Walkthrough: Generate a C# projection from a C++/WinRT component and distribute as a NuGet package for .NET 5+ applications

In .NET 5 and later, consumption of WinMD files is no longer supported. Instead, the C#/WinRT tool can be used to generate a projection assembly for any WinMD files, which then enables consumption of WinRT components from .NET 5+ applications. A projection assembly is also known as an interop assembly. This walkthrough shows how to do the following:

  • Use the C#/WinRT package to generate a C# projection from a C++/WinRT component.
  • Distribute the component along with the projection assembly as a NuGet package.
  • Consume the NuGet package from a .NET 5 console application.

You can download the full sample for this walkthrough from GitHub here.


This walkthrough and the corresponding sample requires the following tools and components:

  • Visual Studio 16.8 (or later) with the Universal Windows Platform development workload installed. In Installation Details > Universal Windows Platform development, check the C++ (v14x) Universal Windows Platform tools option.
  • .NET 5.0 SDK.
  • C++/WinRT VSIX extension for C++/WinRT project templates.

Create a simple C++/WinRT Runtime component

To follow this walkthrough, you must first have a C++/WinRT component for which to generate the C# projection assembly. This walkthrough uses the SimpleMathComponent project in the related sample from GitHub here. This is a Windows Runtime Component (C++/WinRT) project that was created by using the C++/WinRT VSIX extension. After you copy the project to your development computer, open the solution in Visual Studio 2019.

The code in this project provides the functionality for basic math operations shown in the header file below.

// SimpleMath.h
namespace winrt::SimpleMathComponent::implementation
    struct SimpleMath: SimpleMathT<SimpleMath>
        SimpleMath() = default;
        double add(double firstNumber, double secondNumber);
        double subtract(double firstNumber, double secondNumber);
        double multiply(double firstNumber, double secondNumber);
        double divide(double firstNumber, double secondNumber);

Make sure to set the Windows Desktop Compatible property to Yes for the C++/WinRT component project. To do this, right click on the SimpleMathComponent project and select Properties. Under Configuration Properties -> General, set the property Windows Desktop Compatible under Project Defaults to Yes. This ensures that the correct runtime binaries are loaded for consuming .NET desktop apps.

Desktop Compatible property page

For more detailed steps about creating a C++/WinRT component and generating a WinMD file, see Windows Runtime components with C++/WinRT.


If you are implementing IInspectable::GetRuntimeClassName in your component, it must return a valid WinRT class name. Because C#/WinRT uses the class name string for interop, an incorrect runtime class name will raise an InvalidCastException.

Add a projection project to the component solution

If you have cloned the sample from the repo, first delete the SimpleMathProjection project to follow the walkthrough step by step.

  1. Add a new C# library project to your solution.

    1. In Solution Explorer, right click your solution node and click Add -> New Project.
    2. In the Add New Project dialog box, enter Class Library in the search box. Choose C# from the Language list, and then choose Windows from the Platform list. Choose the Class Library template and click Next.
    3. Name the new project SimpleMathProjection and select Next.
    4. On the Additional information page, select .NET 5.0 (Current), and then choose Create.
  2. Delete the empty Class1.cs file from the project.

  3. Install the C#/WinRT NuGet package.

    1. In Solution Explorer, right click your SimpleMathProjection project and select Manage NuGet Packages.
    2. Search for the Microsoft.Windows.CsWinRT NuGet package and install the latest version.
  4. Add a project reference to the SimpleMathComponent project. In Solution Explorer, right click the Dependencies node under the SimpleMathProjection project, select Add Project Reference, and select the SimpleMathComponent project.

After these steps, your Solution Explorer should look similar to this.

Solution Explorer showing projection project dependencies

Build projects out of source

In the related sample, the build output location is configured with the Directory.build.props file to build out of source. This means that files from the build output are generated outside the source folder. It is recommended to build out of source when using the C#/WinRT tool. This prevents the C# compiler from inadvertently picking up all *.cs files under the project root directory, which can cause duplicate type errors (for example when compiling for multiple configurations and/or platforms).

To configure your solution to build out of source:

  1. Right click on your solution, and select Add -> New Item. Select the XML File option and name it Directory.build.props.

  2. Paste the contents below to Directory.build.props:

        <BuildOutDir>$([MSBuild]::NormalizeDirectory('$(SolutionDir)_build', '$(Platform)', '$(Configuration)'))</BuildOutDir>
        <OutDir>$([MSBuild]::NormalizeDirectory('$(BuildOutDir)', '$(MSBuildProjectName)', 'bin'))</OutDir>
        <IntDir>$([MSBuild]::NormalizeDirectory('$(BuildOutDir)', '$(MSBuildProjectName)', 'obj'))</IntDir>

Edit the project file to execute C#/WinRT

Before you can invoke the cswinrt.exe tool to generate the projection assembly, you must first edit the project file to specify a few project properties.

  1. In Solution Explorer, double-click the SimpleMathProjection node to open the project file in the editor.

  2. Update the TargetFramework element to target a specific Windows SDK version. This adds assembly depedencies that are necessary for the interop and projection support. This sample targets the latest Windows 10 SDK version as of this walkthrough, net5.0-windows10.0.19041.0 (also known as Windows 10, version 2004). To allow referencing applications to support earlier Windows 10 SDK versions, you can also set the TargetPlatformMinimumVersion property.



    For this walkthrough and the related sample code, the solution and all projects are configured to build for x64 and Debug. You can choose to build for any other .NET supported platform, but note that you will need to adjust the NuGet spec properties accordingly.

  3. Add a new PropertyGroup element that sets several C#/WinRT properties.


    Here are some details about the settings in this example:

    • The CsWinRTIncludes property specifies which namespaces to project.
    • The CsWinRTGeneratedFilesDir property sets the output directory in which the projection source files are generated. This property is set to OutDir, defined in Directory.build.props from the section above.
  4. Save the SimpleMathProjection.csproj file.

Create a NuGet package with the projection

To distribute the projection assembly for .NET 5+ application developers, you can automatically create a NuGet package when building the solution by adding some additional project properties. For .NET 5+ targets, the NuGet package needs to include the projection assembly and the implementation assembly from the component.

  1. Add a NuGet spec (.nuspec) file to the SimpleMathProjection project.

    1. In Solution Explorer, right-click the SimpleMathProjection node, choose Add -> New Folder, and name the folder nuget.
    2. Right-click the nuget folder, choose Add -> New Item, choose the XML file, and name it SimpleMathProjection.nuspec.
  2. Add the following property group to SimpleMathProjection.csproj to automatically generate the package. These properties specify the NuspecFile and the directory to generate the NuGet package.



    If you prefer generating a package separately, you can also choose to run the nuget.exe tool from the command line. For more information about creating a NuGet package, see Create a package using the nuget.exe CLI.

  3. Open the SimpleMathProjection.nuspec file to edit the package creation properties. Below is an example NuGet spec for distributing the projection assembly from the C++/WinRT component. Note that SimpleMathProjection.dll is specified instead of SimpleMathComponent.winmd for the target lib\net5.0-windows10.0.19041.0\SimpleMathProjection.dll. This behavior is new in .NET 5 and enabled by C#/WinRT. The implementation assembly, SimpleMathComponent.dll, must also be deployed and will be loaded at runtime.

    <?xml version="1.0" encoding="utf-8"?>
    <package xmlns="http://schemas.microsoft.com/packaging/2012/06/nuspec.xsd">
        <authors>Contoso Math Inc.</authors>
        <description>A simple component with basic math operations</description>
          <group targetFramework=".NETCoreApp3.0" />
          <group targetFramework="UAP10.0" />
          <group targetFramework=".NETFramework4.6" />
          <group targetFramework="net5.0-windows10.0.19041.0" />
        <!--Support .NET Core 3, UAP, .NET Framework 4.6, .NET 5, C++ -->
        <!--Architecture-netural assemblies-->
        <file src="..\..\_build\x64\Debug\SimpleMathComponent\bin\SimpleMathComponent\SimpleMathComponent.winmd" target="lib\netcoreapp3.0\SimpleMathComponent.winmd" />
        <file src="..\..\_build\x64\Debug\SimpleMathComponent\bin\SimpleMathComponent\SimpleMathComponent.winmd" target="lib\uap10.0\SimpleMathComponent.winmd" />
        <file src="..\..\_build\x64\Debug\SimpleMathComponent\bin\SimpleMathComponent\SimpleMathComponent.winmd" target="lib\net46\SimpleMathComponent.winmd" />
        <file src="..\..\_build\x64\Debug\SimpleMathProjection\bin\SimpleMathProjection.dll" target="lib\net5.0-windows10.0.19041.0\SimpleMathProjection.dll" />
        <!--Architecture-specific implementation DLLs should be copied into RID-relative folders-->
        <file src="..\..\_build\x64\Debug\SimpleMathComponent\bin\SimpleMathComponent\SimpleMathComponent.dll" target="runtimes\win10-x64\native\SimpleMathComponent.dll" />


    SimpleMathComponent.dll, the implementation assembly for the component, is architecture-specific. If building for other platforms (e.g., x86 or ARM64), you must first build SimpleMathComponent for the desired platforms and add targets for the appropriate RID-relative folder. The projection assembly SimpleMathProjection.dll and the component SimpleMathComponent.winmd are both architecture-neutral.

Build the solution to generate the projection and NuGet package

Before building the solution, make sure to check the Configuration Manager settings in Visual Studio, under Build -> Configuration Manager. For this walkthrough, set the Configuration to Debug and Platform to x64, for both projects and the solution.

At this point you can now build the solution: right click on your solution node and select Build Solution. This will first build the SimpleMathComponent project and then the SimpleMathProjection project. The component WinMD and implementation assembly (SimpleMathComponent.winmd and SimpleMathComponent.dll), the projection source files, and the projection assembly (SimpleMathProjection.dll), will all be generated under the _build output directory. You will also be able to see the the generated NuGet package, SimpleMathComponent0.1.0-prerelease.nupkg, under the nuget folder.

Solution Explorer showing projection generation

Reference the NuGet package in a C# .NET 5 console application

To consume SimpleMathComponent from .NET 5 projects, you can simply add a reference to the newly created NuGet package in your application. The following steps demonstrate how to do this by creating a simple Console app in a separate solution.

  1. Create a new solution with a C# Console Application project.

    1. In Visual Studio, select File -> New -> Project.
    2. In the Add New Project dialog box, search for the Console Application project template. Select the template and click Next.
    3. Name the new project SampleConsoleApp and click Create. Creating this project in a new solution allows you to restore the SimpleMathComponent NuGet package separately.
  2. In Solution Explorer, double-click the SampleConsoleApp node to open the SampleConsoleApp.csproj project file, and update the TargetFramework and Platform properties as shown in the following example.

  3. Add the SimpleMathComponent NuGet package to the SampleConsoleApp project. To restore the SimpleMathComponent NuGet when building the project, you can use the RestoreSources property with the path to the nuget folder in your component solution.

      <PackageReference Include="SimpleMathComponent" Version="0.1.0-prerelease" />

    Note that for this walkthrough, the RestoreSources path for the SimpleMathComponent package assumes that both solution files are in the same directory. You may need to update the path accordingly. Alternatively, you can add a local NuGet package feed to your solution.

  4. Edit the Program.cs file to use the functionality provided by SimpleMathComponent.

    static void Main(string[] args)
        var x = new SimpleMathComponent.SimpleMath();
        Console.WriteLine("Adding 5.5 + 6.5 ...");
        Console.WriteLine(x.add(5.5, 6.5).ToString());
  5. Build and run the console app. You should see the output below.

    Console NET5 output