[C#] Enable dual build for netstandard2.0 & netstandard2.1

Author: pierre-irrmann

csharp/sbe-dll/sbe-dll.csproj

@@ -1,6 +1,6 @@
 <Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
- <TargetFramework>netstandard2.0</TargetFramework>
+ <TargetFrameworks>netstandard2.0;netstandard2.1</TargetFrameworks>
  <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
  <AssemblyName>SBE</AssemblyName>
  <RootNamespace>Org.SbeTool.Sbe.Dll</RootNamespace>
@@ -38,9 +38,13 @@
  </None>
  </ItemGroup>
 
- <ItemGroup>
- <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0-preview.2" PrivateAssets="All" />
- <PackageReference Include="System.Memory" Version="4.5.3" />
- </ItemGroup>
+ <Choose>
+ <When Condition="'$(TargetFramework)' == 'netstandard2.0'">
+ <ItemGroup>
+ <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0-preview.2" PrivateAssets="All" />
+ <PackageReference Include="System.Memory" Version="4.5.3" />
+ </ItemGroup>
+ </When>
+ </Choose>
 
 </Project>