When to use TaskEx.Run vs. TaskEx.RunEx

Question

I'm trying to understand when to use TaskEx.Run. I have provided two code sample i wrote below that produce the same result. What i fail to see is why i would take the Task.RunEx TaskEx.RunEx approach, I'm sure there is a good reason and was hoping someone could fill me in.

async Task DoWork(CancellationToken cancelToken, IProgress<string> progress)
{
    int i = 0;
    TaskEx.RunEx(async () =>
        {
            while (!cancelToken.IsCancellationRequested)
            {
                progress.Report(i++.ToString());
                await TaskEx.Delay(1, cancelToken);
            }
        }, cancelToken);
}
private void Button_Click(object sender, RoutedEventArgs e)
{
    if (button.Content.ToString() == "Start")
    {
        button.Content = "Stop";
        cts.Dispose();
        cts = new CancellationTokenSource();
        listBox.Items.Clear();
        IProgress<string> progress = new Progress<string>(s => 
        {
            listBox.Items.Add(s); 
            listBox.ScrollIntoView(listBox.Items[listBox.Items.Count - 1]);
        });
        DoWork(cts.Token, progress);
    }
    else
    {
        button.Content = "Start";
        cts.Cancel();
    }
}

I can achieve the same results like so

  async Task DoWork(CancellationToken cancelToken)
    {
        int i = 0;
        while (!cancelToken.IsCancellationRequested)
        {
            listBox.Items.Add(i++);
            listBox.ScrollIntoView(listBox.Items[listBox.Items.Count - 1]);
            await TaskEx.Delay(100, cancelToken);

        }
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        if (button.Content.ToString() == "Start")
        {
            button.Content = "Stop";
            cts.Dispose();
            cts = new CancellationTokenSource();
            listBox.Items.Clear();
            DoWork(cts.Token);
        }
        else
        {
            button.Content = "Start";
            cts.Cancel();
        }
    }

Hydrosome

The above thread is a discussion on the reasons for TaskEx.RunEx, its all to do with changes that could not be put into the core .NET functionality for the CTP, but will be integrated properly for final release – Loggia

Changed Task.RunEx to TaskEx.RunEx There is neither Run() nor RunEx() in Async CTP Task class. They are both in TaskEx. Correct me, if I am wrong – Sapele

Answer 1

Use TaskEx.Run when you want to run synchronous code in a thread pool context.

Use TaskEx.RunEx when you want to run asynchronous code in a thread pool context.

Stephen Toub has two blog posts related to the difference in behavior:

• Potential pitfalls to avoid when passing around async lambdas
• Task.Run vs Task.Factory.StartNew

This is only one of several options you have for creating tasks. If you do not have to use Run/RunEx, then you should not. Use simple async methods, and only use Run/RunEx if you need to run something in the background.

Scramble

Actually that's not true... RunEx has to do with value returning lambda expressions... not asynchrony. Run handles async lambda's just fine. – Bandstand

In .Net 4.5 DP, there seems to be only Task.Run(), no Exes. – Standardbearer

TaskEx is in the CTP only. In .Net 4.5 the methods on TaskEx have been integrated into Task. – Tantalum

@svick, @Phil: In .NET 4.5, TaskEx.Run becomes an overload of Task.Run, and TaskEx.RunEx also becomes an overload of Task.Run. – Scramble

@Firoso: RunEx performs an unwrapping of the inner task. If you pass an async lambda to Run in VS2010, it will work "just fine" -- if you don't care about when it completes. However, if you want the returned Task to complete when the async lambda completes, then use TaskEx. See this blog post or this blog post for more info. – Scramble

Answer 2

The difference between your two DoWork() methods is that the first one (that uses TaskEx.RunEx()) is not asynchronous at all. It executes fully synchronously, starts the other task on another thread, and immediately returns a completed Task. If you awaited or Wait()ed on that task, it wouldn't wait until the internal task is completed.

Standardbearer

Answer 3

Task.Run spawns a new thread in most scenarios as I understand it.

It's important to note that simply because you mark a method as async, and use awaiters, this does NOT (necessarily) mean that new threads are being created, completions are scheduled on the SAME thread of execution that they were called from in many cases.

The trick here has to do with the SchedulingContext. If it's set for a multithreaded apartment, then you're going to delegate completions to viable threads on the threadpool. If you're in a singlethreaded apartment as all WPF and WinForms UI code is, then it will return to the calling thread for completion allowing work to be done directly on the UI without visible thread marshalling in the code.

Bandstand

Task.Run queues to the thread pool, which usually does not start a new thread. await schedules its continuations to its current SynchronizationContext or TaskScheduler (not thread) - see the end of this article which still applies as of the CTP v3 (and VS11 dev preview). I also cover async context in my async intro post. This context doesn't have anything to do with MTA or STA, though it's possible to have a context that supports STA or MTA. – Scramble