NetStandardのバージョン判定

ClassLibrary1.csproj

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

  <PropertyGroup>
    <TargetFrameworks>netstandard1.6;netstandard2.0;netstandard2.1</TargetFrameworks>
    <DefineConstants Condition="$(TargetFramework.Contains('netstandard')) and $(TargetFramework.Length) > 12 and 
                     $([System.Version]::Parse('$(TargetFramework.Replace('netstandard',''))').CompareTo($([System.Version]::Parse('2.1')))) >= 0">
      NETSTANDARD2_1_OR_GREATER
    </DefineConstants>
  </PropertyGroup>

</Project>