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>