With .NET 6 Preview6, build netstandard2.0 classlib failed with error: CSC : error CS8630: Invalid 'nullable' value: 'Enable' for C# 7.3. Please use language version '8.0' or greater.

[NicoleWang001]

Repro steps:

1. Install .NET 6 Preview6 SDK
2. Create a new netstandard2.0 classlib and build it

Expected Result:
Classlib could be built successfully

Actual Result:
build netstandard2.0 classlib failed with error:

Microsoft (R) Build Engine version 17.0.0-preview-21304-01+a5ea6d2ca for .NET
Copyright (C) Microsoft Corporation. All rights reserved.
 
  Determining projects to restore...
  All projects are up-to-date for restore.
  You are using a preview version of .NET. See: https://aka.ms/dotnet-core-preview
CSC : error CS8630: Invalid 'nullable' value: 'Enable' for C# 7.3. Please use language version '8.0' or greater. [/home/nicole/myclass1/myclass1.csproj]
 
Build FAILED.
 
CSC : error CS8630: Invalid 'nullable' value: 'Enable' for C# 7.3. Please use language version '8.0' or greater. [/home/nicole/myclass1/myclass1.csproj]
    0 Warning(s)
    1 Error(s)
 
Time Elapsed 00:00:00.83

Note: It not repro with .NET 6 Preview5

Dotnet info:
.NET SDK (reflecting any global.json):
Version: 6.0.100-preview.6.21315.23
Commit: 8f09936962

Runtime Environment:
OS Name: Windows
OS Version: 10.0.19043
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\6.0.100-preview.6.21315.23\

Host (useful for support):
Version: 6.0.0-preview.6.21306.1
Commit: 5b8e1780ad

.NET SDKs installed:
6.0.100-preview.5.21302.13 [C:\Program Files\dotnet\sdk]
6.0.100-preview.6.21315.23 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 6.0.0-preview.5.21301.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 6.0.0-preview.6.21306.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.0-preview.5.21301.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.0-preview.6.21306.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 6.0.0-preview.5.21301.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.0-preview.6.21305.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET runtimes or SDKs:
https://aka.ms/dotnet-download

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[NicoleWang001]

Adding binary log via dotnet build myclass.csproj /bl:out.binlog
out.binilog.zip

[jaredpar]

The project file has the following structure

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

The default language version for “netstandard2.0” is 7.3 and that is not compatible with enable. Either the element needs to be removed or needs to be explicitly set to 8.0 or higher.

Closing as the behavior for the SDK + Compiler is as expected given this project file structure.

[richaverma1]

@NicoleWang001, @jaredpar, this is a template issue then. @dsplaisted who owns the netstandard classlib template?

[dsplaisted]

@jmarolf It looks like #3173 broke the class library template when created for .NET Standard 2.0. Probably it also broke other target frameworks where the default language version doesn't support nullable.

[jmarolf]

The tests don't crate netstandard version of the templates? I'll submit a fix here

[vdanche]

it has been fixed on 6.0.100-preview.6.21323.12(runtime-6.0.0-preview.6.21323.11)

[Amy-Li03]

We also encounter this issue on .NET SDK 6.0.100 - rc.2.21505.57 which inserted into 17.0 Preview 5, when multi- target Frameworks, the error message pops up after build WinForms project.
<TargetFrameworks>net472;net6.0-windows</TargetFrameworks>

[vlada-shubina]

@Amy-Li03 have you followed this guidance when creating the multi-target framework project? dotnet/docs#25383

thanks

[Amy-Li03]

@vlada-shubina We just want to use .NET framework designer in .NET core project, firstly create a WinForms project targeting .NET 6.0 by default, then in XXX.csproj file change TargetFramework from <TargetFramework>net6.0-windows</TargetFramework> to <TargetFrameworks>net48;net6.0-windows</TargetFrameworks>,
after build this project, will pop up above error message, please correct us if we have done something wrong.

[vlada-shubina]

@Amy-Li03 please double check wtih dotnet/winforms for this case, I'm not sure, if it's supported.