Testing C# 9.0 in VS2019 - CS0518 IsExternalInit is not defined or imported … How do I define/import it?

问题

EDIT [Nov 29 2020]: .NET 5.0 is out now, but the solution below is still required if you're targetting .NET Standard 2.1

C# 9.0 is still under development. There are a couple references which lead me to believe it should be testable now (some of it, anyway).

A Microsoft blog by Mr. Awesome himself, introducing the features. https://devblogs.microsoft.com/dotnet/welcome-to-c-9-0/
The language tracking page on github: https://github.com/dotnet/roslyn/blob/master/docs/Language%20Feature%20Status.md

I'm using VS 2019 16.7 Preview 3.1. I've selected the language version as Preview for a project.

Some C# 9 features, I can use. Like: Dictionary<string, object> Stuff = new()

But using the new init feature gives me this error: Error CS0518 Predefined type 'System.Runtime.CompilerServices.IsExternalInit' is not defined or imported

How do I fix this?

Examples of code causing the error:

class Test
{
   public int Hello { get; init; }
}

and

record Test(int hello);

The record definition is shorthand and expands into something that uses init, which is why it's also affected.

The language tracking page I linked to above says the feature was Merged into 16.7p3, which I am using.

Am I just being overly excited? Do I need to wait? Or is there a way to play with these features right now :D

EDIT (requested in comments) - Adding csproj for .net 5.0 console app:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net5.0</TargetFramework>
    <LangVersion>preview</LangVersion>
  </PropertyGroup>
</Project>

EDIT #2: A workaround posted here - https://github.com/dotnet/roslyn/issues/45510

回答1:

This is a bug in the current preview and the latest master branch (June 27). A simple record in sharplab.io creates the same error.

Just add the missing type somewhere in your project

namespace System.Runtime.CompilerServices
{
    public class IsExternalInit{}
}

Records and init will work without problem.

Only LinqPad 6 seems to work without problems, probably because it includes that type too

来源：https://stackoverflow.com/questions/62648189/testing-c-sharp-9-0-in-vs2019-cs0518-isexternalinit-is-not-defined-or-imported

标签

c#-9.0