reactive/Rx.NET/Source/src/System.Reactive/build/System.Reactive.targets

<Project>
  <PropertyGroup>
    <UseWindowsRxVersion Condition="'$(UseWpf)' == 'true' OR '$(UseWindowsForms)' == 'true'" >true</UseWindowsRxVersion>
    <UseWindowsRxVersion Condition="'$(UseWindowsRxVersion)' == '' " >false</UseWindowsRxVersion>
  </PropertyGroup>
  <ItemGroup>
    <Reference Condition="'$(UseWindowsRxVersion)' == 'true' "  Include="$(MSBuildThisFileDirectory)..\..\build\netcoreapp3.0\System.Reactive.dll" />
    <Reference Condition="'$(UseWindowsRxVersion)' != 'true' "  Include="$(MSBuildThisFileDirectory)..\..\lib\netstandard2.0\System.Reactive.dll" />

    <FrameworkReference Condition="'$(UseWindowsRxVersion)' == 'true' " Include="Microsoft.WindowsDesktop.App" />
  </ItemGroup>
</Project>