C# 编译器抛出语言版本 (LangVersion) 引用错误“无效的‘可空’值：‘Enable’ for C# 7.3”

Question

I have solution with couple .NET Standard projects in all I wanted to enable c# 8 and nullable like below:

<PropertyGroup>
    <TargetFramework>netstandard2.1</TargetFramework>
    <LangVersion>8.0</LangVersion>
    <Nullable>enable</Nullable>
  </PropertyGroup>

Note: These settings are found in your.csproj file.

The problem is that some projects are compiling fine and some have error:

Invalid 'nullable' value: 'Enable' for C# 7.3. Please use language version 'preview' or greater

I have Visual Studio 16.2 Preview 2 and .NET Core 3 Preview 6. Is this a bug in preview or I'm doing something wrong?

Answer 1

In my case, I ran into this problem with Visual Studio 2022 when I changed the target framework from .NET Standard 2.1 to .NET Standard 2.0. I solved my problem by removing <Nullable>enable</Nullable> in the.csproj file and restarting Visual Studio.

Original.csproj file:

<PropertyGroup>
  <TargetFramework>netstandard2.1</TargetFramework>
  <Nullable>enable</Nullable>
</PropertyGroup>

New.csproj file:

<PropertyGroup>
  <TargetFramework>netstandard2.0</TargetFramework>
</PropertyGroup>

Answer 2

您应该尝试<LangVersion>preview</LangVersion>因为错误消息提示。

Answer 3

I had error like this "Invalid 'nullable' value: 'Enable' for C# 7.3. Please use language version '8.0' or greater" and I was able to solve it by changing order of specified targeted frameworks.

<TargetFrameworks>net6.0;net48</TargetFrameworks>

To

<TargetFrameworks>net48;net6.0</TargetFrameworks>

Answer 4

There is another solution... add some conditions to your project file to only use the new nullable feature for the target frameworks that support it.

There is an excellent article here ... and all credit to the author for the import code snippets below:

<!-- Set the LangVersion = 8 -->
<PropertyGroup>
    <TargetFrameworks>netstandard2.0;netstandard2.1</TargetFrameworks>
    <LangVersion>8.0</LangVersion>
</PropertyGroup>

<!-- Only enable nullable feature for the supported frameworks -->
<PropertyGroup Condition=" '$(TargetFramework)' != 'netstandard2.0' ">
    <Nullable>enable</Nullable>
</PropertyGroup>

And then to hide warning messages in the unsupported frameworks:

<PropertyGroup Condition=" $(Nullable) != 'enable' ">
  <NoWarn>$(NoWarn);CS8632</NoWarn>
</PropertyGroup>

Answer 5

I had had the same issue when I have to downgrade compile-time supported version of C# language from 11.0 to 7.0. I resolved my issue for .net 7.0 project and was able to compile it within VS2022 with simple replacement from 'enable' to 'disable'.

See the final result:

<PropertyGroup>
   <TargetFramework>net7.0</TargetFramework>
   <ImplicitUsings>disable</ImplicitUsings>
   <Nullable>disable</Nullable>
   <LangVersion>7.0</LangVersion>
</PropertyGroup>