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I have a WSDL generated class

 [System.Diagnostics.DebuggerStepThroughAttribute()]

    [System.CodeDom.Compiler.GeneratedCodeAttribute("System.ServiceModel", "4.0.0.0")]

    public partial class FVZServicesClient : System.ServiceModel.ClientBase<FVZScanningDocumentInfo.FVZServices.FVZServices>, FVZScanningDocumentInfo.FVZServices.FVZServices {

...

}

I need to provide a library that is used in VB6. I use a COM Interface which happens to use the above class.

When I register the assembly with

regasm MyAssembly.dll /tlb:MyAssembly.tlb

I get the error message

Microsoft .NET Framework Assembly Registration Utility version
4.7.3062.0 for Microsoft .NET Framework version 4.7.3062.0 Copyright (C) Microsoft Corporation. All rights reserved.

Types registered successfully Type library exporter warning processing
'MyAssembly.FVZServices.FVZServicesClient, MyAssembly'. Warning: Type
library exporter encountered a type that derives from a generic class
and is not marked as [ClassInterface(ClassInterfaceType.None)]. Class
interfaces cannot be exposed for such types. Consider marking the type
with [ClassInterface(ClassInterfaceType.None)] and exposing an
explicit interface as the default interface to COM using the
ComDefaultInterface attribute. Assembly exported to
'C:UsersMyNameDocumentsScanLibraryPacketMyAssembly.tlb', and the
type library was registered successfully

When I add the Attribute [ClassInterface(ClassInterfaceType.None)] as suggested to FVZServicesClient, I still get the same error message the next time I try the regasm. I know that the generated code can be overwritten at any time, but I verified with DotPeek that the ClassInterface-attribute was still in the compiled code.

Another thing I tried, was to create a partial class of FVZServicesClient that had the ClassInterface-attribute. I also marked the default interface with COMDefaultInterface as the message suggests. The result was the same. Adding the Attribute [ComVisible(false)] for the FVZServicesClient did not help either.

When the library is referenced in VB6, the compile results in an error: "User-defined type not defined".

How can I exclude FVZServicesClient from the COM interface?

Try adding the attribute [ComVisible(false)] to class FVZServicesClient.

I'm not sure how your code generation process works, but as noted in the question you can at least try adding this manually and seeing if it has the desired effect.

The error message was deceiving. In FVZServicesClient there is a method that expects a parameter to be passed by reference.

public ScanBatchItemType CheckBatch(ref ScanBatchItemType scanBatchItemTypes)

I did not pass it by ref. Once I corrected that, the error message disappeared. At the moment there is no COMVisibility or ClassInterface Attribute on FVZServicesClient and COM is Ok with that.