Why disposed object doesn't throw exception on using it after disposing?

Question

Is it legal to call a method on disposed object? If yes, why?

In the following demo program, I've a disposable class A (which implements IDisposable interface).As far as I know, if I pass disposable object to using() construct, then Dispose() method gets called automatically at the closing bracket:

A a = new A();
using (a)
{
   //...
}//<--------- a.Dispose() gets called here!

//here the object is supposed to be disposed, 
//and shouldn't be used, as far as I understand.

If that is correct, then please explain the output of this program:

public class A : IDisposable
{
   int i = 100;
   public void Dispose()
   {
      Console.WriteLine("Dispose() called");
   }
   public void f()
   {
      Console.WriteLine("{0}", i); i  *= 2;
   }
}

public class Test
{
        public static void Main()
        {
                A a = new A();
                Console.WriteLine("Before using()");
                a.f();
                using ( a) 
                {
                    Console.WriteLine("Inside using()");
                    a.f();
                }
                Console.WriteLine("After using()");
                a.f();
        }
}

Output (ideone):

Before using()
100
Inside using()
200
Dispose() called
After using()
400

How can I call f() on the disposed object a? Is this allowed? If yes, then why? If no, then why the above program doesn't give exception at runtime?

---

I know that the popular construct of using using is this:

using (A a = new A())
{
   //working with a
}

But I'm just experimenting, that is why I wrote it differently.

Answer 1

Disposed doesn't mean gone. Disposed only means that any unmanaged resource (like a file, connection of any kind, ...) has been released. While this usually means that the object doesn't provide any useful functionality, there might still be methods that don't depend on that unmanaged resource and still work as usual.

The Disposing mechanism exist as .net (and inheritly, C#.net) is a garbage-collected environment, meaning you aren't responsable for memory management. However, the garbage collector can't decide if an unmanaged resource has been finished using, thus you need to do this yourself.

If you want methods to throw an exception after the object has been diposed, you'll need a boolean to capture the dispose status, and once the object is disposed, you throw the exception:

public class A : IDisposable
{
   int i = 100;
   bool disposed = false;
   public void Dispose()
   {
      disposed = true;
      Console.WriteLine("Dispose() called");
   }
   public void f()
   {
      if(disposed)
        throw new ObjectDisposedException();

      Console.WriteLine("{0}", i); i  *= 2;
   }
}