Programming .NET Components, 2nd Edition

Asynchronous Invocation Pitfalls

By now you have probably come to appreciate the elegance of .NET asynchronous calls, and the ease with which you can turn a synchronous component and its client code into an asynchronous implementation. However, no technology is without its pitfalls. Following is a rundown of the technical pitfalls you are likely to encounter when using .NET asynchronous calls. There is also a major conceptual consequence when dealing with asynchronous calls rather than synchronous calls, which merits a dedicated section of its own at the end of this chapter.

Threading Concurrency and Synchronization

In using asynchronous method calls, you must be aware of potential problems concerning thread concurrency, state corruption, and re-entrance. An asynchronous method is invoked on a thread from the .NET thread pool. When the call is made, the called object may already be servicing a normal call from a synchronous client on another thread, along with additional asynchronous calls on different threads. A completion callback method is also a potential pitfall, because it too is executed on a different thread and can have multiple threads calling it.

In general, you should invoke methods asynchronously only on thread-safe objects—that is, objects that allow multiple threads to safely access them concurrently. Even when using thread-safe objects, you must keep a watchful eye out for race conditions and deadlocks. In addition, the object whose method you invoke asynchronously ...