Warning - ambiguous method call in VSTO

Activating a worksheet is a fairly common task in Office automation; however, in a VSTO project, if you call:


… building the project will give you the following warning:

Ambiguity between method ‘Microsoft.Office.Interop.Excel._Worksheet.Activate()’ and non-method ‘Microsoft.Office.Interop.Excel.DocEvents_Event.Activate’. Using method group.

I don’t like to have warnings in my projects when I can avoid it, but I never got to look into it. After all, it was “just a warning”, so I let it go.

The answer came to me via the Carter & Lippert VSTO book (aka “The Brick”), which I finally started reading through, and highly recommend. The gist of it is that the Worksheet interface implements 2 interfaces, _Worksheet and DocEvents_Event. _Worksheet contains the properties and methods that correspond to the Worksheet, including the Activate() method, while DocEvents_Event owns the events, including Activate, and these two names collide.

To disambiguate the call, you just need to cast the Workbook to the appropriate interface, the one which owns the method you are interested in. In my case, I want to Activate the workbook, and therefore use the following code:


And sure enough, the warning is gone.

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