What is the relationship between the using keyword and the IDisposable interface?

Question

If I am using the using keyword, do I still have to implement IDisposable?

Answer 1

If you use the using statement the enclosed type must already implement IDisposable otherwise the compiler will issue an error. So consider IDisposable implementation to be a prerequisite of using.

If you want to use the using statement on your custom class, then you must implement IDisposable for it. However this is kind of backward to do because there's no sense to do so for the sake of it. Only if you have something to dispose of like an unmanaged resource should you implement it.

// To implement it in C#:
class MyClass : IDisposable {

    // other members in you class 

    public void Dispose() {
        // in its simplest form, but see MSDN documentation linked above
    }
}

This enables you to:

using (MyClass mc = new MyClass()) {

    // do some stuff with the instance...
    mc.DoThis();  //all fake method calls for example
    mc.DoThat();

}  // Here the .Dispose method will be automatically called.

Effectively that's the same as writing:

MyClass mc = new MyClass();
try { 
    // do some stuff with the instance...
    mc.DoThis();  //all fake method calls for example
    mc.DoThat();
}
finally { // always runs
    mc.Dispose();  // Manual call. 
}

Answer 2

You can't have one without the other.

When you write :

using(MyClass myObj = new MyClass())
{
    myObj.SomeMthod(...);
}

Compiler will generate something like this :

MyClass myObj = null;
try
{
    myObj = new MyClass();
    myObj.SomeMthod(...);
}
finally
{
    if(myObj != null)
    {
        ((IDisposable)myObj).Dispose();
    }
} 

So as you can see while having using keyword it is assumed/required that IDisposable is implemented.

Answer 3

You are confusing things. You can only use the "using" keyword on something that implements IDisposable.

Edit: If you use the using keyword, you don't have to explicity invoke Dispose, it will be called automatically at the end of the using block. Others have already posted examples of how the using statement is translated into a try - finally statement, with Dispose invoked within the finally block.

Answer 4

Yes, the using keyword is syntactic sugar for this type of pattern...(from msdn)

  Font font1 = new Font("Arial", 10.0f);
  try
  {
    byte charset = font1.GdiCharSet;
  }
  finally
  {
    if (font1 != null)
      ((IDisposable)font1).Dispose();
  }

Edit: a useful example.

When you find you are doing things in a finally section consistantly, for example resetting a Cursor back to its default after having set it to a wait cursor, this is a candidate for this pattern...

  public class Busy : IDisposable
  {

    private Cursor _oldCursor;

    private Busy()
    {
      _oldCursor = Cursor.Current;
    }

    public static Busy WaitCursor
    {
      get
      {
        Cursor.Current = Cursors.WaitCursor;
        return new Busy();
      }
    }

    #region IDisposable Members

    public void Dispose()
    {
      Cursor.Current = _oldCursor;
    }

    #endregion
  }

Called like...

using(Busy.WaitCursor)
{
  // some operation that needs a wait cursor.
}

Answer 5

Using will only dispose of disposable objects. So wraping a using block around an object that does not implement IDisposable is rather useless will in fact cause a compiler error.

http://msdn.microsoft.com/en-us/library/yh598w02.aspx

As a rule, 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 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.

Answer 6

You must implement IDisposable to use using. If you try to use using() on a type that does not implement IDisposable you get the following compile time error:

error CS1674: 'SomeType': type used in a using statement must be implicitly convertible to 'System.IDisposable'

Answer 7

The using keyword already implements, so if you use the using keyword, you do not have to invoke IDisposable