CODEWITHSUNDEEP
c#.netmultithreading
ubi
ubi

Reputation: 4399

Return async operation result to the original thread

Can the result of an async operation be returned to the thread that initiated it (ie.AsyncCallback invoked in the original thread)?

Here's an example

    void Main(string[] args)
    {

        Func<bool> action = () =>
        {
            Console.WriteLine(Thread.CurrentThread.ManagedThreadId + ": Running");
            return true;
        };

        AsyncCallback onComplete = (r) =>
        {
            bool result = action.EndInvoke(r);
            Console.WriteLine(Thread.CurrentThread.ManagedThreadId + ": Completed");
        };

        Console.WriteLine(Thread.CurrentThread.ManagedThreadId + ": Main");
        IAsyncResult iar = action.BeginInvoke(onComplete, null);
    }

Its output is something like:

5: Main
6: Running
6: Completed

What I'd like to achieve is the onComplete callback executing in the main thread (eg. Completed and Main will have the same thread id printed in this example)

(I know this can be achieved with a BackgroundWorker ie. it's RunWorkerCompleted event is fired in the original thread and therefore the return value of the function invoked in the background thread can be returned to the original thread)

Upvotes: 2

Views: 419

Answers (1)

Marc Gravell
Marc Gravell

Reputation: 1062650

This is largely a question for the SynchronizationContext and TaskScheduler implementations that you choose to use. In the case of many UI frameworks (winforms, WPF, etc), then yes: if you start work on the UI thread, the callback can happen on the UI thread - but this does not apply in the general case, where a ThreadPool thread is more likely to win. Ultimately, you can't simply force work back to a particular thread, unless that thread is by design checking some queue of work, and dequeing / executing packets of work as they arrive (which is exactly what UI frameworks do).

Upvotes: 5

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