Embedded Interop REGDB_E_CLASSNOTREG error on client machine

I've compiled my app as x86. I'm using a VB6 COM Interop in my app. I've set the reference to "Embed Interop type=Yes", "Isolated=No".

I don't need to install anything on the client machine, not even the "Interop.MyDLL.dll", it magically worked without installing anything.

I can test this easily on my test computer, it works just fine.

Now, on a client machine the error "System.TypeInitializationException, class not registered, HRESULT: 0x80040154 (REGDB_E_CLASSNOTREG)" is thrown when I try to access the DLL.

It occured after I recompiled the app and deployed it. Before that, it worked fine.

What might be the culprit here?

There's not "magic" with COM, you must register the VB6 COM component (dll, ocx, etc.).

The interop .NET dll is not a COM component, it just contains metadata/binding from .NET to COM, so it doesn't need to be registered.

PS: in fact, you don't have to register, you just need to give clients a way to find you, so there's also the possibility to use Registration-Free Activation of COM Components: A Walkthrough, but it can be more work than simply register a bunch of dlls.