This one had me stumped for a while, especially as I couldn't find anything with the internal knowledge search tools. The symptoms are an Excel add-in that runs perfectly on it's own but crashes when run in the Visual Studio 2003 debugger. Actually, if you enable native debugging, it crashed Visual Studio too...

It turns out that by default Microsoft Office applications try to host the most recent version of the .NET runtime. That means you can't attach a .NET 1.1 debugger if you have .NET 2.0 installed side-by-side. The details are here but you basically have three options:

• Develop with Visual Studio 2005 and .NET 2.0
• Use "Start Without Debugging" from VS2003 and then attach a 2.0 compatible debugger
• Create a configuration file for the relevant office application and ensure there is only a <supportedRuntime> element for version="v1.1.4322"

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