I have had the same problem sice yesterday, I was spending today to find out waht the problem is. Beside some solutions I hvae found (duplicate guid for assembly and class) but did not work for me, I found the solution for me: I am using a vb6 com-class library dll as a reference in my .NET project. I have changed that com class library dll by removing a function from one of the classes, because it is no more used/needed. After adding this function and recompiling the vb6 com library, registering again in .NET project, the problem was gone. It seams, that the type library exporter remebers some information somewhere, that even a clean did not swipe out. My be the old version of the type library was still hanging aaround somewhere in the registry. Afterwards I again removed the unnecessary function from the dll, unregistered to old one registered the new one and, even then the problem was gone. So seams to be a windows registry mystery. I hope it helps some folks, so they do not search and loose hours and hours like i did. Wolfiko