VBA UDF evaluates after every change - excel

Question

I have an issue, I thought would be a pretty simple one, but now can't handle it so I guess was wrong.
I have a UDF that calculates the average of exchange rates between 2 dates
Option Explicit
Public Function averageFromRange() As Double
Dim sh As Worksheet
Set sh = ThisWorkbook.Worksheets("Exchange Rates")
Dim dateStart As Date: dateStart = sh.range("G1").Value
Dim dateEnd As Date: dateEnd = sh.range("G2").Value
Dim myRange As String
Dim rangeStart As range
Dim rangeEnd As range
Set rangeStart = sh.range("A:A").Find(What:=CStr(dateStart), LookAt:=xlWhole, LookIn:=xlValues).Offset(0, 1)
Set rangeEnd = sh.range("A:A").Find(What:=CStr(dateEnd), LookAt:=xlWhole, LookIn:=xlValues).Offset(0, 1)
If rangeStart Is Nothing Then
MsgBox ("Date " & dateStart & " out of range")
End If
If rangeEnd Is Nothing Then
MsgBox ("Date " & dateEnd & " out of range")
End If
If Not (rangeStart Is Nothing Or rangeEnd Is Nothing) Then
myRange = rangeStart.Address & ":" & rangeEnd.Address
averageFromRange = Application.WorksheetFunction.Average(range(myRange))
End If
End Function
Any change in the entire workbook (apart from the sheet in which the function is called) re-evaluates the function to #VALUE!. I tried both parametrizing the UDF to have these dates as input params, and activating the sheet. I have no other ideas how to handle this issue. Could you help me out?

Answer 1

The Function returns #VALUE! when any of the dateStart or dateEnd is not found in column [A:A] because of these lines:
Set rangeStart = sh.range("A:A").Find(What:=CStr(dateStart), LookAt:=xlWhole, LookIn:=xlValues).Offset(0, 1)
Set rangeEnd = sh.range("A:A").Find(What:=CStr(dateEnd), LookAt:=xlWhole, LookIn:=xlValues).Offset(0, 1)
Those lines are trying to set the Offset(0, 1) of Nothing (i.e. Find returns Nothing and the lines are still trying to return the Offset)
Solution: First find the Cell containing the Dates then if the dates are found, set the Offset range.
Also you may want the UDF be Volatile if Column [A:A] or the Dates (start & end) are updated by formulas.
Try this code:
Public Function averageFromRange() As Double
Dim dDateIni As Date, dDateEnd As Date
Dim rINI As Range, rEND As Range
Application.Volatile 'Comment this line is VOLATILE is not required
With ThisWorkbook.Worksheets("Exchange Rates")
dDateIni = .Range("G1").Value
dDateEnd = .Range("G2").Value
With .Columns(1)
Set rINI = .Find(What:=CStr(dDateIni), LookAt:=xlWhole, LookIn:=xlValues)
Set rEND = .Find(What:=CStr(dDateEnd), LookAt:=xlWhole, LookIn:=xlValues)
End With
End With
If rINI Is Nothing Then MsgBox ("Date " & dDateIni & " out of range")
If rEND Is Nothing Then MsgBox ("Date " & dDateEnd & " out of range")
If Not (rINI Is Nothing And rEND Is Nothing) Then
averageFromRange = Application.Average(Range(rINI.Offset(0, 1), rEND.Offset(0, 1)))
End If
End Function
Resources used:
Worksheet.Range,
With statement