When the callback method in invoked, the call has already been completed,
it's too late. Not calling EndRead will simply create a memory leak. You
MUST call EndXXX for every BeginXXX that you've called or you'll end up
with memory (and possibly other ressources as well) leaks. The only
exception to this rule is the Control.BeginInvoke() method which doesn't
require you to call the corresponding Control.EndInvoke() method. As for
the OP question, i'm afraid that unless the class that you are using
provides an explicit way to cancel an asynchronous operation, there is
nothing that you can do apart from ignoring the results in your callback
function.