Question

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 thread-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
}

Answer 1

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.