Is there a .NET Core CLI pre before build task?

Question

The question

I assume when msbuild is about to compile source code files, it produces a list of files to build, based on Include and

Answer 1

The "default items" as the .NET SDK calls it are part of the static evaluation of the project file - before any target is run. So you'll need a target that is run before the @(Compile) items are needed.

The trick is to include files added to the filesystem after the custom tool is run. This can be done by re-scanning all files and excluding those already part of the project inside a target that is run before the build:

  <Target Name="GenerateSomeFiles" BeforeTargets="BeforeBuild">
    <Exec Command="dotnet my-tool" />
    <ItemGroup>
      <Compile Include="**/*$(DefaultLanguageSourceExtension)"
               Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder);$(BaseIntermediateOutputPath)**;$(BaseOutputPath)**;@(Compile)" />
    </ItemGroup>
  </Target>