IAsyncResult

Question

Why does IAsyncResult require I keep a reference to the Delegate that BeginInvoked it?

I would like to be able to write something like:

new GenericDelegate(DoSomething).BeginInvoke(DoSomethingComplete);

void DoSomethingComplete(IAsyncResult ar)
{
    ar.EndInvoke();
}

Answer 1

You don't need to keep your own reference to a delegate when doing a normal¹ delegate BeginInvoke; you can cast the IAsyncResult to an AsyncResult and retrieve the delegate from the AsyncDelegate property. And before anybody says "that's a dirty hack", it's documented as being valid at MSDN.

The AsyncResult class is used in conjunction with asynchronous method calls made using delegates. The IAsyncResult returned from the delegate's BeginInvoke method can be cast to an AsyncResult. The AsyncResult has the AsyncDelegate property that holds the delegate object on which the asynchronous call was invoked.

So you could write:

new GenericDelegate(DoSomething).BeginInvoke(DoSomethingComplete);

void DoSomethingComplete(IAsyncResult ar)
{
    ((GenericDelegate)((AsyncResult)ar).AsyncDelegate)).EndInvoke();
}

Note that you do still have to know the type of the original delegate (or at least, I haven't found a way around this limitation; then again I haven't tried).

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¹ By "normal" here I mean a BeginInvoke on a delegate instance, using the compiler-generated method. This technique of casting to AsyncResult is not guaranteed to work when using pre-defined methods, i.e. when using a class which declares its own BeginX/EndX methods. This is because the class may be doing something more clever internally such as blocking on IO completion ports, and may therefore use a different type of IAsyncResult. However, in the scenario as posited, it will work just fine.