Asynchronous method invocation

One can also pass a callback method to BeginAccomplish, to be invoked when the long-running method completes. It typically calls EndAccomplish to obtain the return value of the long-running method. A problem with the callback mechanism is that the callback function is naturally executed in the worker thread (rather than in the original calling thread), which may cause race conditions.

In the .NET Framework documentation, the term event-based asynchronous pattern refers to an alternative API style (available since .NET 2.0) using a method named AccomplishAsync instead of BeginAccomplish. A superficial difference is that in this style the return value of the long-running method is passed directly to the callback method. Much more importantly, the API uses a special mechanism to run the callback method (which resides in an event object of type AccomplishCompleted) in the same thread in which BeginAccomplish was called. This eliminates the danger of race conditions, making the API easier to use and suitable for software components; on the other hand this implementation of the pattern comes with additional object creation and synchronization overhead.