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MS-Windows Troubleshooting

Compiler error: User-defined type not defined (IUnknown)


When compiling a Visual Basic 6 application the following error is generated when VB encounters the type "IUnknown":

Compiler error:
User-defined type not defined


This error means simply that the VB compiler has encountered a type that it does not recognise. The compiler is assuming this to be a user-defined type because it is not a system defined one.

In the case of IUnknown this error indicates that the VB project is missing a reference to the OLE Automation library.


These notes apply specifically to the case where the unknown type is "IUnknown": add the following reference to the project:


substitute C:\Windows if using Windows 9x or XP.

These notes have been tested with Visual Basic 6 running under Windows NT and Windows 2000.

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