Tables interfere with VBA range variables depending on scope

Question

An Excel file includes VBA-coded user-defined functions (UDFs) that are deployed in tables (VBA listobjects). Now, for reasons that escape me, if the UDF module contains

Answer 1

Declaring module-level variables simply to save the two lines in each UDF that would otherwise be required is indeed bad coding practice. However, if that is your thinking, why not go all the way and save four lines per UDF by avoiding setting them in each as well!

You can do this by using pseudo-constant functions as seen in the following code:

Option Explicit

Private Function rng_ItemNumber() As Range
    Set rng_ItemNumber = Sheet1.Range("A4:A6")
End Function
Private Function rng_ItemName() As Range
    Set rng_ItemName = Sheet1.Range("B4:B6")
End Function

Public Function ITEM_NAME(varItemNumber As Variant) As String
' Returns Item Name as a function of Item Number.
  With Application.WorksheetFunction
    ITEM_NAME = .Index(rng_ItemName, .Match(varItemNumber, rng_ItemNumber))
  End With
End Function

The cost, of course, is the overhead of a function call.

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If you are planning on using the ListObject class for the final design, then why not use it now, and also use dynamic named ranges (the hard-coded ranges in the example are there so it actually works as is - these should be replaced with the named ranges):

Option Explicit

Private Function str_Table1() As String
    Static sstrTable1 As String
    If sstrTable1 = vbNullString Then
      sstrTable1 = Sheet1.Range("A4:B6").ListObject.Name
    End If
    str_Table1 = sstrTable1
End Function
Private Function str_ItemNumber() As String
    Static sstrItemNumber As String
    If sstrItemNumber = vbNullString Then
      sstrItemNumber = Sheet1.Range("A4:A6").Offset(-1).Resize(1).Value2
    End If
    str_ItemNumber = sstrItemNumber
End Function
Private Function str_ItemName() As String
    Static sstrItemName As String
    If sstrItemName = vbNullString Then
      sstrItemName = Sheet1.Range("B4:B6").Offset(-1).Resize(1).Value2
    End If
    str_ItemName = sstrItemName
End Function

Public Function ITEM_NAME(varItemNumber As Variant) As String
  'Returns Item Name as a function of Item Number.
  Dim ƒ As WorksheetFunction: Set ƒ = WorksheetFunction
  With Sheet1.ListObjects(str_Table1)
    ITEM_NAME _
    = ƒ.Index _
      ( _
        .ListColumns(str_ItemName).DataBodyRange _
      , ƒ.Match(varItemNumber, .ListColumns(str_ItemNumber).DataBodyRange) _
      )
  End With
End Function

Once the logic/design is ready, you can replace the functions with module-level constants of the same name if speed is critical and you need to reclaim the function call overhead. Otherwise, you can just leave everything as is.

Note that the use of static variables is not required, but should reduce execution time. (Static variables could also have been used in the first example as well, but I left them out to keep it short.)

It's probably not really necessary to extract out the table names into pseudo-constants, but I have done so for completeness sake.

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EDIT: (v2)

Following up on Egalth's two brilliant suggestions, leads to the follow code which obviates the need for named ranges, or even hard-coded cell addresses, altogether as we leverage the builtin dynamism of the ListObject table itself.

I have also changed the parameter name to match* the relevant column header name so when the user presses Ctrl+Shift+A a hint as to which column to use appears. (This tip and, if required, more info on how to add Intellisense tool-tips and/or get a description to appear in the Function Arguments dialog can be seen here.)

Option Explicit

Private Function str_Table1() As String
    Static sstrTable1 As String
    If sstrTable1 = vbNullString Then sstrTable1 = Sheet1.ListObjects(1).Name ' or .ListObjects("Table1").Name
    str_Table1 = sstrTable1
End Function
Private Function str_ItemNumber() As String
    Static sstrItemNumber As String
    If sstrItemNumber = vbNullString Then
      sstrItemNumber = Sheet1.ListObjects(str_Table1).HeaderRowRange(1).Value2
    End If
    str_ItemNumber = sstrItemNumber
End Function
Private Function str_ItemName() As String
    Static sstrItemName As String
    If sstrItemName = vbNullString Then
      sstrItemName = Sheet1.ListObjects(str_Table1).HeaderRowRange(2).Value2
    End If
    str_ItemName = sstrItemName
End Function

Public Function ITEM_NAME(ByRef Item_ID As Variant) As String
  'Returns Item Name as a function of Item Number.
  Dim ƒ As WorksheetFunction: Set ƒ = WorksheetFunction
  With Sheet1.ListObjects(str_Table1)
    ITEM_NAME _
    = ƒ.Index _
      ( _
        .ListColumns(str_ItemName).DataBodyRange _
      , ƒ.Match(Item_ID, .ListColumns(str_ItemNumber).DataBodyRange) _
      )
  End With
End Function

Note the usage of .Value2. I have always used .Value2 ever since I found out about the performance drag and other issues caused by the implicit type conversion done when using .Value (or when relying on it as the default property).

_{* Make sure to update the column header names in the code when the logic/design of the project is finished.}

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EDIT: (re-boot)

Re-reading your own comments to your posted Question, I noted this one:

I might adopt that approach eventually, but I'm still in the design process and moving columns around a lot so the index number might also change

Whilst the last example above allows the header names to be changed dynamically, moving/inserting columns changes the indexes, requiring the code to be modified.

Looks like we're back to using named ranges. However, this time we only need static ones pointing to the column headers.

It also turns out that, for this new case, static variables are a bad idea in the design stage. Since the column indexes are cached, inserting a new column breaks the UDF until the project is reset.

I have also incorporated a shortened version of the sheet-less table reference hack from the quote in your posted Question:

Option Explicit

Private Function str_Table1() As String
    str_Table1 = Sheet1.ListObjects(1).Name
End Function
Private Function str_ItemNumber() As String
    With Range(str_Table1).ListObject
      str_ItemNumber = .HeaderRowRange(.Parent.Range("A3").Column - .HeaderRowRange.Column + 1).Value2
    End With
End Function
Private Function str_ItemName() As String
    With Range(str_Table1).ListObject
      str_ItemName = .HeaderRowRange(.Parent.Range("B3").Column - .HeaderRowRange.Column + 1).Value2
    End With
End Function

Public Function ITEM_NAME(ByRef Item_ID As Variant) As String
  'Returns Item Name as a function of Item Number.
  Dim ƒ As WorksheetFunction: Set ƒ = WorksheetFunction
  With Range(str_Table1).ListObject
    ITEM_NAME _
    = ƒ.Index _
      ( _
        .ListColumns(str_ItemName).DataBodyRange _
      , ƒ.Match(Item_ID, .ListColumns(str_ItemNumber).DataBodyRange) _
      )
  End With
End Function

Note that you can't use Item_name for one of the named ranges as it is the same as the UDF (case is ignored). I suggest using a trailing underscore, eg, Item_name_, for your named ranges.

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All the above methods would also have solved the original issue that you had. I'm awaiting the last pieces of info in order to make an educated guess as to why this issue was occurring in the first place.