Proposal: Exclude CompilerGeneratedFilesOutputPath from Compilation by Default

[kasperk81]

Problem

When CompilerGeneratedFilesOutputPath is set in csproj, the .NET SDK emits source generator output to disk. However, these files are not excluded from compilation by default, leading to errors when running dotnet build a second time.

Currently, developers must manually add:

<ItemGroup>
    <Compile Remove="$(CompilerGeneratedFilesOutputPath)/**/*.cs" />
</ItemGroup>

Without this, builds fail due to duplicate type definitions: https://andrewlock.net/creating-a-source-generator-part-6-saving-source-generator-output-in-source-control/#excluding-emitted-files-from-the-compilation

Proposal

The .NET SDK should automatically exclude the CompilerGeneratedFilesOutputPath from compilation when it is set, preventing these errors.

Would love to hear thoughts on whether this can be supported natively!

[kasperk81]

@jjonescz @jaredpar

[jjonescz]

I imagine some users might be running generators explicitly (via some MSBuild property), including their outputs in the Git-tracked source tree, disabling the generators again, and from then on compiling normally without generators. I guess that's not a very good approach. But just wanted to note that adding <Compile Remove="$(CompilerGeneratedFilesOutputPath)/**/*.cs" /> would break these users (if they exist).

[kasperk81]

Would exclusion by default with bailout option ExcludeGeneratedFilesFromCompilation (default true) work for those cases?

[jaredpar]

Would exclusion by default with bailout option ExcludeGeneratedFilesFromCompilation (default true) work for those cases?

It would work but I'm not sure that is the direction that we want to go. The compiler has lots of knobs today, every one that we add is yet another knob that we have to track, support, etc ... Given that this issue already has a single line fix I don't think a new knob is the best way to go here.