如何为C＃类库多目标以同时支持NET_STANDARD和NET_COREAPP？

Question

I have a class library which is currently multi-targeting NET40 and NETSTANDARD2.0 :

<TargetFrameworks>net40;netstandard2.0</TargetFrameworks>

However, I now need to also support some new api which was added as part of NETCOREAPP2.1 which is not covered by netstandard .

My initial thought is to simply expand the current frameworks to include NETCOREAPP2.1 :

<TargetFrameworks>net40;netstandard2.0;netcoreapp2.1</TargetFrameworks>

<PropertyGroup Condition=" '$(TargetFramework)' == 'netcoreapp2.1'">
    <DefineConstants>NETCORE</DefineConstants> 
</PropertyGroup>

and in the code I can use the newly added API like so:

#if NETCORE
    // Use the api added in NETCORE 2.1
#endif

However what will happen once the library is used in the app targeting later versions of .NET Core ? eg .NET Core 2.2 ? Do I have to create new constants for every newly released version?

In an ideal world NETCOREAPP2.1 would implement NETSTANDARD2.1 but unfortunately THIS is not the case.

Answer 1

just create a .net standard 2.0 library. this lib can be used by .net (>= 4.6.1) or core (>= 2.0) applications. the downwards compatibility of an lib is guaranteed.

https://docs.microsoft.com/en-us/dotnet/standard/net-standard#net-platforms-support

Answer 2

You can use .Net string methods inside your condition:

<PropertyGroup Condition="$(TargetFramework.StartsWith('netcore'))">
    <DefineConstants>NETCORE</DefineConstants> 
</PropertyGroup>