您的控件相关代码需要在 UI 线程上运行.试试:

Your controls related code needs to be run on a UI thread. Try:

[TestMethod]
async public Task CreateThumbnail_EmptyLayout_ReturnsEmptyGrid()
{
    int count = 0;
    await ExecuteOnUIThread(() =>
    {
        Layout l = new Layout();
        ThumbnailCreator creator = new ThumbnailCreator();
        Grid grid = creator.CreateThumbnail(l, 192, 120);
        count = grid.Children.Count;
    });

    Assert.AreEqual(count, 0);
}

public static IAsyncAction ExecuteOnUIThread(Windows.UI.Core.DispatchedHandler action)
{
    return Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(Windows.UI.Core.CoreDispatcherPriority.Normal, action);
}

以上内容应该适用于 MS Test.我不知道 NUnit.

The above should work on MS Test. I don't know about NUnit.