异步调用如何知道何时在新线程中开始完成事件或使用调用者线程

Question

Let's say we have a service TestService with DoWork method. On the client, it will generate DoWorkCompleted and DoWorkAsync methods.

This is my observation if you call DoWorkAsync from UI thread, DoWorkCompleted will be executed on UI thread as expected.

But if I start BackgroundWorker and call service DoWorkAsync from it then DoWorkCompleted is executed on new thread other than caller thread. Example:

1. Start BackgroundWorker instance <- From UI Thread
2. Worker DoWork method gets executed <- Thread 2
3. Call to service DoWorkAsync from worker DoWork <- From thread 2
4. Service runs and DoWorkCompleted gets executed <-- Thread 3
5. Worker DoWorkCompleted gets executed <- UI thread

How does Service client know when to start new thread and when to post it back to caller thread?

Answer 1

This has to do with the availability of a SynchronizationContext on the thread. In Windows Forms, I assume Silverlight is the same, when you start an application the main thread that will become the UI thread is provided with a SynchronizationContext that allows calls to be marshaled back to the UI thread.

In your first scenario, since you call DoWorkAsync from a thread with a synchronization context available the DoWorkCompleted is marshaled back to the initial thread.

In your second scenario you are on a thread pool thread without a synchronization context so the DoWorkCompleted is not marshaled back to the initial thread.