Improve compilation symbols when multi-targeting

[yaakov-h]

The SDK defines a bunch of compilation symbols when multi-targeting.

For example, when targeting .NET Standard 1.3, developers can use #if NETSTANDARD1_3 to conditionally use APIs that are available with that target framework.

However, when multi-targeting across different versions, developers have to use multiple of these. For example:

#if NETSTANDARD1_3 || NETSTANDARD1_6 || NETSTANDARD2_0

It would be nice to have more inclusive compilation symbols, so that a developer could, for example, use:

#if NETSTANDARD1_3_OR_GREATER

This would be defined when targeting .NET Standard 1.3, 1.4, 1.5, 1.6. 2.0, 2.1, etc., but not when targeting .NET Standard 1.0, 1.1, 1.2, or .NET Framework, Xamarin, .NET Core etc.

Doing so would decrease the friction when introducing a newer platform (e.g. .NET Standard 3.0 one day) to a multi-targeting library that aims to keep backwards compatibility for older runtimes, but also taking advantage of newer APIs where they are available.

Inspired by dotnet/csharplang#1182

[dasMulli]

Also saw this on SO once: https://stackoverflow.com/questions/44208788/conditionally-compiling-for-a-minimum-version-of-net/44209523#44209523

At the moment this needs to be done through a custom target like

<Target Name="AddNetStdMinDefine" 
        BeforeTargets="CoreCompile"
        Condition="('$(TargetFrameworkIdentifier)' == '.NETStandard' and '$(TargetFrameworkVersion.Substring(1))' &gt;= '1.4')">
   <PropertyGroup>
     <DefineConstants>$(DefineConstants);IS_MIN_NETSTANDARD1_4</DefineConstants>
   </PropertyGroup>
</Target>

[github-actions]

Due to lack of recent activity, this issue has been labeled as 'stale'. It will be closed if no further activity occurs within 30 more days. Any new comment will remove the label.

[yaakov-h]

This was added a few years ago:

https://learn.microsoft.com/en-us/dotnet/standard/frameworks#preprocessor-symbols