For .NET Standard 2.0 library, why add .NET Core 3.1 target to enable NRT?

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For .NET Standard 2.0 library, why add .NET Core 3.1 target to enable NRT?

Vernou 86

I try to find the good way to enable NRT (nullable reference type) in my .NET Standard library. I found nothing about it in Library guidance.

But I see this in this blog post Embracing nullable reference types :

What should library authors do?

As a library writer you always face a dilemma between reach of your library and the feature set you can depend on in the runtime. In some cases you may feel compelled to split your library in two so that one version can target e.g. the classic .NET Framework, while a “modern” version makes use of e.g. new types and features in .NET Core 3.1.

However, with Nullable Reference Types specifically, you should be able to work around this. If you multitarget your library (e.g. in Visual Studio) to .NET Standard 2.0 and .NET Core 3.1, you will get the reach of .NET Standard 2.0 while benefitting from the nullable annotations of the .NET Core 3.1 libraries.

You also have to set the language version to C# 8.0, of course, and that is not a supported scenario when one of the target versions is below .NET Core 3.0. However, you can still do it manually in your project settings, and unlike many C# 8.0 features, the NRT feature specifically happens to not depend on specific elements of .NET Core 3.1. But if you try to use other language features of C# 8.0 while targeting .NET Standard 2.0, all bets are off!

I make some test and enable NRT with :

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
    <LangVersion>8.0</LangVersion>
    <Nullable>enable</Nullable>
  </PropertyGroup>
</Project>

.NET Framework project that reference this library haven't nullable reference type error, by .NET Core project that reference that library have nullable reference type error. That is the expected behavior.

In this case, why to add .NET Core 3.1 as target framework?

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Answer 1

Bruce (SqlWork.com) 77,686 Volunteer Moderator

here is a more complete post:

https://www.meziantou.net/how-to-use-nullable-reference-types-in-dotnet-standard-2-0-and-dotnet-.htm

note: a request add nullable annotations to .netstandard 2.0 was closed as no changes to netstandard are planned:

https://github.com/dotnet/standard/issues/1118

Vernou 86 Reputation points

2023-09-19T12:27:23.3633333+00:00
Thank, the link explain perfectly the advantage to add .NET Core 3 as target. I recommend reading the entire article, but there is a short summary.

.NET Standard 2.0 haven't nullable annotations. Then this safe code provides a warning :

void Foo(string? str) { if(!string.IsNullOrEmpty(str)) { _ = str.LEngth //Warning : deference of a possibly null reference. } }

.NET Core 3 have nullable annotations like in String.cs :

public static bool IsNullOrEmpty([NotNullWhen(false)] string? value) { return value == null || value.Length == 0; }

So the compiler that the value is not null when the result is false.

In resume, this help the author library, but it isn't mandatory.

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For .NET Standard 2.0 library, why add .NET Core 3.1 target to enable NRT?

What should library authors do?

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