use of memory and can contribute to memory pressure. When the garbage collector encounters an unused object that requires finalization, it moves it to the "ready-to-be-finalized" list. Cleanup of the object's memory is deferred until after the single specialized finalizer thread can execute the registered finalizer method on the object. After the finalizer runs, the object is removed from the queue and literally dies a second death. At that point, it is collected along with any other objects. Use a finalizer only on objects that hold unmanaged resources across client calls.