OnClick in Excel VBA

In order to trap repeated clicks on the same cell, you need to move the focus to a different cell, so that each time you click, you are in fact moving the selection.

The code below will select the top left cell visible on the screen, when you click on any cell. Obviously, it has the flaw that it won't trap a click on the top left cell, but that can be managed (eg by selecting the top right cell if the activecell is the top left).

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
  'put your code here to process the selection, then..
  ActiveWindow.VisibleRange.Cells(1, 1).Select
End Sub

Clearly, there is no perfect answer. However, if you want to allow the user to

1. select certain cells
2. allow them to change those cells, and
3. trap each click,even repeated clicks on the same cell,

then the easiest way seems to be to move the focus off the selected cell, so that clicking it will trigger a Select event.

One option is to move the focus as I suggested above, but this prevents cell editing. Another option is to extend the selection by one cell (left/right/up/down),because this permits editing of the original cell, but will trigger a Select event if that cell is clicked again on its own.

If you only wanted to trap selection of a single column of cells, you could insert a hidden column to the right, extend the selection to include the hidden cell to the right when the user clicked,and this gives you an editable cell which can be trapped every time it is clicked. The code is as follows

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
  'prevent Select event triggering again when we extend the selection below
  Application.EnableEvents = False
  Target.Resize(1, 2).Select
  Application.EnableEvents = True
End Sub

SelectionChange is the event built into the Excel Object model for this. It should do exactly as you want, firing any time the user clicks anywhere...

I'm not sure that I understand your objections to global variables here, you would only need 1 if you use the Application.SelectionChange event. However, you wouldn't need any if you utilize the Workbook class code behind (to trap the Workbook.SelectionChange event) or the Worksheet class code behind (to trap the Worksheet.SelectionChange) event. (Unless your issue is the "global variable reset" problem in VBA, for which there is only one solution: error handling everywhere. Do not allow any unhandled errors, instead log them and/or "soft-report" an error as a message box to the user.)

You might also need to trap the Worksheet.Activate() and Worksheet.Deactivate() events (or the equivalent in the Workbook class) and/or the Workbook.Activate and Workbook.Deactivate() events so that you know when the user has switched worksheets and/or workbooks. The Window activate and deactivate events should make this approach complete. They could all call the same exact procedure, however, they all denote the same thing: the user changed the "focus", if you will.

If you don't like VBA, btw, you can do the same using VB.NET or C#.

[Edit: Dbb makes a very good point about the SelectionChange event not picking up a click when the user clicks within the currently selected cell. If you need to pick that up, then you would need to use subclassing.]