Cast using is twice, or use as once and create a new variable

Question

In a previous question today these two different approaches was given to a question as answers.

We have an object that might or might not implement IDisposable. If it does, we want to dispose it, if not we do nothing. The two different approaches are these:

1)

if(toDispose is IDisposable)
  (toDispose as IDisposable).Dispose();

2)

IDisposable disposable = toDispose as IDisposable;
if( disposable != null )
  disposable.Dispose();

Mainly, from the comments it sounds like the consensus was that 2) was the best approach.

But looking at the differences, we come down to this:

1) Perform a cast twice on toDispose.

2) Only perform the cast once, but create a new intermediate object.

I would guess that 2 will be marginally slower because it has to allocate a new local variable, so why is this regarded as the best solution in this case? It is solely because of readability issues?

Answer 1

Not really answering your question but I would recommend you this construct:

using (toDispose as IDisposable)
{
    ...
}

and let the compiler worry about the ifs, finally and Dispose calls.

Answer 2

My rules of thumb around casting:

• If it's an error/bug for the value not to be of the right type, just cast
• Otherwise use as, like in your second case
• If you're dealing with a value type, you can either use as with the nullable type (for consistency) or use is and a direct cast

Note that your second form doesn't create a "new intermediate object". It creates a new intermediate variable. So does your first approach really - it's just that the variable is hidden by the compiler, effectively. It's still present in the IL as a stack location, but it doesn't have any representation in source code.

Having said all of that, in this particular case (where you just want to dispose), Darin's approach is the best one, I believe.