Adding a VB6 reference in Visual Studio gives "Type library importer encountered an error during type verification"

Question

I am in the process of convering a rather large project written in VB6 into C#. Given the size of the project being moved, it is being done in phases over the course of 18-months. I am running into an issue with adding a reference of a VB6 ActiveX dll to a .Net project.

If you follow exactly these steps, you too should be able to recreate the problem.

I have written an interface in .Net that is COM visible:

<ComVisible(True)>
Public Interface ITestInterface
    Property A As String
    Function TestMethod() As String
End Interface

By selecting "Register for COM interop" in Compile tab of project properties, you get a TLB file.

I've created a VB6 project that references this TLB and a class that implements the interface exposed.

Implements ITestInterface

Private mA As String

Public Property Get ITestInterface_A() As String
    ITestInterface_A = mA
End Property

Public Property Let ITestInterface_A(ByVal value As String)
    mA = value
End Property

Public Function ITestInterface_TestMethod() As String
    ITestInterface_TestMethod = "From VB6"
End Function

If I set the Component tab of project properties in VB6 to use "Remote Server Files" then a TLB is automatically created when compiling. I can view that TLB in OleView and see the following (in addition to the details of the concrete implementation done in VB6 of the interface defined in the .Net project):

// typelib filename: TestVB6Interface.dll

[
  uuid(**EF005573-BFC7-436D-A382-F906CA09F94A**),
  version(3.0)
]

// ... some other stuff

// TLib :     // TLib :  : {79EC733A-0267-4506-8D38-C4D4655E0755}
importlib("SimpleDotNetLibrary.tlb");

Now, I create a completely new .Net project. If I add a reference to the VB6 dll, I get the following error:

Could not resolve COM reference "ef005573-bfc7-436d-a382-f906ca09f94a" version 3.0. The type library importer encountered an error during type verification. Try importing without class members.

However, if I launch a Visual Studio Command Prompt and run the following:

tlbimp TestVB6Interface.tlb /out:TestVB6Interface.MyInterop.dll

Then I can add that dll as a reference in my .Net solution and it works perfectly fine.

My question. What is tlbimp doing on the command line that is not being done when I just add the reference directly? When the message in Visual Studio says "try importing without class members" how exactly do I do that within Visual studio? I know how to do that in tlbimp.

I apologize for the wall of text, but I wanted to describe the situation as best I could keeping the information I felt was relevant.

Answer 1

The Visual Studio IDE definately takes a different path when registering DLLs for COM Interop then it does when running the command line tools from a command prompt.

I doubt that Microsoft has documented this anywhere. However, my years of experience have proven this to be the case. I once ran into a situation in which a "regsvcs" command from the .NET 2.0 Framework would actually cause an infinite loop. If you Google it you'll probably find others that have had this problem. I was able to make it one step further by using the VS IDE to perform the COM registration of a .NET Serviced Component. However, it inevitably ended in error. The error was a step forward over the infinite loop. Either way it proved to me that the VS IDE takes a different code path / business logic when dealing with COM Interop and registry entries.