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copied!<blockquote> <p>Is there a way I can go back to thread A while in Thread B's completion handler?</p> </blockquote> <p>No, you don't have the plumbing in place to marshal a call from one thread to another. That kind of plumbing is provided by the main thread of a GUI app. Out of necessity, user interfaces are pretty fundamentally thread-unsafe. There are several implementation details of a UI thread that support such marshaling. It acts like a consumer in a typical implementation of a <a href="http://en.wikipedia.org/wiki/Producer-consumer_problem">producer/consumer</a> threading model.</p> <p>Which requires a thread-safe queue and a loop in the consumer that reads the queue. You may recognize this as the message queue in a Windows GUI app. With the message loop that calls GetMessage to read notifications and act on them. Marshaling a call is now simple, you just post a message to the message queue, the UI thread reads it and executes the request. Posting is implemented by Control.BeginInvoke for Winforms and Dispatcher.BeginInvoke for WPF.</p> <p>You certainly can implement this synchronization mechanism yourself. The .NET 4 BlockingCollection class makes it easy. But <em>do</em> keep in mind that you have to fundamentally alter the way thread A executes. Staying responsive to requests posted to the queue is important. The kind of problem that a class like BackgroundWorker tries to solve. Keep in mind that the GUI message loop exists because it is <em>necessary</em>, UI isn't thread-safe. A console app doesn't (typically) have the same kind of burden, the console is thread-safe.</p>

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