VBA UDF gives different answer when called from Excel vs VBA - excel

Question

The following VBA function counts the number of cells containing formulas in a given range. It works correctly when called from a VBA sub. When called from Excel, it returns the total number of cells in the range.
The call from Excel is =CountFormulas(A1:C7), which returns 21 even though only two cells with formulas are in the range.
What is causing this discrepancy?
Public Function CountFormulas(ByRef rng As Range) As Long
CountFormulas = rng.SpecialCells(xlCellTypeFormulas).Count
End Function
Public Sub CountFormulasFromSub()
Dim rng As Range
Dim res As Integer
Set rng = Sheet1.Range("a1:c7")
res = CountFormulas(rng)
End Sub

Answer 1

This isn't possible. The following link has the things that won't work inside of a UDF.
Here - http://support.microsoft.com/kb/170787
EDIT: A manual way of counting works though.
Public Function CountFormulas(rng As Range) As Integer
Dim i As Integer
Dim cell As Range
For Each cell In rng
If cell.HasFormula Then
i = i + 1
End If
Next
CountFormulas = i
End Function
Change Integer to Long if you think it will exceed 32767.

Answer 2

If I were to send worksheet.cells to the function, it would check all cells in the entire worksheet, quite many and quite slow. Although Excel 2007+ supports 16384*1048576 rows, only cells which have actually been used are loaded to memory. There would be no need to go through all the other 17 billion cells to check. The closest I could get to identifying these was using Worksheet.UsedRange to restrict an arbitrary range input. It is not perfect though, in cases when cells far apart have been used. E.g. if cells A1 and XFD1048576 contain data, the entire worksheet would be included in UsedRange. Any tips on how to restrict the range to actually used cells (merely two cells in the above example) would be greatly appreciated.
Utilizing UsedRange I built a function which I'll share in case anyone else can make use of it:
Public Function CountIfFormula(ByRef rng As Range, Optional ByVal matchStr As String) As Long
'Counts the number of cells containing a formula and optionally a specific string (matchStr) in the formula itself.
Dim i As Long
Dim isect As Range
'Restricts the range to used cells (checks only cells in memory)
Set isect = Application.Intersect(rng, rng.Parent.UsedRange)
For Each cell In isect
If cell.HasFormula Then
If InStr(1, cell.Formula, matchStr) Then i = i + 1
End If
Next
CountIfFormula = i
End Function
Use of the function:
Sub GetNrOfCells()
Dim i As Long
Dim ws As Worksheet
For Each ws In ThisWorkbook.Worksheets
i = i + CountIfFormula(ws.Cells, "=SUM(")
Next
'i will now contain the number of cells using the SUM function
End Sub
Best regards, and thank you for your reply.
Fossie