question

they are correct, core 2.0-3.0 sdk is not supported (3.1 ends in December, net 5 in May). but .netstandard 2.0 is its own framework and still supported. you can build .netstandard 2.0 with classic 4.8 or .net 6.0 runtimes.

dotnet new classlib -f netstandard2.0

That command is to create a netstandard project.

Is the command to build dotnet build -f netstandard2.0 ?

no. it creates a project file with the framework set to netstandard 2.0

 <Project Sdk="Microsoft.NET.Sdk"> 
     
   <PropertyGroup> 
     <TargetFramework>netstandard2.0</TargetFramework> 
   </PropertyGroup> 
     
 </Project> 

so just:

dotnet build

Our servers have already .NET Core 3.1 SDK . The project also has the setting that you shown above. But the command dotnet build fails!

We have nuget package manager which fails with the following error message. This build failure happened after .NET 2.0 SDK was uninstalled. So, we assume the corresponding dotnet SDK is unavailable.

Is it possible that the issue is with the nuget package manager?

is the error from building a .netstandard 2.0 library project, or from an application build using the library. A .netstandard library should not have a reference to core sdk.

If it’s a library, then probably only worked with core projects. Just change from standard to a valid .net core frame, and update the nuget package to compatible versions.

Also, why 3.1? It’s support ends in December.

There are two projects (1) .NET Core 3.1 (2) .netstanadard2.0 in the same solution. The error is happening in the .netstandard project when the build tries to build, pack and publish to nuget. It does not have a reference to .net core 3.1.

The nuget pack is the command run to build and pack the assemblies. When that command is run, nuget may be trying to build the project by looking at the targetframework in the csproj file. It can't find the version of msbuild or dotnet for netstandard2.0 framework and may be erroring out.

1. Can a netstandard2.0 assembly be compiled with .net core 3.1 SDK by using the command dotnet build?
   

2. Are we using an outdated nuget tools which can't understand the netstandard2.0 framework?

We have a plan to migrate the apps to .NET 6 by the end of this year.

.net standard 2.0 and the final version 2.1 are supported by .net 3.1+

But nuget pack does not work with modern msbuild files, only the old 4.* format. You should use the dotnet pack or msbuild -t:pack commands instead.