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Bug: Cannot winrt::box_value a wchar_t #1566

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Bug: Cannot winrt::box_value a wchar_t#1566

Description

@HO-COOH
HO-COOH
opened on Apr 13, 2026

Version

No response

Summary

From this documentation, it should be able to expose a Char type from the idl, but doing this will cause a compile error:

        static_assert(impl::has_category_v<T>, "T must be WinRT type.");

Reproducible example

Use this idl

    [default_interface]
    runtimeclass MainPage : Windows.UI.Xaml.Controls.Page
    {
        MainPage();
        Char MyProperty;
    }

Expected behavior

Compiles

Actual behavior

No response

Additional comments

No response

Activity

  1. Link1J commented on Apr 13, 2026

    @Link1J
    Jared Irwin (Link1J)
    on Apr 13, 2026

    Char is char16_t, not wchar_t. So, you can't box a wchar_t, but you should be able to box a char16_t.

  2. HO-COOH commented on Apr 14, 2026

    @HO-COOH
    HO-COOH
    on Apr 14, 2026
    Author

    Nope. The code generated by cppwinrt from the idl was using wchar_t from what I see in the build error.

    And the winrt::impl::reference_traits template does not have a specialization for either wchar_t or char16_t

  3. Link1J commented on Apr 14, 2026

    @Link1J
    Jared Irwin (Link1J)
    on Apr 14, 2026

    winrt::impl::reference_traits does have a char16_t specialization.

    cppwinrt/strings/base_reference_produce.h

    Lines 180 to 185 in f23b41e

    template <>
    struct reference_traits<char16_t>
    {
    static auto make(char16_t value) { return Windows::Foundation::PropertyValue::CreateChar16(value); }
    using itf = Windows::Foundation::IReference<char16_t>;
    };

    As for the error message, winrt::impl::category also has a specialization for char16_t.

    cppwinrt/strings/base_identity.h

    Line 594 in f23b41e

    template <> struct category<char16_t> { using type = basic_category; };

    It has been this way for 7 years, so it isn't anything new.

  4. HO-COOH commented on Apr 14, 2026

    @HO-COOH
    HO-COOH
    on Apr 14, 2026
    Author

    My bad. I can winrt::box_value a char16_t. But using Char in idl gives me this error:

    2>     3>C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\impl\Windows.Foundation.1.h(145,29): error C2338: static assertion failed: 'T must be WinRT type.'
2>         (compiling source file '/Generated%20Files/XamlTypeInfo.g.cpp')
2>             C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\impl\Windows.Foundation.1.h(145,29):
2>             the template instantiation context (the oldest one first) is
2>                 C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\XamlTypeInfo.g.cpp(224,1):
2>                 see reference to function template instantiation 'winrt::Windows::Foundation::IInspectable winrt::UWPMain::implementation::GetValueTypeMember_MyProperty<winrt::UWPMain::MainPage,wchar_t>(const winrt::Windows::Foundation::IInspectable &)' being compiled
2>                 C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\XamlTypeInfo.g.cpp(71,21):
2>                 see reference to function template instantiation 'winrt::Windows::Foundation::IInspectable winrt::box_value<wchar_t,0>(const T &)' being compiled
2>                 with
2>                 [
2>                     T=wchar_t
2>                 ]
2>                 C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\Windows.Foundation.h(3093,47):
2>                 see reference to class template instantiation 'winrt::impl::reference_traits<wchar_t>' being compiled
2>                 C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\Windows.Foundation.h(2680,21):
2>                 while compiling class template member function 'auto winrt::impl::reference_traits<wchar_t>::make(const T &)'
2>                 with
2>                 [
2>                     T=wchar_t
2>                 ]
2>                     C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\Windows.Foundation.h(3093,47):
2>                     see the first reference to 'winrt::impl::reference_traits<wchar_t>::make' in 'winrt::box_value'
2>                     C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\XamlTypeInfo.g.cpp(71,21):
2>                     see the first reference to 'winrt::box_value' in 'winrt::UWPMain::implementation::GetValueTypeMember_MyProperty'
2>                 C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\Windows.Foundation.h(2680,58):
2>                 see reference to function template instantiation 'auto winrt::make<winrt::impl::reference<T>,const T&>(const T &)' being compiled
2>                 with
2>                 [
2>                     T=wchar_t
2>                 ]
2>                 C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\base.h(7870,28):
2>                 see reference to variable template 'const bool is_destructible_v<winrt::impl::reference<wchar_t> >' being compiled
2>                 C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\base.h(7870,28):
2>                 see reference to class template instantiation 'winrt::impl::reference<T>' being compiled
2>                 with
2>                 [
2>                     T=wchar_t
2>                 ]
2>                 C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\Windows.Foundation.h(2572,24):
2>                 see reference to class template instantiation 'winrt::implements<winrt::impl::reference<T>,winrt::Windows::Foundation::IReference<T>,winrt::Windows::Foundation::IPropertyValue>' being compiled
2>                 with
2>                 [
2>                     T=wchar_t
2>                 ]
2>                 C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\base.h(7930,31):
2>                 see reference to alias template instantiation 'winrt::impl::producers<D,winrt::Windows::Foundation::IReference<T>,winrt::Windows::Foundation::IPropertyValue>' being compiled
2>                 with
2>                 [
2>                     D=winrt::impl::reference<wchar_t>,
2>                     T=wchar_t
2>                 ]
2>                 C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\base.h(6638,41):
2>                 see reference to alias template instantiation 'winrt::impl::tuple_if<winrt::impl::is_interface,std::tuple<winrt::Windows::Foundation::IReference<T>,winrt::Windows::Foundation::IPropertyValue>>' being compiled
2>                 with
2>                 [
2>                     T=wchar_t
2>                 ]
2>                 C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\base.h(6555,60):
2>                 see reference to class template instantiation 'winrt::impl::tuple_if_base<winrt::impl::is_interface,std::tuple<winrt::Windows::Foundation::IReference<T>,winrt::Windows::Foundation::IPropertyValue>>' being compiled
2>                 with
2>                 [
2>                     T=wchar_t
2>                 ]
2>                 C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\base.h(6552,122):
2>                 see reference to class template instantiation 'winrt::impl::is_interface<winrt::Windows::Foundation::IReference<T>>' being compiled
2>                 with
2>                 [
2>                     T=wchar_t
2>                 ]
2>                 C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\base.h(6558,32):
2>                 see reference to class template instantiation 'std::disjunction<std::is_base_of<winrt::Windows::Foundation::IInspectable,T>,winrt::impl::is_classic_com_interface<T>>' being compiled
2>                 with
2>                 [
2>                     T=winrt::Windows::Foundation::IReference<wchar_t>
2>                 ]
2>                 C:\Program Files\Microsoft Visual Studio\18\Community\VC\Tools\MSVC\14.50.35717\include\xtr1common(162,79):
2>                 see reference to class template instantiation 'winrt::Windows::Foundation::IReference<T>' being compiled
2>                 with
2>                 [
2>                     T=wchar_t
2>                 ]
2>
2>Build FAILED.
2>
2>       "C:\Users\Peter\Desktop\UWPWithWin32\Package\Package.wapproj" (build target) (1) ->
2>       "C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\UWPMain.vcxproj" (default target) (3:7) ->
2>       (CompileXamlGeneratedFiles target) -> 
2>         C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\UWPMain.h(32,67): warning C4244: 'argument': conversion from 'int32_t' to 'T', possible loss of data
2>       C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\UWPMain.h(32,67): warning C4244:         with
2>       C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\UWPMain.h(32,67): warning C4244:         [
2>       C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\UWPMain.h(32,67): warning C4244:             T=char16_t
2>       C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\UWPMain.h(32,67): warning C4244:         ]
2>
2>
2>       "C:\Users\Peter\Desktop\UWPWithWin32\Package\Package.wapproj" (build target) (1) ->
2>       "C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\UWPMain.vcxproj" (default target) (3:7) ->
2>       (CompileXamlGeneratedFiles target) -> 
2>         C:\Users\Peter\Desktop\UWPWithWin32\UWPMain\Generated Files\winrt\impl\Windows.Foundation.1.h(145,29): error C2338: static assertion failed: 'T must be WinRT type.'

    It's the same for winui3. Should I file this issue to winui's repo instead?

  5. Link1J commented on Apr 14, 2026

    @Link1J
    Jared Irwin (Link1J)
    on Apr 14, 2026

    It definitely is a bug for the WinUI team, but I do consider C++/WinRT's use of char16_t instead of wchar_t for Char a bug as well.
    It is a bit surprising that winrt::hstring uses wchar_t for its character type, but the actual character type is char16_t.

  6. hoshiizumiya commented on Apr 14, 2026

    @hoshiizumiya
    hoshiizumiya
    on Apr 14, 2026

    It is a confusing thing:
    At the ABI level, the HSTRING underlying data for WinRT is char16_t (unsigned 16 bit integer).
    At the C++/WinRT level, the public interface of winrt:: hstring uses wchar_t.

  7. sylveon commented on Apr 14, 2026

    @sylveon
    Charles Milette (sylveon)
    on Apr 14, 2026
    Contributor

    hstring probably uses wchar_t for convenience because of the large amount of legacy APIs that take and hand wchar_t strings.

    In this case the bug is on WinUI for interpreting Char in metadata as wchar_t, it looks like this wasn't even tested.

  8. YexuanXiao commented on Apr 14, 2026

    @YexuanXiao
    Yexuan Xiao (YexuanXiao)
    on Apr 14, 2026
    Contributor

    From the perspective of C++'s future evolution, char16_t should be used instead of wchar_t, because the width of wchar_t is configurable, making it unsuitable for cross-platform UTF-16 representation. As a result, future C++ Unicode facilities will only provide APIs using char16_t. I believe changing hstring's value_type to char16_t is a potential improvement for C++/WinRT 4.0, while still allowing construction of hstring from wchar_t to support legacy code.

  9. HO-COOH commented on Oct 9, 2026

    @HO-COOH
    HO-COOH
    on Oct 9, 2026
    Author

    This is fixed from winui3's repo: microsoft/microsoft-ui-xaml#11069

  10. HO-COOH
    closed this as completedon Oct 9, 2026
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