Add user defined function to Visual Studio Excel Add-in

Question

In visual studio I have an Excel 2010 Add-in project. How can I have that project create the following module:

Answer 1

I dont think that VSTO supports Excel UDF's, the general recommendation is to use Automation Add-in's (as Sid's link suggests).

Another option is to call a managed VSTO function from VBA. Once again this is not recommended but possible.

(Recap of tutorial from link) Here is any easy way to call Managed functions from VBA.

Create a class with your functions in VSTO

 _
Public Class MyManagedFunctions
    Public Function GetNumber() As Integer
        Return 42
    End Function
End Class

Wire up your class to VBA in VSTO

Private Sub ThisWorkbook_Open() Handles Me.Open
    Me.Application.Run("RegisterCallback", New MyManagedFunctions)
End Sub

Create Hook for managed code and a wrapper for the functions in VBA

In a VBA module in your spreadsheet or document

Dim managedObject As Object

Public Sub RegisterCallback(callback As Object)
    Set managedObject = callback
End Sub

Public Function GetNumberFromVSTO() As Integer
    GetNumberFromVSTO = managedObject.GetNumber()
End Function

Now you can enter =GetNumberFromVSTO() in a cell, when excel starts the cell value should be 42.

http://blogs.msdn.com/b/pstubbs/archive/2004/12/31/344964.aspx