Task.Delay never completing

Question

The following code will freeze forever.

public async Task DoSomethingAsync()
{
    await Task.Delay(2000);
}

private void Button_Click(object sender, RoutedEventArgs e)
{
    DoSomethingAsync().Wait();
    // Task.Delay(2000).Wait();
}

If I switch the call to DoSomethingAsync with the commented out code, it behaves as expected. I suspect that somehow the nested awaits are causing a deadlock, but I'm not sure why, or how to fix it.

Answer 1

Assuming Button_Click runs in the GUI thread you have a deadlock on your hands.

When you use Wait on a task you are synchronously blocking the thread until the task ends, but the task will never end because the continuation (the completion of Task.Delay(2000);) must run on the GUI thread as well (which is blocked on Wait).

You have several solutions. Either use ConfigureAwait(false) to not capture the GUI thread's SynchronizationContext:

public async Task DoSomethingAsync()
{
    await Task.Delay(2000).ConfigureAwait(false);
}

Or (which I recommend) use an async void event handler (which is the only appropriate place for an async void method):

private async void Button_Click(object sender, RoutedEventArgs e)
{
    await DoSomethingAsync();
}

public async Task DoSomethingAsync()
{
    await Task.Delay(2000);
}