async test + Apartment(ApartmentState.STA) => await not returning on STA thread

[yar-shukan]

There is the old post on SO:
http://stackoverflow.com/questions/18838116/nunit-async-test-requiressta-await-not-returning-on-sta-thread
It seems like this issue has been there long time ago, but we faced it only after upgrading to NUnit 3.0.1.

We have tests that use async/await with Apartment(ApartmentState.STA) and before the upgrade to NUnit 3.0.1 they used to work, but after the upgrade they fail because WPF components don't run on STA thread after await statement.

[rprouse]

Are you saying that when the test is marked STA, the thread is no longer STA after an await in NUnit 3, or are you saying that the workarounds in the SO post no longer work?

[yar-shukan]

Yes, it is no longer STA after await in NUnit 3.2.1 (the same issue in 3.0.1 as well).
Marking test method with [Apartment(ApartmentState.STA)] as well as obsolete [RequiresSTA] doesn't work. The managed thread ID is different after await.

[keithallenjackson]

My current work project has ran into this very issue. I'm curious if any work has been done to address this. I'm interested in at least looking into it, but wanted to touch base and get a status update to not duplicate work effort.

[Rincoln]

I've had to do the same thing to work with proprietary ODBC drivers. My solution is a little convoluted, but works.

public Task<T> RunSTAThread<T>(Func<T> action)
{
var tcs = new TaskCompletionSource<T>();
System.Threading.Thread runThread = new System.Threading.Thread(new System.Threading.ThreadStart(() =>
{
try
{
var result = action();
tcs.TrySetResult(result);
}
catch (Exception ex)
{
tcs.TrySetException(ex);
}
}));
runThread.SetApartmentState(System.Threading.ApartmentState.STA);
runThread.Start();
return tcs.Task;
}

Then you simply run your method requiring STA thread by:

var result = await RunSTAThread(MethodRequiringSTA);

[CharliePoole]

This was still marked as confirm and nothing has been done. I guess we should at least consider it confirmed.

[jnm2]

There's an easy fix for this, I just did this the other day. We need to set a synchronization context and message pump on the STA thread. Otherwise, await schedules continuations on a random thread pool thread.

[jnm2]

Stephen Toub gives an excellent example:
https://blogs.msdn.microsoft.com/pfxteam/2012/01/20/await-synchronizationcontext-and-console-apps/

[jnm2]

Here's a workaround you all can use so that you don't waste threads:

Change

[Apartment(ApartmentState.STA)]
public async Task MyTestAsync()
{
    await FirstThing;
    // Still on STA thread
    await SecondThing;
    // etc
}

to

[Apartment(ApartmentState.STA)]
public void MyTest()
{
    SingleThreadedSynchronizationContext.Await(async () =>
    {
        await FirstThing;
        // Still on STA thread
        await SecondThing;
        // etc
    });
}

And use:

public sealed class SingleThreadedSynchronizationContext : SynchronizationContext
{
    private readonly BlockingCollection<(SendOrPostCallback d, object state)> queue = new BlockingCollection<(SendOrPostCallback, object)>();

    public override void Post(SendOrPostCallback d, object state)
    {
        queue.Add((d, state));
    }

    public static void Await(Func<Task> taskinvoker)
    {
        var originalContext = Current;
        try
        {
            var context = new SingleThreadedSynchronizationContext();
            SetSynchronizationContext(context);

            var task = taskinvoker.Invoke();
            task.ContinueWith(_ => context.queue.CompleteAdding());

            while (context.queue.TryTake(out var work, Timeout.Infinite))
                work.d.Invoke(work.state);

            task.GetAwaiter().GetResult();
        }
        finally
        {
            SetSynchronizationContext(originalContext);
        }
    }
}

[jnm2]

@nunit/framework-team The fix for this in NUnit will be to install the SingleThreadedSynchronizationContext before each invocation of a test method. This will collide with my PR #2181 so I should probably assign this to myself.

[CharliePoole]

Cool!