Creating and starting a task on the UI thread

Question

When a method that gets called on a worker thread needs to run code on the UI thread and wait for it to complete before doing something else, it can be done like this:

    public int RunOnUi(Func<int> f)
    {
        int res = Application.Current.Dispatcher.Invoke(f);

        return res;
    }

But what if I wanted to do it with tasks? Is there a way for the RunOnUi method to create a task that is started on the UI and return it so that the caller (which runs on a worker thread) can wait for it? Something that will fit the following signature: public Task<int> StartOnUi(Func<int> f) ?

One way to do it is as follows:

public Task<int> RunOnUi(Func<int> f)
{
    var task = new Task<int>(f);
    task.Start(_scheduler);

    return task;
}

Here, assume that _schduler holds the ui TaskScheduler. But I am not too comfortable with creating "cold" tasks and using the start method to run them. Is that the "recommended" way or is there a more elegant way to do it?

Answer 1

Just use InvokeAsync instead of Invoke then return the Task<int> inside the DispatcherOperation<int> the function returns.

//Coding conventions say async functions should end with the word Async.
public Task<int> RunOnUiAsync(Func<int> f)
{
    var dispatcherOperation = Application.Current.Dispatcher.InvokeAsync(f);
    return dispatcherOperation.Task;
}

If you do not have access to .NET 4.5 it is a little more complicated. You will need to use BeginInvoke and a TaskCompletionSource to wrap the DispaterOperation that BeginInvoke returns

    public Task<int> RunOnUi(Func<int> f)
    {
        var operation = Application.Current.Dispatcher.BeginInvoke(f);
        var tcs = new TaskCompletionSource<int>();
        operation.Aborted += (sender, args) => tcs.TrySetException(new SomeExecptionHere());
        operation.Completed += (sender, args) => tcs.TrySetResult((int)operation.Result);

        //The operation may have already finished and this check accounts for 
        //the race condition where neither of the events will ever be called
        //because the events where raised before you subscribed.
        var status = operation.Status;
        if (status == DispatcherOperationStatus.Completed)
        {
            tcs.TrySetResult((int)operation.Result);
        }
        else if (status == DispatcherOperationStatus.Aborted)
        {
            tcs.TrySetException(new SomeExecptionHere());
        }

        return tcs.Task;
    }