Calling TaskCompletionSource.SetResult in a non blocking manner

Question

I\'ve discovered that TaskCompletionSource.SetResult(); invokes the code awaiting the task before returning. In my case that result in a deadlock.

This

Answer 1

A little late to the party, but here's my solution which i think is added value.

I've been struggling with this also, i've solved it by capturing the SynchronizationContext on the method that is awaited.

It would look something like:

// just a default sync context
private readonly SynchronizationContext _defaultContext = new SynchronizationContext();

void ReceiverRun()
{
    while (true)    // <-- i would replace this with a cancellation token
    {
        var msg = ReadNextMessage();
        TaskWithContext task = requests[msg.RequestID];

        // if it wasn't a winforms/wpf thread, it would be null
        // we choose our default context (threadpool)
        var context = task.Context ?? _defaultContext;

        // execute it on the context which was captured where it was added. So it won't get completed on this thread.
        context.Post(state =>
        {
            if (msg.Error == null)
                task.TaskCompletionSource.SetResult(msg);
            else
                task.TaskCompletionSource.SetException(new Exception(msg.Error));
        });
    }
}

public static Task SendAwaitResponse(string msg)
{
    // The key is here! Save the current synchronization context.
    var t = new TaskWithContext(SynchronizationContext.Current); 

    requests.Add(GetID(msg), t);
    stream.Write(msg);
    return t.TaskCompletionSource.Task;
}

// class to hold a task and context
public class TaskWithContext
{
    public SynchronizationContext Context { get; }

    public TaskCompletionSource TaskCompletionSource { get; } = new TaskCompletionSource();

    public TaskWithContext(SynchronizationContext context)
    {
        Context = context;
    }
}