System.ObjectModel.csproj

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>$(NetCoreAppCurrent)</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <Compile Include="System.ObjectModel.cs" />
    <Compile Include="System.ObjectModel.Forwards.cs" />
  </ItemGroup>
  <ItemGroup>
    <ProjectReference Include="..\..\System.Runtime\ref\System.Runtime.csproj" />
    <ProjectReference Include="..\..\System.Collections\ref\System.Collections.csproj" />
  </ItemGroup>
</Project>