How to implement unit tests for WinUI3 (C++) application

Question

At the moment, I have created a test project using a project template from Visual Studio Unit Test App (Winui 3) and my code looks like this

namespace TestMyWinrtApp
{
    TEST_CLASS(CppUnitTests)
    {
    public:
        TEST_METHOD(CppTestOne)
        {
            try {
                auto bp = winrt::make< winrt::MyUI::implementation::BlankPage>();
                Button btn = bp.FindName(L"myButton").as<winrt::Microsoft::UI::Xaml::Controls::Button>();
                winrt::hstring btnTag = btn.Tag().as<winrt::hstring>();
                Assert::IsTrue(btnTag == L"MyTag");
            } catch (const winrt::hresult_error& ex) {
                MessageBox(
                    NULL,                  
                    ex.message().c_str(), 
                    L"Error", 
                    MB_OK | MB_ICONINFORMATION
                );
            }
        }
    };
}

However, after starting the tests, I get the exception: "The Application Called An Interface that was Marshalled for a Different Thread". As far as I know, this exception thrown is due to the fact that I try to create an winrt::make< winrt::MyUI::implementation::BlankPage>() UI element outside the UI thread. I found a solution for C# which is to add an [UITestMethod] attribute for the test method. But how i can do unit tests like this in C++ test project?

Answer 1

The Problem is solved. As @Simon Mourier & @György Kőszeg suggested in comments, we can We can add a task to the ui thread queue to use the ui elements there. To access the ui thread queue, I used a global variable because I did not find any other way to access the ui thread queue other than through a global variable whose value is assigned in the OnLaunched method in the App class after activating the main window. Finally, solution looks like this:

#include <winrt/base.h>
#include <winrt/Microsoft.UI.Xaml.Controls.h>
using namespace winrt::Microsoft::UI::Xaml::Controls;
extern winrt::Microsoft::UI::Dispatching::DispatcherQueue UIDispatcherQueue;

TEST(UITEST, first)
{
    std::promise<bool> p;
    std::future<bool> f = p.get_future();
    bool enqueued = UIDispatcherQueue.TryEnqueue(winrt::Microsoft::UI::Dispatching::DispatcherQueuePriority::High, [&p]()
    {
        auto bp = winrt::make< winrt::WinUIGtest::implementation::BlankPage>();
        Button btn = bp.FindName(L"myButton").as<Button>();
        winrt::hstring btnTag = btn.Tag().as<winrt::hstring>();
        bool isEquals = (btnTag == L"MyTag");
        p.set_value(isEquals);
    });

    bool res = f.get();
    ASSERT_EQ(res, true);
}

As you can see, I was also changed test framework to google test. If you want to do so, then you need:

1. Install google test in your project
2. Run google tests manually (you can do it in the App::OnLaunched method)

Note: If you want to run tests which should enqueue callback in the UI thread, you should run tests in the separate thread, otherwise this action seems like produced deadlock.

    // allocate console to see gtest output
    if (AllocConsole()) {
        FILE* pStdout, * pStderr;
        freopen_s(&pStdout, "CONOUT$", "w", stdout);
        freopen_s(&pStderr, "CONOUT$", "w", stderr);
    }
    
    // run gtest in separate thread
    std::thread([]
    {
        int argc = 1;
        char** fakeArgv = new char* [1];
        char* path = new char[1024];
        strcpy_s(path, 1024, R"(C:\Users\UserName\source\repos\WinUIGtest\x64\Debug\WinUIGtest\WinUIGtest.exe)");
        fakeArgv[0] = path;
        ::testing::InitGoogleTest(&argc, fakeArgv);
        //::testing::InitGoogleMock(&argc, fakeArgv);
    
        RUN_ALL_TESTS();
    }).detach();
    
    // disable standard tests
    //winrt::Microsoft::VisualStudio::TestPlatform::TestExecutor::WinRTCore::UnitTestClient::Run(m_args);