targets.props

<Project>
  <Choose>
    <When  Condition="'$(VisualStudioVersion)' == '16.0'">
      <PropertyGroup>
        <AppTargetFramework>netcoreapp3.1</AppTargetFramework>
        <TestsTargetFrameworks>net462;netcoreapp2.1;netcoreapp3.1</TestsTargetFrameworks>
        <LibTargetFrameworks>net462;netstandard2.0;netstandard2.1</LibTargetFrameworks>
        <LibxTargetFrameworks>net462;netcoreapp2.0;netstandard2.1</LibxTargetFrameworks>
      </PropertyGroup>
    </When>
    <Otherwise>
      <PropertyGroup>
        <AppTargetFramework>netcoreapp2.1</AppTargetFramework>
        <TestsTargetFrameworks>netcoreapp2.1</TestsTargetFrameworks>
        <LibTargetFrameworks>net462;netstandard2.0</LibTargetFrameworks>
        <LibxTargetFrameworks>net462;netcoreapp2.0</LibxTargetFrameworks>
      </PropertyGroup>
    </Otherwise>
  </Choose>
</Project>