async/await deadlock when using WindowsFormsSynchronizationContext in a console app

Question

As a learning exercise, I'm trying to reproduce an async/await deadlock that occurs in a normal windows form, but using a console app. I was hoping the code below would cause this to happen, and indeed it does. But the deadlock also happens unexpectedly when using await.

using System;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;
static class Program
{
    static async Task Main(string[] args)
    {
        // no deadlocks when this line is commented out (as expected)
        SynchronizationContext.SetSynchronizationContext(new WindowsFormsSynchronizationContext()); 
        Console.WriteLine("before");
        //DoAsync().Wait(); // deadlock expected...and occurs
        await DoAsync(); // deadlock not expected...but also occurs???
        Console.WriteLine("after");
    }
    static async Task DoAsync()
    {
        await Task.Delay(100);
    }
}

I'm mostly curious if anyone knows why this is happening?

Answer 1

This happens because the WindowsFormsSynchronizationContext depends on the existence of a standard Windows message loop. A console application does not start such a loop, so the messages posted to the WindowsFormsSynchronizationContext are not processed, the task continuations are not invoked, and so the program hangs on the first await. You can confirm the non-existence of a message loop by querying the boolean property Application.MessageLoop.

Gets a value indicating whether a message loop exists on this thread.

To make the WindowsFormsSynchronizationContext functional you must start a message loop. It can be done like this:

static void Main(string[] args)
{
    EventHandler idleHandler = null;
    idleHandler = async (sender, e) =>
    {
        Application.Idle -= idleHandler;
        await MyMain(args);
        Application.ExitThread();
    };
    Application.Idle += idleHandler;
    Application.Run();
}

The MyMain method is your current Main method, renamed.

---

Update: Actually the Application.Run method installs automatically a WindowsFormsSynchronizationContext in the current thread, so you don't have to do it explicitly. If you want you can prevent this automatic installation, be configuring the property WindowsFormsSynchronizationContext.AutoInstall before calling Application.Run.

The AutoInstall property determines whether the WindowsFormsSynchronizationContext is installed when a control is created, or when a message loop is started.