"The calling thread must be STA, because many UI components require this" exception during unit test

Question

I have a class called OptionsWindow which inherits from Window which is for picking from options in a window. And a Dialog Class which deals with these dialogs. In my test, im trying to mock the choice picked from the dialog.

[TestMethod]
public async Task Test()
{
    dialog.Setup(e => e.ShowDialog(It.IsAny<Window>(), It.IsAny<IntPtr>()))
                .Returns(true)
                .Callback<Window, IntPtr>((w, ip) => {
                    if (w.DataContext != null && w.DataContext is OptionsViewModel ovm)
                        ovm.Result = -1;
                    });
    await tester.ShowWindow();
    //assert....
}

Then in the class being tested, i have these methods.

public async Task ShowWindow()
{
    var res = ShowDialog();
    //do other stuff...
}

private int ShowDialog()
{
    OptionsViewModel vm = //.....
    dialog.ShowDialog(new OptionsWindow(vm));
    return vm.Result;
}

however, i get the error "The calling thread must be STA, because many UI components require this" when it tries to set the Result of the OptionsViewModel.

During manual testing, everything worked fine and there were no threading issues so im not sure why im getting these here... any help is great. thanks

(im using Microsoft.VisualStudio.TestTools.UnitTesting btw)

Answer 1

My use case was similar and what worked for me was creating an STA thread inside the test method. In this way, I was able to exercise a Winforms UI that wouldn't run otherwise.

[TestMethod("JUST THE BASIC: Microsoft.VisualStudio.TestTools.UnitTesting")]
public async Task TestMethod1()
{
    // Use a semaphore to prevent the [TestMethod] from returning prematurely.
    SemaphoreSlim ss = new SemaphoreSlim(1);
    await ss.WaitAsync();
    Thread thread = new Thread(() =>
    {
        // Verify
        Assert.IsTrue(Thread.CurrentThread.GetApartmentState() == ApartmentState.STA);

        // Log a message to the Unit Test
        Console.WriteLine($"Thread State is {Thread.CurrentThread.GetApartmentState()}.");

        // I personally needed to test a Winforms UI and
        // the DragDrop COM wouldn't register without STA.
        var myUI = new System.Windows.Forms.Form();
        myUI.HandleCreated += (sender, e) =>
        {
            AutomateMyUI(myUI);
        };
        System.Windows.Forms.Application.Run(myUI);

        // Signal that the [TestMethod] can return now.
        ss.Release();
    });
    // Just make sure to set the apartment state BEFORE starting the thread:
    thread.SetApartmentState(ApartmentState.STA);
    thread.Start();           
    await ss.WaitAsync();

    Console.WriteLine("All done!");
}

Then to prove out the concept I made a short routine that cycles the colors.

/// <summary>
/// DEMO: Sweeps though some UI colors before closing UI.
/// </summary>
private async void AutomateMyUI(System.Windows.Forms.Form myUI)
{
    await Task.Delay(1000);
    myUI.BackColor = Color.LightSalmon;
    await Task.Delay(1000);
    myUI.BackColor = Color.LightGreen;
    await Task.Delay(1000);
    myUI.BackColor = Color.LightYellow;
    await Task.Delay(1000);
    myUI.BackColor = Color.LightBlue;

    myUI.Close();
}

Answer 2

In my test, im trying to mock the choice picked from the dialog.

Usually, if writing tests is difficult, that indicates the code should be better designed.

In this case, direct dependence on UI components is not ideal. There's a pattern called ports and adapters (a.k.a. Hexagonal Architecture, a.k.a. Clean Architecture) that would help here. In summary, you define interfaces from the application's perspective, and then have small adapter objects implement those interfaces.

So you could have the application define an interface that provides what it needs:

public interface IUserInteraction
{
  int ModalOptionsWindow();
}

with an implementation:

public sealed class WpfUserInteraction : IUserInteraction
{
  int ModalOptionsWindow()
  {
    OptionsViewModel vm = //.....
    dialog.ShowDialog(new OptionsWindow(vm));
    return vm.Result;
  }
}

What exactly the interface covers is up to you. Generally, I like to keep my ViewModels on the application side of the port, and only have views on the UI side of the port.

Once you have an interface, inject the IUserInteraction and have your code call that. After that, the unit testing is simplified.

---

However, if you are in a legacy code scenario, where you need to write tests before refactoring, then you can unit test UI code. It's just not easy. See WpfContext or WindowsFormsContext in this old archive for a way to create an STA thread and pump messages from within a unit test.