Change cell value from an Add-In

runs OK when placed in a worksheet

Of course. The event is local to the worksheet class so the event is fired, but only for that worksheet, There is a corresponding event in the ThisWorkbook class which will respond to events on any worksheet in that workbook.

So the events are scoped to the Workbook or worksheet. An Add-in cannot respond to events in another workbook (generally speaking) so your Add-in knows nothing about the selection change on the worksheet.

General answer as the comment

workbook that references the Add-In

is not strictly correct. No workbook 'references' an add-in, they are simply 2 workbooks loaded in the same instance of Excel and each knows nothing about the other to start with.

It is possible to have an Add-in respond to events in another workbook, but you need some way to set that up. I've no idea why you want the code in an Add-in when it seems to refer specifically to one particular worksheet (testx) so perhaps you might explain the logic behind this.

For info, between your posting this and me replying the Pearson page you linked replies with "The resource you are looking for has been removed, had its name changed, or is temporarily unavailable."

Cytop, thanks for the reply. The reason I want the code in an Add-In is because we have users that all have the same Excel file as a "template" for their work which includes the "testx" worksheet. I cannot place the code in the "testx" worksheet because the VBA code for it is password-protected and our Company will not allow me access. However, I can include an Add-In to run my code. As far as the Pearson site, try http://www.cpearson.com/excel/AppEvent.aspx. I think the link I previously provided included a comma at the end of the URL which was giving the error message.

Thanks again, Cytop. For your benefit and the forum members, I am posting this solution. In the "ThisWorkbook" module of the Add-In, this code was placed:

Private WithEvents mApp As Application

Private Sub mApp_SheetSelectionChange(ByVal Sh As Object, ByVal Target As Range)
    
' This will place an "x" in cell A1 if it is empty in the "testx" worksheet when cell A1 is clicked and will
' make cell A1 empty if it contains an "x" in the "testx" worksheet when cell A1 is clicked.
' Change "x", worksheet name, cell location, Add-In name, and Add-In Class Module name as needed.
    
mApp.EnableEvents = False
    
 
    If Application.ActiveSheet.Name = "testx" Then
      
        Set mc = ActiveWorkbook.Worksheets("testx").Cells(1, 1)
         
   End If
        
      
    If Application.ActiveSheet.Name = "testx" Then
      
        If Target.Address = mc.Address Then
  
            Select Case Target.Value
            Case "x"
            Target.Value = ""
            Case Else
            Target.Value = "x"
            End Select
  
        End If
    End If
   
  
mApp.EnableEvents = True


End Sub

Private Sub Workbook_Open()

   Set mApp = Application
   
End Sub

In the Class Module named "CExcelEvents" of the Add-In, this code was placed:

Private Sub Class_Initialize()
End Sub

After entering the code, you may have to save and close Excel, then open it.