C# compiler throws Language Version (LangVersion) reference error "Invalid 'nullable' value: '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?

Epidemic

You have <LangVersion>8.0</LangVersion>, not <LangVersion>preview</LangVersion>. See this page for how to specify preview, latest, etc... – Davies

With preview error message is the same – Epidemic

No repro. I use 8.0 too. Do you have a global.json in those failing projects that points to an earlier SDK? – Vial

For repro I have my code github - for examle this project is failing - github.com/bigdnf/HomeCenter/blob/Core3/Core/HomeCenter.Model/…. I don't recall any globals. – Epidemic

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>

Enigmatic

Didn't need to restart Visual Studio 17.5.3. – Suppress

Wow. Most confusing, yet accurate error message I've ever seen! – Hasdrubal

BTW if this is a new project you've just created which you'd already referenced from another project BEFORE you made this fix you may need to remove the reference and re-add it again. I had to do this to bypass some extremely annoying error messages about namespaces not being found. – Hasdrubal

Answer 2

You should try <LangVersion>preview</LangVersion> as the error message suggests.

Hollis

With preview error message is the same – Epidemic

preview is 8.0 at this point in time. – Vial

It is not recognized as 8.0 till after release. – Hollis

If you right click on your project, navigate to "Properties" >> "Build" >> "Advanced" >> "Language Version" you will notice that visual studio does not provide UI to change the value, You may need to Navigate to the csproj file (by right click on each of your project in visual studio and select "Unload Project", to open up the csproj file) of all your projects under your solution and add <LangVersion>preview</LangVersion>. This settings will help your latest C# compiler determine the default language version based on your projects targets framework. – Avowal

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>

Clearwing

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>

Spokesman

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 by simple replacement from 'enable' to 'disable'.

See the final project configuration:

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

Plague

Answer 6

To solve this,

1. In visual studio, right click on your project file, go to Properties
2. Build --> Events --> Advanced --> uncheck the Deterministic

Now build the project.

Hendeca

Answer 7

I was testing AvaloniaUI With version Visual studio 2022 and it must have changed it. Inside file "Directory.Build.props" was the property nullable enable

Just deleted the file and it works!

Weakminded

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