docs/code-samples/serving/hello-world/helloworld-csharp/helloworld-csharp.csproj

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

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <RootNamespace>helloworld_csharp</RootNamespace>
  </PropertyGroup>

</Project>