Core 3.1: Cannot run dotnet ef migrations in .DLL project (framework not found)

I ran into a problem after upgrading a project to Core 3.1.

I have a EF Core setup in a Class Library (DLL). When I run

dotnet ef migrations add Init --context TransferDbContext --verbose

I get the error:

Build started...
Build succeeded.
It was not possible to find any compatible framework version
The framework 'Microsoft.NETCore.App', version '3.1' was not found.
  - The following frameworks were found:
      3.1.7 at [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]

You can resolve the problem by installing the specified framework and/or SDK.

The specified framework can be found at:
  - https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=3.1&arch=x64&rid=win10-x64  

The call failing is:

dotnet exec 
    --depsfile C:\tfs\DigitalEksamenIntegration\DigitalEksamenIntegrationBusiness\bin\Debug\netcoreapp3.1\win-x64\DigitalEksamenIntegrationBusiness.deps.json 
    --additionalprobingpath C:\Users\dalby\.nuget\packages --additionalprobingpath "C:\Program Files\dotnet\sdk\NuGetFallbackFolder" 
    --runtimeconfig C:\tfs\DigitalEksamenIntegration\DigitalEksamenIntegrationBusiness\bin\Debug\netcoreapp3.1\win-x64\DigitalEksamenIntegrationBusiness.runtimeconfig.json 
    C:\Users\dalby\.dotnet\tools\.store\dotnet-ef\3.1.0\dotnet-ef\3.1.0\tools\netcoreapp3.1\any\tools\netcoreapp2.0\any\ef.dll migrations 
    add Init --context TransferDbContext 
    --assembly C:\tfs\DigitalEksamenIntegration\DigitalEksamenIntegrationBusiness\bin\Debug\netcoreapp3.1\win-x64\DigitalEksamenIntegrationBusiness.dll 
    --startup-assembly C:\tfs\DigitalEksamenIntegration\DigitalEksamenIntegrationBusiness\bin\Debug\netcoreapp3.1\win-x64\DigitalEksamenIntegrationBusiness.dll 
    --project-dir C:\tfs\DigitalEksamenIntegration\DigitalEksamenIntegrationBusiness\ 
    --language C# 
    --working-dir C:\tfs\DigitalEksamenIntegration\DigitalEksamenIntegrationBusiness 
    --verbose 
    --root-namespace DigitalEksamenIntegration.Business

My

dotnet --info

Lists following:

.NET Core SDK (reflecting any global.json):
 Version:   3.1.401
 Commit:    5b6f5e5005

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.18363
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\3.1.401\

Host (useful for support):
  Version: 3.1.7
  Commit:  fcfdef8d6b

.NET Core SDKs installed:
  3.1.401 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
  Microsoft.AspNetCore.All 2.1.21 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.7 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

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

and dotnet-core-uninstall list lists

This tool can not uninstall versions of the runtime or SDK that are 
    - SDKs installed using Visual Studio 2019 Update 3 or later.
    - SDKs and runtimes installed via zip/scripts.
    - Runtimes installed with SDKs (these should be removed by removing that SDK).
The versions that can be uninstalled with this tool are:

.NET Core SDKs:
  3.1.401  x64    [Used by Visual Studio. Specify individually or use --force to remove]

.NET Core Runtimes:
  3.1.7  x64
  2.2.7  x64  

I have been trying to setup a global.json but it did not seem to have an effect.

My cjsproj looks like this (reduced version):

        <Project Sdk="Microsoft.NET.Sdk">
          <PropertyGroup>
            <TargetFramework>netcoreapp3.1</TargetFramework>
            <RootNamespace>DigitalEksamenIntegration.Business</RootNamespace>
            <RuntimeFrameworkVersion>3.1</RuntimeFrameworkVersion>
            <RuntimeIdentifier>win-x64</RuntimeIdentifier>
          </PropertyGroup>

          <PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
            <PlatformTarget>AnyCPU</PlatformTarget>
          </PropertyGroup>


          <ItemGroup>
            <Reference Include="EntityFramework">
              <HintPath>..\DigitalEksamenIntegration.Web\Lib\EntityFramework.dll</HintPath>
            </Reference>
          </ItemGroup>
          <ItemGroup>
            <Folder Include="Migrations\" />
          </ItemGroup>
          <ItemGroup>
            <WCFMetadata Include="Connected Services" />
          </ItemGroup>
          <ItemGroup>
            <None Update="MailReport_Layout.template.txt">
              <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
              <ExcludeFromSingleFile>true</ExcludeFromSingleFile>
            </None>
            <None Update="MailReport_StudentsMissingInRegistrations.template.txt">
              <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
              <ExcludeFromSingleFile>true</ExcludeFromSingleFile>
            </None>
          </ItemGroup>

        </Project>

Some reports suggested that this was a tooling problem so I tried:

   dotnet tool uninstall -g dotnet-ef
   dotnet tool install -g dotnet-ef --version 3.1.7

But that did not work either.

Any help is appreciated.

Thanks in advance.

Kind regards Soeren

I have found a solution - or rather a cirumvention:

I edited the runtimeconfig.json which resides in the ouput directory. In my case: bin\Debug\netcoreapp3.1\win-x64\<ProjectName>.runtimeconfig.json (replace <ProjectName> with the name of your project).

The field version previously contained the version number 3.1. I edited it to contain the value 3.1.7 :

    {
      "runtimeOptions": {
        "tfm": "netcoreapp3.1",
        "framework": {
          "name": "Microsoft.NETCore.App",
          "version": "3.1.7"
        }
      }
    }

And I was finally able to migrate my database.