Threads spun with Task.Run always exit with exit code 259 [duplicate]

Question

As a simple example I have a WPF application with a single button on the Main Window and code behind thus:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    async void Button_Click(object sender, RoutedEventArgs e)
    {
        await Task<bool>.Run(() => this.DoOnThread());
    }

    async Task<bool> DoOnThread()
    {
        Thread.CurrentThread.Name = "MyTestThread";
        Thread.Sleep(1000);
        return true;
    }
}

If I break at "return true" via VisualStudio threads window I can get the ThreadID, if I continue and let the code run to completion and wait a little till the thread exits, I get "The thread 0x9ad34 has exited with code 259 (0x103)" displayed in the VS output window.

What am I doing wrong and how do I ensure I get a thread exit code of 0?

Answer 1

Task.Run does not create a thread. It schedules a delegate to run on a ThreadPool thread. The threads in a threadpool are created or destroyed according to the CPU load.

The exit code you see has nothing really to do with your code: it may simply be a Visual Studio debug message, or a ThreadPool thread that exited.

Additionally, async doesn't mean that a method will run asynchronously. It is syntactic sugar that allows the compiler to create code to wait asynchronously for any asynchronous methods marked with await. In your case, DoOnThread has no asynchronous calls or await so it will run syncrhonously.

In fact, the compiler will even emit a warning that DoOnThread doesn't contain await so it will run synchronously