我应该为 .NET Core 中的库项目选择哪个目标框架

Question

My current mindset is to use .NET Standard xx for libraries and concrete frameworks for executable projects. So with this in mind let's say I have one common (fictional) project Common.csproj

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFrameworks>netstandard2.0;net5.0</TargetFrameworks>
  </PropertyGroup>

  <ItemGroup Condition="'$(TargetFramework)' == 'net5.0'">
    <PackageReference Include="FluentValidation" Version="9.5.4" />
  </ItemGroup>
  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
    <PackageReference Include="FluentValidation" Version="8.6.0" />
  </ItemGroup>

One library project Lib.csproj

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
        <TargetFramework>netstandard2.0</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
      <ProjectReference Include="Common.csproj" />
  </ItemGroup>

And one executable project Cli.csproj

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
        <TargetFramework>net5</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
      <ProjectReference Include="Lib.csproj" />
  </ItemGroup>

So, in this case if FluentValidation library has different method signatures in different versions, compilation in some cases goes thru, but you get error at runtime.

What are the recommendations for cases like this? Should I just support all needed target frameworks in Lib.csproj ?

Answer 1

I think that what you're after is using pre-processor directives with target frameworks:

#if NETSTANDARD
    // Code targeting .Net Standard
#elif NET
    // Code targetting .Net Core / .Net
#elif NETFRAMEWORK
    // Code targetting .Net Framework
#endif

You can get as specific as you like ie targeting .Net Standard 2.0 exactly:

#if NETSTANDARD2_0
   // .Net Standard 2.0 specific code
#elif
   // anything else!
#endif

This will let Common.csproj use the correct method calls for the appropriate version of FluentValidation that you need.

Personally, I find it a little messy - but sometimes, needs must.