For managed Excel addins that contain UDF worksheet functions, ExcelDNA ( http://exceldna.codeplex.com) is the best tool. You can still you the NetOffice libraries to give you versionindependent
access to the Excel COM object model. But ExcelDNA integrates with Excel through the C API, which gives the best performance and flexibility for UDFs  including customizing the Function Arguments dialog.
With ExcelDNA you could write your function as:
Imports ExcelDna.Integration
Public Module MyFunctions
<ExcelFunction(Description:="My number adding function", Category:="My useful functions")> _
Public Function AddThem(x As Double, y As Double) As Double
AddThem = x + y
End Function
End Module
Govert
