What is an "async IO operation" in .NET?

Question

To cut a long story short - read this article first and then this article. In short - it's the old issue about ASP.NET and randomly switching among threads. Well, not so randomly actually. As the second article explains, this only happens "when your thread performs an async IO operation". So... what the heck is an async IO operation in this case? I'm familiar with the standard Begin...End...IAsyncResult and the event model for asynchronous IO. But I can't see how this could be tied together with a thread switch.

The point is - I'm simply afraid of any "hidden" asynchronous IO that my software could be doing (and which would then provoke a thread switch). But how to identify them?

Answer 1

Async IO operations refer to situations where the calling thread does not block in order to wait for IO. Examples include the BeginXXX operations on streams, network connections etc.

When you set up a call using BeginXXX a threadpool thread will be used to execute your callback while a device driver typically handles the actual IO. Thus a number of different threads may be involved.