Sample of generating a .cs file during build and adding it to the compilation

AddGeneratedFile.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net472</TargetFramework>
  </PropertyGroup>

  <PropertyGroup>
    <GeneratedText><![CDATA[
using System%3B

public class Hello$(TargetFramework)
{
    public void Print()
    {
        Console.WriteLine("Hello $(TargetFramework)!")%3B
    }
}
      ]]></GeneratedText>
  </PropertyGroup>

  <Target Name="AddGeneratedFile" BeforeTargets="BeforeCompile;CoreCompile" Inputs="$(MSBuildAllProjects)" Outputs="$(IntermediateOutputPath)GeneratedFile.cs">
    <PropertyGroup>
      <GeneratedFilePath>$(IntermediateOutputPath)GeneratedFile.cs</GeneratedFilePath>
    </PropertyGroup>
    <ItemGroup>
      <Compile Include="$(GeneratedFilePath)" />
      <FileWrites Include="$(GeneratedFilePath)" />
    </ItemGroup>
    <WriteLinesToFile Lines="$(GeneratedText)" File="$(GeneratedFilePath)" WriteOnlyWhenDifferent="true" Overwrite="true" />
  </Target>

</Project>

@rainersigwald,

possibly skipping them at the target level based on target inputs and outputs.

Would this explain why the following generates GeneratedFile.cs the first time, but never regenerates it, even after a Rebuild?

<Target ... Inputs="$(MSBuildAllProjects)" Outputs="$(IntermediateOutputPath)GeneratedFile.cs">

Does Rebuild force the up-to-date check to return false, but MSBuild decides the target still doesn't need to execute because GeneratedFile.cs was updated more recently than $(MSBuildAllProjects)?

Rebuild does Clean;Build. I think in that case, because the generated file isn't added to @(FileWrites), it's not getting deleted/cleaned up on Clean, so Rebuild doesn't reset its state.

Incremental build for targets is unconfigurable: it's always on.

A log at detailed or diagnostic level should show the target being skipped as up to date.