Different behavior when using ContinueWith or Async-Await

Question

When I use an async-await method (as the example below) in a HttpClient call, this code causes a deadlock. Replacing the async-await method with a t.C

Answer 1

Granted, my answer is only partial, but I'll go ahead with it anyway.

Your Task.ContinueWith(...) call does not specify the scheduler, therefore TaskScheduler.Current will be used - whatever that is at the time. Your await snippet, however, will run on the captured context when the awaited task completes, so the two bits of code may or may not produce similar behaviour - depending on the value of TaskScheduler.Current.

If, say, your first snippet is called from the UI code directly (in which case TaskScheduler.Current == TaskScheduler.Default, the continuation (logging code) will execute on the default TaskScheduler - that is, on the thread pool.

In the second snippet, however, the continuation (logging) will actually run on the UI thread regardless of whether you use ConfigureAwait(false) on the task returned by GetStringAsync, or not. ConfigureAwait(false) will only affect the execution of the code after the call to GetStringAsync is awaited.

Here's something else to illustrate this:

    private async void Form1_Load(object sender, EventArgs e)
    {
        await this.Blah().ConfigureAwait(false);

        // InvalidOperationException here.
        this.Text = "Oh noes, I'm no longer on the UI thread.";
    }

    private async Task Blah()
    {
        await Task.Delay(1000);

        this.Text = "Hi, I'm on the UI thread.";
    }

The given code sets the Text within Blah() just fine, but it throws a cross-threading exception inside the continuation in the Load handler.