Wookbooks.open method works in editor but not from Excel - excel

Question

I am trying to develop a function in VBA that returns the result to the current worksheet. It is intended that the function opens up another spreadsheet, extracts some data, does some processing and returns a value to the worksheet that called the function.
Everything works well when I call the function from the "immediate" window in the VBA editor. However, when I transfer the call to a worksheet the function behavior deviates from the expected when an attempt is made to open the other workbook (AreaBook). The object, AreaBook, remains as a pointer to nothing.
I've tried hard coding the filename; again a call to the function works from the immediate window but not when called from a workbook.
Any ideas?
Public Function pointInWhichArea(FileName As String, SheetName As String, areaID As String, ByVal pointLong As Single, ByVal pointLat As Single) As Variant ', testPointLon As Single, testPointLat As Single) As Variant
Dim a, b, c As Integer
Dim colAreaID, colLat, colLon As Integer
Dim AreaBook As Workbook
Dim AreaSheet As Worksheet
Dim polygonPoints() As pointType
Dim testPoint As pointType
Dim found As Boolean
' extract the point details
testPoint.x = pointLong
testPoint.y = pointLat
' set the workbook and sheet objects
FileName = filePath + FileName ' open the Area definition file
Set AreaBook = Workbooks.Open(FileName) ' <<<< PROBLEM HERE
Set AreaSheet = AreaBook.Sheets(SheetName)
a = 1 ' identify the Polygon ID, latitude and longitude columns column
While AreaSheet.Cells(1, a).Value <> ""
Select Case Worksheets(SheetName).Cells(1, a).Value
Case Is = areaID
colAreaID = a
Case Is = "Latitude"
colLat = a
Case Is = "Longitude"
colLon = a
End Select
a = a + 1
Wend
a = 2 ' loop through all points in the area list
b = a ' remember the beginning of the polygon
found = False
While (AreaSheet.Cells(a, colAreaID).Value <> "" And found = False)
If AreaSheet.Cells(a, colAreaID).Value <> AreaSheet.Cells(a + 1, colAreaID).Value Then ' test for the end of this polygon
c = a ' remember the end of the polygon
ReDim polygonPoints(b To c) As pointType ' array to capture the poylgon
For a = b To c ' loop through each point
polygonPoints(a).x = AreaSheet.Cells(a, colLon).Value ' extract the longitude of the point
polygonPoints(a).y = AreaSheet.Cells(a, colLat).Value ' extract the latitude of the point
Next a
b = a ' remember the beginning of the next polygon
If pointInArea(testPoint, polygonPoints) = True Then ' test if the point is in the current polygon
pointInWhichArea = AreaSheet.Cells(a - 1, colAreaID).Value ' return the area label
found = True
End If
Else
a = a + 1
End If
Wend
AreaBook.Close
End Function

Answer 1

I'm afraid a worksheet function cannot be used to affect other cells or workbooks - you can't add a function to cell A1 and expect the result to appear in cell B2.
In the same way you can't add a function in cell A1 and expect it to open another workbook to get its answer.
That's why it works in the immediate window and not as an Excel function.
You may be able to define a link to the other workbook and then reference that, but you can't get the function to physically open the other workbook.

Answer 2

Worksheet Functions (and user defined variants of these) are limited in terms of what they can and cannot do, here is a small excerpt from a Microsoft article regarding the matter:
https://support.microsoft.com/en-us/kb/170787
A user-defined function called by a formula in a worksheet cell cannot
change the environment of Microsoft Excel. This means that such a
function cannot do any of the following:
Insert, delete, or format cells on the spreadsheet.
Change another cell's value.
Move, rename, delete, or add sheets to a workbook.
Change any of the environment options, such as calculation mode or screen views.
Add names to a workbook.
Set properties or execute most methods.
The page goes on to state that:
Any environmental changes should be made through the use of a Visual Basic subroutine.
So in short, you cannot do what you're attempting to do with a worksheet UDF (User-Defined Function) and will need to change it to a sub routine.