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Customize a build to handle generated files

In any given build, files that get generated during the build behave differently from static files (such as source files). For this reason, it's important to understand How MSBuild Builds Projects. The two phases are the evaluation phase and the execution phase. During the evaluation phase, MSBuild reads your project, imports everything, creates properties, expands globs for items, and sets up the build process. During the execution phase, MSBuild performs the build by running targets and tasks with the data it parsed during the evaluation phase.

Files generated during execution don't exist during the evaluation phase, therefore they aren't included in the build process. To solve this problem, you must manually add the generated files into the build process. The recommended way to do this is by adding the new file to the Content or None items before the BeforeBuild target, as in the following example:

<Target Name="MyTarget" BeforeTargets="BeforeBuild">
  
  <!-- Some logic that generates your file goes here -->
  <!-- Generated files should be placed in $(IntermediateOutputPath) -->
  <WriteLinesToFile
      File="$(IntermediateOutputPath)GeneratedFile.cs"
      Lines='enum MyEnum { A, B }'
      Overwrite="true" />
    <ItemGroup>
      <Compile Include="$(IntermediateOutputPath)GeneratedFile.cs" />
    </ItemGroup>

  <ItemGroup>
    <!-- If your generated file was placed in `obj\` -->
    <None Include="$(IntermediateOutputPath)GeneratedFile.cs" TargetPath="GeneratedFile.cs" CopyToOutputDirectory="PreserveNewest"/>
    <!-- If you know exactly where that file is going to be, you can hard code the path. -->
    <None Include="some\specific\path\my-generatedfile" CopyToOutputDirectory="PreserveNewest"/>
    
    <!-- If you want to capture "all files of a certain type", you can glob like so. -->
    <None Include="some\specific\path\*.xyz" CopyToOutputDirectory="PreserveNewest"/>
    <None Include="some\specific\path\*.*" CopyToOutputDirectory="PreserveNewest"/>
  </ItemGroup>
</Target>

<Target Name="CleanGeneratedCode" AfterTargets="CoreClean">
  <Delete Files="$(IntermediateOutputPath)GeneratedFile.cs" />
</Target>

Adding your generated file to None or Content is sufficient for the build process to see it. You also want to ensure it gets added at the right time. Ideally, your target runs before BeforeBuild. AssignTargetPaths is another possible target, as it is the final opportunity to modify None and Content items (among others) before they are transformed into new items. See Common Item Types.

Copy the above, paste it into a file, and call it buildcodegen.targets. Then, run dotnet new console, import the file, and build it to see how it works.

<Project Sdk="Microsoft.NET.Sdk">
  <Import Project="buildcodegen.targets"/>
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net9.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>
</Project>

Run msbuild.exe and look at the output to verify that your file was generated and copied to the output folder. You can use ildasm.exe to confirm that your output binaries include the generated code MyEnum:

ildasm CodeGen.dll

Next steps

This example could be improved to support more realistic use cases. For example, to support incremental builds when the generated code depends on an input file, Inputs and Outputs should be provided to the target. Such a target would only regenerate the file if the date of the input file or files is more recent than the output file. Often when customizing for code generation, it's recommended to create a custom task. See Create a custom task for code generation.

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