I downloaded the latest version of .Net Core 3.0 and 3.1 and had the same issue. For me, the fix seemed to download the latest update for Visual Studio 2019 (to version 16.4.2).

This also restarted my computer and the error went away.

I received the same error, but I had simply forgotten to include the

<LangVersion>8.0</LangVersion>

attribute in ALL the .csproj files in the solution. The following is my current c# 8 setup:

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
    <LangVersion>8.0</LangVersion>
    <Nullable>enable</Nullable>
    <NullableContextOptions>enable</NullableContextOptions>
  </PropertyGroup>

I found the following documents to be the most helpful when migrating from core 2.2 to 3.x:

MSDN 2.2 -> 3.0

MSDN 3.0 -> 3.1

I had to update Visual Studio to version from 16.3.X to 16.4.2. This resolved the problem and I didn't have to add any LangVersion.

Credits: https://github.com/aspnet/AspNetCore.Docs/issues/16047

I did the following and it solved my issue:

1. create a file named "Directory.Build.props" in the solution directory and writing this code:

   <Project>
    <PropertyGroup>
     <LangVersion>latest</LangVersion>
    </PropertyGroup>
   </Project>
   

2. Deleted the .vs folder (it is hidden in the solution directory)

3. Restart the Visual Studio

2021

Cause: You may be targeting .NET Standard 2.0, which uses C# 7.3.

Fix: Under the project's Properties, click on the Application panel and choose .NET Standard 2.1 as the Target framework.

After the above change, Visual Studio 2019 will fix the issue by itself, and no LangVersion setting will be necessary.

See: C# language versioning

You must forget this tag in one of your PropertyGroup 's in your *.csproj file

<LangVersion>8.0</LangVersion>

If you install resharper, it will automatically modify the csproj files for you ;)

Check that you have valid configuration on both machines (Debug/Release, x64/Any CPU). This could also result to this error.