Bug in AxImp.exe

I ran into what seems to be a bug in AXIMP.EXE (and the corresponding process in Visual Studio .NET) recently. I tried using one of our older, MFC-based ActiveX controls in Visual Studio .NET toolbox. It barfed generating the interop DLLs, giving the error "Cannot apply indexing with [] to an expression of type <classname>. AxImp.exe generated the same error.

Looking at the generated source code for the wrapper control (which AxImp is kind enough to keep around for you) revealed the problem. The AxImp process generates two DLLs - an interop DLL for the types in the COM type library (the equivalent of doing a tlbimp.exe), and a control DLL which contains Windows Forms control wrappers. The control wrappers delegate their calls to a contained instance of the interop type. So if you have an ActiveX control called MyControl, two classes are generated - an interop class MyControl, and a wrapper Windows Forms control called AxMyControl. AxMyControl basically delegates most of its work to a contained instance of the interop class MyControl.

Well, in this case apparently the code that generates the interop dll and the code that generates the wrapper control disagree on how to interpret the type library. Our ActiveX control has a property on it called Item. The wrapper control was converting that call into an indexer (this[]) - presumably based on the name. But the interop dll doesn't - it leaves it has get_Item(). This seems to be because the Item property doesn't have a DISPID of DISPID_VALUE (don't ask why, it's a weird historical quirk). If I change the DISPID to be DISPID_VALUE, everything works.

I guess it's a pretty obscure case, but it's a pain.

I was able to hand edit the generated wrapper control source to get it to compile. Unfortunately, I then discovered the second bit of silliness - if you run AxImp by hand, it doesn't add the ToolboxItem attribute to the generated controls. It seems that you have to add it by hand, which isn't required when using Visual Studio .NET. That makes the standalone tool a lot less convenient to use.

1 Comment

  • I have used aximp.exe for activeX controls that I need to embed dynamically into winforms. No problems encountered.





    &quot;Item&quot; sounds like a good property for screwing automated processes up. It should probably be a protected name. So the converter was probably making some assumptions about it's existence there. (?)





    I'm sure you'll hear from one of the interop pros on this one.

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