Creating new C# version of existing COM object to use in Delphi program

Question

We had a bit of a problem where we have lost some source code for a very old ActiveX component we had. We've got a Delphi7 program that calls the Active X component. Thats a bit of a moster and can't be changed too much. However, some of the functionality in the Active X component needs to be updated. Since we've been moving our apps to C#, its been decided to redo the ActiveX as a C# class library.

I've got a basic C# class library working to prove the concept. I can hack the GUIDs and registry so it just looks liek the old one. If I look at the type lib for the old one and the new one, they are basically the same. Couple of slight differences but not anything i would have thought would be a problem.

Now as far as I can see there are three ways to use the new class library:

1. Late binding by looking up the ProgId. This works ok.

2. Import the type library. (Project| Import Type Library) This works ok.

3. Import as an ActiveX component. This generates a tlb file. This doesn't work. When I go to create the component I get 'No such interface exists'. The class is loading to a point but not completely.

Now unfortunately, the Delphi program is using option 3. So I have to try to get my class library working in this circumstance. Any ideas how I get past that error? I googled and found some references but nothing that says what the problem is.

Second question. I should know how to do this since I've done it once but I seemed to have fluked it. When I am in Delphi I do an Import ActiveX component to generate a TLB file. Now the first C# class library generated is showing up in the list of ActiveX controls and I can Create Unit for it. But I've created another simplier class library for testign purposes and damned if I can get the library to show in the list of ActiveX controls and so I can't import it.

The Create Unit for the Active X component generates a different format of TLB.pas to the Import Type Library format and its the ActiveX one that I require.

Answer 1

An ActiveX Control does way more then just implementing your interfaces. It is more like an TComponent or TWinControl. It requires you to implement a lot more ActiveX Control interfaces. Your C# object exposed through COM/ActiveX is not a Control, but a simple class.

It does not show up in Import ActiveX Control since it is not an ActiveX Control. I do not think it is feasible to make your C# class an Active Control.

Answer 2

Another possibility, although probably not the most elegant, would be to create a new ActiveX com object in Delphi which in turn calls your C# class by either method which you already have working.

Since you stated that this is for an existing Delphi 7 application, you might want to just use Com Interop on the C# side, expose the object as com visible, and the consume it directly from your Delphi 7 application.

The first option would probably allow you to get by with minimal changes to your existing application, but does add an additional layer that most likely is not necessary.