Version

2.0.250303.1

Summary

If a project has an .idl file that isn't "modern", WinMD merging fails.
To reproduce the issue:

1. Create a simple C++ project in Visual Studio
2. Add a simple non-WinRT .idl file to the project, it doesn't have to contain anything
3. Add the metadata <OutputDirectory>$(MSBuildThisFileDirectory)</OutputDirectory> to the Midl item of the .idl file
4. Set CppWinRTModernIDL and CppWinRTGenerateWindowsMetadata properties to false in the project.

I attached a sample solution that does exactly this here: CppWinRtTest.zip

Reproducible example

Simple C++ project with:

  <PropertyGroup>
    <CppWinRTModernIDL>false</CppWinRTModernIDL>
    <CppWinRTGenerateWindowsMetadata>false</CppWinRTGenerateWindowsMetadata>
  </PropertyGroup>
  <ItemGroup>
    <Midl Include="test.idl">
      <OutputDirectory>$(MSBuildThisFileDirectory)</OutputDirectory>
    </Midl>
  </ItemGroup>

Sample `test.idl`:
import "oaidl.idl";
import "ocidl.idl";

[object, uuid(93242F75-651F-42FA-8C60-9B7DAD8A163D), local]
interface Test: IUnknown {};

Expected behavior

Project builds successfully

Actual behavior

Project build fails with: MDMERGE : error MDM2025: The input directory or file C:\Users\Bugaleb\source\repos\CppWinRtTest" -o CppWinRtTest\x64\Debug\Merged -partial is not valid.

Additional comments

The issue seems to be with the GetCppWinRTMdMergeInputs target doing this:

            <_MdMergeInputs Include="@(Midl)" Condition="'%(Midl.ExcludedFromBuild)' != 'true'">
                <WinMDPath>%(Midl.OutputDirectory)%(Midl.MetadataFileName)</WinMDPath>
                <MdMergeOutputFile>$(CppWinRTProjectWinMD)</MdMergeOutputFile>
            </_MdMergeInputs>

basically, unconditionally adding all Midl items to _MdMergeInputs.

A potential solution would be to have a Midl.ExcludedFromWinMD metadata or something similar, though since CppWinRTModernIDL is a global property anyway, it can probably be used here as well to avoid attempting to merge anything.

A potential workaround is to add the following target in the project:

  <Target Name="WorkaroundCppWinRtBug" AfterTargets="GetCppWinRTMdMergeInputs" BeforeTargets="CppWinRTMergeProjectWinMDInputs">
    <ItemGroup>
      <CppWinRTMdMergeInputs Remove="@(CppWinRTMdMergeInputs)" />
    </ItemGroup>
  </Target>