Excel VBA UDF Executes in Immediate Window, Fails on Worksheet - vba

Question

UDF "NAV()" is designed to find the correct report on a network drive based on the first argument (always a date), then loop through all worksheets to find a piece of data with the same row as second argument and same column as third argument (second and third can be text or numbers).
Works reliably in the immediate window. Always returns #VALUE! when used on worksheet, e.g. =NAV(D7,D8,D9) or =NAV(2/19/2016,"Net Asset Value","221-I").
In general it looks like one could get this behaviour if trying to alter other cells in a UDF, but my functions don't do that. Also, I believe all range references specify which workbook and worksheet, so I don't think that is the problem either. I'm not sure where to look next.
Function also attempts to email me a report through Outlook when it fails to find what the user is looking for. I don't know if that is relevant.
Again, what is perplexing is that this code seems to work fine in the immediate window, but only gives #VALUE! when used on a worksheet.
Where else can I look in my code below to determine what would cause NAV() to function correctly in the immediate window, but always yield #VALUE! when used on a worksheet?
Option Explicit
Function NAV(ByVal NAVDate As Date, ByVal matchRow As Variant, ByVal matchColumn As Variant) As Variant
'Application.ScreenUpdating = False
Application.Volatile True
NAV = FindItemOnWorksheet(NAVDate, matchRow, matchColumn)
'Application.ScreenUpdating = True
End Function
Function FindItemOnWorksheet(ByVal NAVDate As Date, ByVal ItemSpecies As Variant, ByVal ItemGenus As Variant) As Variant
' Finds Item by opening NAV workbook with correct date, activating correct worksheet, and searching for correct row and column
Dim startingRange As Range
Dim ws As Worksheet
Dim wb As Workbook
Dim theDate As Date
Dim theItemSpecies As String
Dim theItemGenus As String
theDate = NAVDate
theItemSpecies = ItemSpecies
theItemGenus = ItemGenus
Set wb = GetWB(NAVDate)
'Loop through ws
Dim WS_Count As Integer
Dim i As Integer
WS_Count = wb.Worksheets.Count
For i = 1 To WS_Count
Set ws = wb.Worksheets(i)
Set startingRange = ws.Range("A1:Z100")
Dim theRow As Range
Dim theColumn As Range
Set theRow = startingRange.Cells.Find(theItemSpecies, SearchDirection:=xlPrevious, lookat:=xlWhole)
If Not (theRow Is Nothing) Then
Set theColumn = startingRange.Cells.Find(theItemGenus, SearchDirection:=xlPrevious, lookat:=xlWhole)
If Not (theColumn Is Nothing) Then
FindItemOnWorksheet = ws.Cells(theRow.Row, theColumn.Column).Value
wb.Close
Exit Function
End If
End If
Next i
'Loop if no hit on either row or column Find()
'following executes only if no match found
MsgBox "No Match Found. Make sure you are entering arguments--" & vbNewLine & _
" The Date of NAV, " & vbNewLine & _
" the entry found in the right row of NAV workbooks (e.g. 'Net Asset Value'), " & vbNewLine & _
" the right column (e.g. 'Fund')." & vbNewLine & _
" This function will only find exact matches." & vbNewLine & vbNewLine & _
"Now emailing developer to ask for a fix."
Dim OutApp As Object
Dim OutMail As Object
Dim strbody As String
Set OutApp = CreateObject("Outlook.Application")
Set OutMail = OutApp.CreateItem(0)
strbody = "User attempted" & _
"=FindItemOnWorksheet( " & theDate & ", " & theItemSpecies & ", " & theItemGenus & " )" & vbNewLine & _
"theDate type " & TypeName(theDate) & vbNewLine & _
"theItemSpecies type " & TypeName(theItemSpecies) & vbNewLine & _
"theItemGenus type " & TypeName(theItemGenus)
On Error Resume Next
With OutMail
.To = <Address Removed>
.CC = ""
.BCC = ""
.Subject = "FindItemOnWorksheet Error"
.Body = strbody
'.Attachments.Add ("C:\file.xlsx")
.Send
End With
On Error GoTo 0
Set OutMail = Nothing
Set OutApp = Nothing
FindItemOnWorksheet = "Error"
'wb.Close
Exit Function
End Function
Function GetWB(ByVal NAVDate As Date) As Workbook
'Open requested workbook, return to parent procedure
Dim wbPath As String
Dim wbYear As String
Dim wbMonth As String
Dim wbWeek As String
Dim wbFile As String
Dim wbString As String
Dim wb As Workbook
Dim BackADay As Boolean
Dim OriginalNAVDateRequested As Date
OriginalNAVDateRequested = NAVDate
BackADay = True
'Loop through possible file tree structures and dates to find the closest NAV in the past to the date requested.
Do While BackADay = True
'Don't go back to a previous week if cannot find current NAV
If OriginalNAVDateRequested - NAVDate > 4 Then
BackADay = False
End If
wbPath = <Network Path Removed>
wbYear = CStr(Year(NAVDate)) & "\"
wbMonth = MonthName(Month(NAVDate)) & " " & wbYear
wbWeek = DateFormat(NAVDate) & "\"
wbFile = Dir(wbPath & wbYear & wbMonth & wbWeek & "*Valuation Package*.xlsx")
'Pricings with distributions have differing tree structure
If wbFile = "" Then
wbWeek = wbWeek & "POST Distribution " & wbWeek
wbFile = Dir(wbPath & wbYear & wbMonth & wbWeek & "*Valuation Package*.xlsx")
If wbFile = "" Then
NAVDate = NAVDate - 1
Else: BackADay = False
End If
Else: BackADay = False
End If
Loop
wbString = wbPath & wbYear & wbMonth & wbWeek & wbFile
Set wb = Workbooks.Open(wbString, UpdateLinks:=False, ReadOnly:=True)
Set GetWB = wb
End Function
Function DateFormat(ByVal X As Date) As String
'Appends leading zeroes if needed to achieve form "00" for any two digit integer, and converts to string
Dim MM As String
Dim DD As String
Dim YYYY As String
If Month(X) < 10 Then
MM = "0" & CStr(Month(X))
Else
MM = CStr(Month(X))
End If
If Day(X) < 10 Then
DD = "0" & CStr(Day(X))
Else
DD = CStr(Day(X))
End If
YYYY = CStr(Year(X))
DateFormat = MM & "." & DD & "." & YYYY
End Function

Answer 1

You can Open Workbooks within a Worksheet_Change Event.
For demonstration, if a change in Sheet1!A2, Excel will try open the workbook name with that cell value, then Output the status to Sheet1!A4.
Put below in a Module:
Option Explicit
Function TryOpenWB(ByVal oItem As Variant) As Variant
Dim sOut As String
Dim oWB As Workbook
On Error Resume Next
Set oWB = Workbooks.Open(CStr(oItem))
If oWB Is Nothing Then
sOut = "Cannot open """ & CStr(oItem) & """"
Else
sOut = "Opened """ & CStr(oItem) & """ successfully."
'oWB.Close
End If
TryOpenWB = sOut
End Function
Then below in Worksheet Module (I used Sheet1 for demonstration):
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = Range("A2").Address Then
Application.EnableEvents = False
Range("A4").Value = TryOpenWB(Target)
Application.EnableEvents = True
End If
End Sub
So this idea is to open the Workbook only if some cell address is matched.