"T must be WinRT type" error for GUID properties

[osolo]

Adding GUID properties in an .idl files leads to the following error:

C2338 T must be WinRT type.

Sample .idl:

namespace CppWinRTXaml
{
    [default_interface]
    runtimeclass BlankUserControl : Windows.UI.Xaml.Controls.UserControl
    {
        BlankUserControl();
        Guid MyProperty;
    }
}

The error stems from the generated XamlTypeInfo.Impl.g.cpp using GUID and not the newer winrt::guid class:

const MemberInfo MemberInfos[] = 
{
    //   0 - CppWinRTXaml.BlankUserControl.MyProperty
    L"MyProperty",
    &GetValueTypeMember_MyProperty<::winrt::CppWinRTXaml::BlankUserControl, GUID>,
    &SetValueTypeMember_MyProperty<::winrt::CppWinRTXaml::BlankUserControl, GUID>,
    0, // Guid
    -1,
    false, false, false,
};

As a temporary workaround, I added the following to my pch.h (but I cannot guarantee correctness):

namespace winrt
{
    template <>
    inline Windows::Foundation::IInspectable box_value<GUID>(GUID const& value)
    {
        return Windows::Foundation::IReference<winrt::guid>(winrt::guid(value));
    }
}

[kennykerr]

@danzil do you know whether this is an issue with the Xaml compiler generated code?

Indeed, the box_value helper doesn't automatically convert GUID to winrt::guid. That's probably a good idea, but I'm mostly asking about XamlTypeInfo.Impl.g.cpp in the issue above.

[danzil]

@kennykerr, is the guidance for developers to use winrt::guid in their code? If yes, then the generated code should also use winrt::guids and I'll make this a bug on WinUI 3. If no, then I guess cppwinrt should add support for that.

[kennykerr]

@danzil it's both. In general, C++/WinRT lets you use GUID where winrt::guid is expected. However, as a type parameter, winrt::guid must be defined as it is used in various recursive calculations following type instantiation. So I'll update winrt::box_value to likewise handle GUID, but if you're generating code that generates GUID types then you should prefer to use winrt::guid as only the latter is strictly speaking a "WinRT type".