Is there any way to perform a callback on a specified thread?

I am writing a library, and would like to be able to fire a callback on a specified thread, so the end-user does not have to worry about th开发者_高级运维read-safety.

I tried using ExecutionContext, but that didn't work out too well, it would fire in the specified context (a new thread), but not on the thread that originally called the function.

The code should work like this:

void Connect() {
  // This should be in the same thread ..
  SocketAsyncEventArgs.Completed += eventHandler;
  Socket.ConnectAsync(SocketAsyncEventArgs)
}

void eventHandler() {
  // .. as this
}

You can't just run your code on some existing thread. That thread is already executing other code. But, it can provide you some way to run your code on it. The main thread in a WPF application does this using Dispatcher.Invoke(). The main thread of a WinForms application uses Control.Invoke().

There is a more general way to do this: use Synchronization.Context.Current. This would work for the main thread of WPF or WinForms application, but would execute the callback on a thread pool thread otherwise. (Unless there is some sort of custom synchronization context, which I think is very rare.)

But this is the best you can do. Like I said, you can't run your code on some other thread when you want. The code in that other thread has to allow you to do that.

That's the thing about asynchronous functions -- you can't guarantee when you'll get called back, or what thread will be running your callback function. Consider that the cost of being able to "set it and forget it".

There's usually no need for that much control anyway. If you "need" to have a specific thread run your callback, what you really need is to review why that's necessary. If it's something that needs to run on the UI thread, there's Control.Invoke. (The UI thread anticipates needing to be handed stuff to do, because of how the architecture works, so controls have a way to pass callbacks to run on that thread. You can't just up and do that with arbitrary threads -- they have to be expecting to be passed a callback like that.) Otherwise, if you have an issue with locks or something, chances are you're trying to use asynchronous functionality to do stuff that should really be done synchronously in a separate thread.