Excel VBA Can't access sheet on external workbook

Question

I have created a custom function in Excel using VBA. I'm trying to get data from a different workbook using the Workbooks.Open(path) command. Here's my code:

Option Explicit

Function TestFunction() As String
  mySub
  TestFunction = "Success."
End Function


Sub mySub()

  Dim path As String
  Dim wk As Workbook

  path = "C:\Users\jg\Desktop\machine_data.xlsm"
  Set wk = Workbooks.Open(path)

  Dim ws As Worksheet
  Set ws = wk.Sheets(1)
  Debug.Print ws.Range("A2")
End Sub


Sub Test()
  Debug.Print (TestFunction())
End Sub

Now my problem is the following:

When I run the Sub Test() within the VBA environment from Excel everything works as planned. machine_data.xlsm gets opened and the field A2 shows up in debug.

Once I go to the workbook where I defined this module in and type =TestFunction() into a cell, I get a #VALUE! . The file also doesn't get opened.

If I comment these two lines:

  Set ws = wk.Sheets(1)
  Debug.Print ws.Range("A2")

the cell will show Success! , but the file still doesn't open.

What am I doing wrong? Both workbooks are .xlsm files. I am using Microsoft Office Excel 2007.

Answer 1

Just throw everything from mySub into the test function and if everything is successful have test function return the value of the cell. So testFunc = ws.Range("A2").

Answer 2

As DaveU already stated UDFs can only return values. I found a different workaround simply calling the function from within the VBA environment which lets me modify cell contents wherever I'd like.