I don't want my Excel Add-In to return an array (instead I need a UDF to change other cells)

Question

I've created an Excel Add-In, and one of the functions of this Add-In, lets say New_Years currently takes in 2 years and outputs every New Years day between those 2 years as an array in Excel. So New_Years(2000,2002) would return Jan 1st 2000, Jan 1st 2001, and Jan 1st 2002 in the last cell.

The problem is that I have to know there are going to be 3 dates in that time, select 3 cells, enter my formula in the top cell, and then hit Ctrl + Shift + Enter to fill out the array.

I use XLW version 5 to convert my C++ code to an .xll file. I would really like it if there was some way I could just fill in one square with my formula, and Excel would fill in the squares below as needed with the appropriate dates. Anyone know if this is possible? Or impossible?

Many thanks!

Answer 1

It is actually possible albeit complex. I am reposting this piece of magic from Kevin Jones aka Zorvek as it sits behind the EE Paywall (link attached if anyone has access)

While Excel strictly forbids a UDF from changing any cell, worksheet, or workbook properties, there is a way to effect such changes when a UDF is called using a Windows timer and an Application.OnTime timer in sequence. The Windows timer has to be used within the UDF because Excel ignores any Application.OnTime calls inside a UDF. But, because the Windows timer has limitations (Excel will instantly quit if a Windows timer tries to run VBA code if a cell is being edited or a dialog is open), it is used only to schedule an Application.OnTime timer, a safe timer which Excel only allows to be fired if a cell is not being edited and no dialogs are open.

The example code below illustrates how to start a Windows timer from inside a UDF, how to use that timer routine to start an Application.OnTime timer, and how to pass information known only to the UDF to subsequent timer-executed routines. The code below must be placed in a regular module.

Private Declare Function SetTimer Lib "user32" ( _
      ByVal HWnd As Long, _
      ByVal nIDEvent As Long, _
      ByVal uElapse As Long, _
      ByVal lpTimerFunc As Long _
   ) As Long

Private Declare Function KillTimer Lib "user32" ( _
      ByVal HWnd As Long, _
      ByVal nIDEvent As Long _
   ) As Long

Private mCalculatedCells As Collection
Private mWindowsTimerID As Long
Private mApplicationTimerTime As Date

Public Function AddTwoNumbers( _
      ByVal Value1 As Double, _
      ByVal Value2 As Double _
   ) As Double

' This is a UDF that returns the sum of two numbers and starts a windows timer
' that starts a second Appliction.OnTime timer that performs activities not
' allowed in a UDF. Do not make this UDF volatile, pass any volatile functions
' to it, or pass any cells containing volatile formulas/functions or
' uncontrolled looping will start.

   AddTwoNumbers = Value1 + Value2

   ' Cache the caller's reference so it can be dealt with in a non-UDF routine
   If mCalculatedCells Is Nothing Then Set mCalculatedCells = New Collection
   On Error Resume Next
   mCalculatedCells.Add Application.Caller, Application.Caller.Address
   On Error GoTo 0

   ' Setting/resetting the timer should be the last action taken in the UDF
   If mWindowsTimerID <> 0 Then KillTimer 0&, mWindowsTimerID
   mWindowsTimerID = SetTimer(0&, 0&, 1, AddressOf AfterUDFRoutine1)

End Function

Public Sub AfterUDFRoutine1()

' This is the first of two timer routines. This one is called by the Windows
' timer. Since a Windows timer cannot run code if a cell is being edited or a
' dialog is open this routine schedules a second safe timer using
' Application.OnTime which is ignored in a UDF.

   ' Stop the Windows timer
   On Error Resume Next
   KillTimer 0&, mWindowsTimerID
   On Error GoTo 0
   mWindowsTimerID = 0

   ' Cancel any previous OnTime timers
   If mApplicationTimerTime <> 0 Then
      On Error Resume Next
      Application.OnTime mApplicationTimerTime, "AfterUDFRoutine2", , False
      On Error GoTo 0
   End If

   ' Schedule timer
   mApplicationTimerTime = Now
   Application.OnTime mApplicationTimerTime, "AfterUDFRoutine2"

End Sub

Public Sub AfterUDFRoutine2()

' This is the second of two timer routines. Because this timer routine is
' triggered by Application.OnTime it is safe, i.e., Excel will not allow the
' timer to fire unless the environment is safe (no open model dialogs or cell
' being edited).

   Dim Cell As Range

   ' Do tasks not allowed in a UDF...
   Application.ScreenUpdating = False
   Application.Calculation = xlCalculationManual
   Do While mCalculatedCells.Count > 0
      Set Cell = mCalculatedCells(1)
      mCalculatedCells.Remove 1
      Cell.Offset(0, 1).Value = Cell.Value
   Loop
   Application.Calculation = xlCalculationAutomatic
   Application.ScreenUpdating = True
   End Sub