C# XAML Metro app using WinRT component calling task.wait() or task.get() on Async method throws: Unhandled exception 0xc0000409

Question

I have been experimenting a lot lately with Windows 8, writing C# XAML Metro style apps using WinRT components written in C++/CX for better performance and to use functionality not available in C#, specifically DirectX.

While loading resources from my app package in my WinRT component, my app threw the following:

Unhandled exception at 0x0fd58ae3 (MSVCR110D.dll) in TestResources.exe: 0xC0000409: 0xc0000409.

I was trying to make asynchronous calls to the new StorageFile API (Windows::Storage::StorageFile::GetFileFromApplicationUriAsync chained to other calls for reading the file contents) and synchronize the resulting task chain using concurrency::task.get().

It didn't seem to be working. If I didn't call concurrency::task.get() or concurrency::task.wait(), then the problem did not occur, but I needed the result synchronously because of how my DirectX code was written.

Answer 1

The reason is that you are calling concurrency::task.wait() or concurrency::task.get() from the UI thread! The framework throws an exception to prevent you from freezing the app. See: Creating Asynchronous Operations in C++ for Metro style Apps, toward the bottom there are three warnings. The last warning says:

Do not call concurrency::task::wait in the body of a continuation that runs on the STA. Otherwise, the runtime throws concurrency::invalid_operation because this method blocks the current thread and can cause the app to become unresponsive.

I wrote a test app and verified that I could make everything work by calling my WinRT component from a separate thread!

Details below:

My test app is C# XAML Metro app calling WinRT component to load a string from a file. It has a Button and a TextBlock.

The resource loader looks like this:

class WstringCaselessLess : std::binary_function< std::wstring, std::wstring, bool >
{
public:
    bool operator()( const std::wstring& s1, const std::wstring& s2 )
    {
        return _wcsicmp( s1.c_str(), s2.c_str() ) < 0;
    }
};

public ref class ComponentResourceLoader sealed
{
public:
    Platform::String^ GetStringResource( Platform::String^ uri )
    {
        auto key = std::wstring( uri->Data(), uri->Length() );
        auto findit = m_resourceMap.find( key );
        if ( findit != std::end( m_resourceMap ) )
            return ref new Platform::String( findit->second.c_str() );
        auto uriObj = ref new Windows::Foundation::Uri( uri );
        auto fileOp = Windows::Storage::StorageFile::GetFileFromApplicationUriAsync( uriObj );
        return concurrency::create_task( fileOp )
                .then( [this, &key]( Windows::Storage::StorageFile^ file )
                       -> Windows::Foundation::IAsyncOperation< Windows::Storage::Streams::IBuffer^ >^
                    {
                        return Windows::Storage::FileIO::ReadBufferAsync( file );
                    } )
                .then( [this, &key]( Windows::Storage::Streams::IBuffer^ buffer )
                       -> Platform::String^
                    {
                        auto reader = Windows::Storage::Streams::DataReader::FromBuffer( buffer );
                        auto str = reader->ReadString( buffer->Length );
                        this->m_resourceMap[key] = std::wstring( str->Data(), str->Length() );
                        return str;
                    } ).get();
    }
private:
    std::map< std::wstring, std::wstring, WstringCaselessLess > m_resourceMap;
};

The broken button click handler looks like this:

private void WinRT_Click_1(object sender, RoutedEventArgs e)
{
    TextContent.Text = m_loader.GetStringResource(@"ms-appx:///Assets/Hello.xml");
}

If I change the button handler to load the string in a separate thread, it works:

private async void WinRT_Click_1(object sender, RoutedEventArgs e)
{
    var text = await Task.Run<string>(() => RunWinrtLoader());
    TextContent.Text = text;
}
private string RunWinrtLoader()
{
    return m_loader.GetStringResource(@"ms-appx:///Assets/Hello.xml");
}

Hope this is helpful to someone! It sure had me pretty angry for a while, because there is no indication from the error to the real problem.