How to instantiate a ComVisible class in a .NET Core assembly from .NET Framework application?

Question

We have a software product that is currently released that is a .NET Framework 4.7.2 application (the "legacy" app). The legacy client-server implementation is built on System.Runtime.Remoting, which is not supported in .NET 5 and later, so the .NET 5 implementation is gRPC.

It is necessary to instantiate each of the two COM servers in turn because the legacy and the .NET 5 COM servers can only connect to the comm (not COM) server application that implements the same communications framework, which are System.Runtime.Remoting and gRPC, respectively.

The COM servers are used by third party applications to interface with the comm server application, so I am currently working on creating a static class that returns the interface from the COM server that can connect to the currently running instance of the comm server.

I have a .NET 5 WPF implementation of the product almost complete, but I've hit a roadblock in that, I am unable to register the .NET COM server.

I found these two articles: Exposing .NET Core Components to COM GitHub Issue

I have now been able to:

Create a Type Library

I found a comment from @SimonMourier suggesting copying the .NET 5 COM server code into a .NET Framework project and use RegAsm to export the type library to be used in the .NET 5 project. The type library was added to the .NET 5 COM server project folder and "" was added to an ItemGroup in the .csproj file per the first referenced article.

Register the .NET 5 COM server

This required using the "dotnet publish -r win-x64 -c Debug" command in the project folder from the Visual Studio Developer Command Line. I was then able to use regsvr32 to register the WinCalRemoting.comhost.dll in the "bin\Debug\net5.0\win-x64\publish" project directory.

Create an Instance of the COM Class

After registering the COM server, I am now able to create an instance of the COM class, but haven't been successful at getting the interface from it:

    public static IWinCalClient LoadCompatibleRemotingClient(bool useClientEventWindow, string serverName, int serverPort, bool connectToServer = true)
    {
        UseClientEventWindow = useClientEventWindow;
        WinCalServerName = serverName;
        WinCalServerPort = serverPort;
        ClassIdList = new Guid[]
        {
            LegacyWinCalClientClsId, // CAN'T GET INTERFACE FROM THIS COM SERVER
            //WinCal5ClientClsId // THE .NET 5 COM SERVER WORKS
        };

        if (RemotingClassObject != null)
        {
            UnloadClient();
        }

        foreach (Guid clsId in ClassIdList)
        {
            try
            {
                RemotingClassObject = Activator.CreateInstance(Type.GetTypeFromCLSID(clsId, true));
            }
            catch (Exception e)
            {
                continue;
            }

            if (RemotingClassObject != null)
            {
                RemotingInterface = (IWinCalClient)RemotingClassObject;

                if (RemotingInterface == null)
                {
                    UnloadClient();
                    continue;
                }

                if (CanClientConnect(_RemotingInterface, connectToServer))
                {
                    break;
                }
            }

            if (Marshal.IsComObject(RemotingClassObject))
            {
                Marshal.FinalReleaseComObject(RemotingClassObject);
            }

            RemotingClassObject = null;
        }

        return RemotingInterface;
    }

Update on the exception

After correcting the "bitness" of the test COM Client application that @SimonMourier clued me to, I am able to get the interface from the .NET 5 COM server. I have updated the code from the method.

HOWEVER, I'm now struggling with getting the interface from the .NET Framework COM server in the same way I get it from the .NET 5 COM server. I successfully register it using RegAsm.exe, but I get the following exception: System.InvalidCastException: 'Unable to cast object of type 'CMI.WinCalRemoting.cWinCalClient' to type 'CMI.WinCalRemoting.IWinCalClient'.'.

I've done an exhaustive search to try to find out how to fix the .NET Framework COM project so that it can be used in the same way that the .NET 5 COM server is used so that it doesn't matter whether the COM client is a .NET Framework or a .NET Core assembly.

I added a .NET Framework COM server project to the shared directory below to replicate what I'm seeing. With the .NET Framework COM server.

I also switched the test application to be 32-bit to replicate how our sister application will be using the COM servers.

All of the projects are located here: .NET 5 COM Interop

Unable to Add .NET Framework COM Type Library Reference

For a .NET Framework client assembly, I've attempted to add a reference to the .NET Framework COM server that was registered with regasm.exe, but that fails with the following message: