New Post: How to correctly dispose excel when using multiple threads?

From an Excel-DNA add-in, I suggest you never call back to Excel from another thread or Task. When you need to update Excel, just call the Excel-DNA helper ExcelAsyncUtil.QueueAsMacro with a delegate that does the Excel-related work. This delegate will then run on the main Excel thread, and you won't need any COM cleanup code anywhere.

-Govert