Heartbeat/watchdog service in MS Excel

We have already covered automating Excel tasks at work -- and indeed it can be a life saver (or rather, work-life balance saver) not to have to be present at work every single day at time XX:XX no matter what only to have to push a few buttons.

However, there is an inconvenience that no automation is 100% fool-proof, especially in a corporate environment. The automation computer may be restarted (by your colleagues or your IT department who decided to push some security updates). It may lose power (because the cleaning personnel got a bit too vigorous with their brooms). It may crash, or freeze, or have a hardware failure. It is particularly nasty is the automation computer is a shared terminal that only gets used once in a while, so a user log-off due to restart may not be immediately detected. And of course once the user is logged off, no code nothing written by that user can be invoked any more, and needless to say that admin access to all computers in the corporate is strictly verboten.

One way of getting around the inconvenience would be to implement a "heartbeat" or "watchdog" service that would periodically query the availability of your automation machine and "phone home" if it goes offline.

The general idea is as follows:

• The "server" is an Excel workbook that does nothing and resides in the user's private folder on the shared drive (most corporate environments have that), so that it's accessible from any computer in the office but only by the particular user. This worksheet contains no code, but is configured to open at startup on the automation machine.
• The "client" is another Excel workbook that runs on the user's regular desktop (which is assumed to be operational). 
• The client polls the server by trying to open the server workbook (assuming that if it's locked for editing, it's open on the automation machine so the latter is operational). 
• Should the server workbook become unlocked, this means that the automation machine has logged the user off and automation won't run. The client then informs the user by sending an email. 

To determine whether a file is open on the automation machine, I use the following macro (adapted from this idea):

Function IsFileOpen(filename As String)
    Dim filenum As Integer, errnum As Integer
    On Error Resume Next   ' Turn error checking off.
    filenum = FreeFile()   ' Get a free file number.
    ' Attempt to open the file and lock it.
    Open filename For Input Lock Read As #filenum
    Close filenum          ' Close the file.
    errnum = Err           ' Save the error number that occurred.
    On Error GoTo 0        ' Turn error checking back on.
    ' Check to see which error occurred. Open for 
    IsFileOpen = (errnum = 70)
End Function

The periodic polling code may look something like this:

Sub Heartbeat()
 Dim monitor As String
 Dim span, tick, fail, restart As Integer
 With ThisWorkbook.Worksheets("Control")
  .Calculate
  monitor = .Range("monitor").Value
  span = .Range("span").Value: fail = .Range("numfail").Value: restart = .Range("autoreset").Value
  If IsFileOpen(monitor) Then
    .Range("tick").Value = 0
    .Range("retick").Value = 0
    Call LogMessage("INFO", "Heartbeat server detected")
  Else
    .Range("tick").Value = .Range("tick").Value + 1
    Call LogMessage("WARN", "Heartbeat server NOT detected")
  End If
  If .Range("tick").Value < fail Then
     Application.OnTime Now + span / 86400, "Heartbeat"
  Else
     Call LogMessage("FAIL", "Heartbeat server has failed, REPORTING FAILURE")
     Call SendEmail(.Range("ReportMail"))
     Application.OnTime Now + restart / 86400, "RestartHeartbeat"
  End If
 End With
End Sub

The idea is that the macro will restart itself using Application.OnTime if all goes well, until the server is not detected several times in a row (to guard against false positives due to intermittent network failures). In that case, a report is set via Outlook (adapted from here and possibly here) using this function:

Sub SendEmail(params As Range)
    Dim OutApp As Object, OutMail As Object, strbody As String, toaddr As String, ccaddr As String, subj As String
    strbody = "": toaddr = "": ccaddr = "": subj = "":
    Dim here As Range
    Set OutApp = CreateObject("Outlook.Application")
    Set OutMail = OutApp.CreateItem(0)
    Set here = params(1, 1)
    While Len(here.Value) > 0
       If UCase(Left(Trim(here.Value), 2)) = "TO" Then toaddr = here.Offset(0, 1).Value
       If UCase(Left(Trim(here.Value), 2)) = "CC" Then ccaddr = here.Offset(0, 1).Value
       If UCase(Left(Trim(here.Value), 2)) = "SU" Then subj = here.Offset(0, 1).Value
       If UCase(Left(Trim(here.Value), 2)) = "BO" Then strbody = here.Offset(0, 1).Value
       If UCase(Left(Trim(here.Value), 1)) = ":" Then strbody = strbody & vbNewLine & here.Offset(0, 1).Value
    Set here = here.Offset(1, 0)
    Wend
    On Error Resume Next
    With OutMail
        .to = toaddr
        .CC = ccaddr
        .BCC = ""
        .Subject = subj
        .Body = strbody
        .Send   'or use .Display
    End With
    On Error GoTo 0
    Set OutMail = Nothing: Set OutApp = Nothing
End Sub

Note that the parameters for the email are conveniently stored on the spreadsheet for easy editing, so that only the range pointing to the parameter block needs to be passed. The email can then be easily defined using something like this:

After the failure report is sent, a slightly different macro is invoked:

Sub ReStartHeartbeat()
 Dim monitor As String, span As Integer, autoreset As Integer
 With ThisWorkbook.Worksheets("Control")
  .Calculate
  monitor = .Range("monitor").Value
  span = .Range("span").Value
  autoreset = .Range("autoreset").Value
  If IsFileOpen(monitor) Then
    .Range("tick").Value = 0
    .Range("retick").Value = 0
    Call LogMessage("OK", "Heartbeat server restarted, monitoring resumed")
    Call SendEmail(.Range("ReSetupMail"))
    Application.OnTime Now + span / 86400, "Heartbeat"
  Else
    .Range("retick").Value = .Range("retick").Value + 1
    Call LogMessage("FAIL", "Heartbeat server NOT restarted, keeping on trying")
    Call SendEmail(.Range("ReSetupErrorMail"))
    Application.OnTime Now + autoreset / 86400, "ReStartHeartbeat"
  End If
 End With
End Sub

Here the idea is to keep reminding the user that the server is still down, and getting  back on track once the user has successfully corrected the problem.

The entire process is set into motion by two macros:

Sub StartHeartbeat()
 Dim monitor As String, span As Integer
 Call LogMessage("INFO", "Service started")
 With ThisWorkbook.Worksheets("Control")
  .Calculate
  monitor = .Range("monitor").Value
  span = .Range("span").Value
  If IsFileOpen(monitor) Then
    .Range("tick").Value = 0
    Call LogMessage("OK", "Heartbeat server detected, monitoring started")
    Call SendEmail(.Range("SetupMail"))
    Application.OnTime Now + span / 86400, "Heartbeat"
  Else
    Call LogMessage("WARN", "Service not started, heartbeat server not detected")
    Call SendEmail(.Range("SetupErrorMail"))
  End If
 End With
End Sub

Private Sub Workbook_Open() ' Put this in the Workbook code rather than Module1
 If ThisWorkbook.ReadOnly Then Exit Sub ' DO NOT execute if read only OR
 If Workbooks.Count > 1 Then
  Call LogMessage("WARN", "Started in non-clean session, entering setup mode. Service NOT starting. For production, run in a CLEAN session.")
  ThisWorkbook.Save
  Exit Sub ' ONLY execute in clean, dedicated session
 End If
 ThisWorkbook.Worksheets("Control").Calculate
 Application.OnTime Now + TimeValue("00:00:05"), "StartHeartbeat"
End Sub

The safeguards in place make sure that opening the client for debugging or viewing (or by accident) do not start spurious monitoring processes (.OnTime is nasty, once set it will persist untill that particular Excel session is ended, even after the workbook containing the .OnTime was closed). So the client only starts if the file is only opened in a clean, dedicated Excel session.

Finally, an auxiliary subroutine, purely aesthetic, is used to log the monitoring actions. Here goes:

Sub LogMessage(code As String, msg As String)
 Dim here As Range, c As Integer, level As Integer
 level = 255
 If UCase(Left(Trim(code), 3)) = "INF" Then level = 10
 If UCase(Left(Trim(code), 2)) = "OK" Then level = 5
 If UCase(Left(Trim(code), 3)) = "WAR" Then level = 2
 If UCase(Left(Trim(code), 3)) = "FAI" Then level = 1
 If UCase(Left(Trim(code), 3)) = "ERR" Then level = 0
 
 For c = 1 To 2
  Set here = ThisWorkbook.Worksheets(IIf(c = 1, "Log", "Errors")).Range("A1")
  If c = 2 And level >= 2 Then Exit For
  While Len(here.Value) > 0
   Set here = here.Offset(1, 0)
  Wend
  With ThisWorkbook.Worksheets("Control")
   .Calculate
   here.Value = IIf(Len(code) > 0, code, "___")
   here.Offset(0, 1).Value = Now
   here.Offset(0, 2).Value = .Range("monitor").Value
   here.Offset(0, 3).Value = .Range("span").Value
   here.Offset(0, 4).Value = .Range("numfail").Value
   here.Offset(0, 5).Value = .Range("tick").Value
   here.Offset(0, 6).Value = .Range("autoreset").Value
   here.Offset(0, 7).Value = .Range("retick").Value
   here.Offset(0, 8).Value = msg
   Set here = ThisWorkbook.Worksheets(IIf(c = 1, "Log", "Errors")).Range(here, here.Offset(0, 8))
   Select Case level
   Case 10
    here.Font.Color = RGB(200, 200, 200)
   Case 5
    here.Font.Color = RGB(0, 128, 0)
   Case 2
    here.Font.Color = RGB(255, 128, 0)
   Case 1
    here.Font.Color = RGB(128, 0, 0)
   Case 0
    here.Font.Color = RGB(255, 0, 0)
   Case Else
    here.Font.Color = RGB(255, 0, 255)
   End Select
  End With
  If c = 2 Then ThisWorkbook.Save ' only save on error
 Next c
End Sub

Excel VBA: Automatic macro execution for Windows Task Scheduler (and a few pranks to boot)

Despite all modern trends towards workflow automation, manual Excel-based workflows still abound in many areas of industry, notably finance. In our previous post we learned how to simplify and automate some of them, reducing manual copy-and-paste to a button push.

The next step of this would be further automation, so that the"button" gets pushed automatically, for example on a daily basis at a predetermined time.

One intuitive approach would be to invoke your code using an Auto Macro such as Workbook_Open, and then have Windows Task Scheduler open your workbook on schedule as required.

However, there is a catch here. Workbooks used for automated workflow are very often also used for ad-hoc manual tasks, or are simply opened to view some data, with no intention of performing any (re)calculations. Now your auto macro code would run every time the workbook is opened. If the automated workflow involves some data modification and/or email notification, you will have side effects very time you open your workbook just to see what is in there.

Worse still, if your workbook is on a shared drive, other used may stumble upon it by accident, triggering execution of the auto macros when they never expect it.

Of course, you could run two copies of your spreadsheet - one for automation and one for manual inquiries, naming the "automation" one something like "never_ever_open_this_manually.xlsm", but what happens in practice in such cases is that the two workbooks fall out of sync, and one of them is always out of date (or worse, they are both partially out of date).

A much more elegant solution would be for the auto macro to detect that the workbook was invoked by the Task Scheduler and not manually, and run the workflow only in this case.

After some probing and looking for inspiration I have come up with this solution:

Private Function IsScheduled() As Boolean
 ' Determine if the workbook was opened manually or as part of a Task Scheduler routine
 On Error GoTo Decided
 Dim Result As Boolean
 Result = True ' An invisible or programmatically started session is always assumed scheduled
  If Not Application.Visible Then GoTo Decided
  If Not Application.UserControl Then GoTo Decided
 Result = False ' A session with more that one workbooks is always assumed manual
  If Application.Workbooks.Count > 1 Then GoTo Decided
 ' otherwise assume scheduled task if the workbook name was supplied in the command line
 Dim wname As String, cmdline As String
 cmdline = UCase(Trim(GetCommandLine)): wname = UCase(Trim(ThisWorkbook.Name))
 Result = InStr(cmdline, wname) > 0
Decided: IsScheduled = Result
 On Error GoTo 0
End Function

Private Sub Workbook_Open()
 If Is Scheduled Then
  ' Run your automated workflow here
 End If
End Sub

It basically relies on the discovery that if a workbook is started by the Task Scheduler, its command line would be

C:\Program Files\OFFICE##\Excel.exe v:\path_to_workbook\workbook.xlsm

whereas for most other scenarios of workbook opening (directly from Excel or from Windows Explorer) the command line would look like

C:\Program Files\OFFICE##\Excel.exe

or

C:\Program Files\OFFICE##\Excel.exe /dde

i.e. would not contain the workbook name (instead, it passes the workbook via DDE)

To get to the command line, we need to implement the following API function (taken from here):

#If Win64 Then
    Private Declare PtrSafe Function GetCommandLineL Lib "kernel32" Alias "GetCommandLineA" () As LongPtr
    Private Declare PtrSafe Function lstrcpyL Lib "kernel32" Alias "lstrcpyA" (ByVal lpString1 As String, ByVal lpString2 As LongPtr) As Long
    Private Declare PtrSafe Function lstrlenL Lib "kernel32" Alias "lstrlenA" (ByVal lpString As LongPtr) As Long
#Else
    Private Declare Function GetCommandLineL Lib "kernel32" Alias "GetCommandLineA" () As Long
    Private Declare Function lstrcpyL Lib "kernel32" Alias "lstrcpyA" (ByVal lpString1 As String, ByVal lpString2 As Long) As Long
    Private Declare Function lstrlenL Lib "kernel32" Alias "lstrlenA" (ByVal lpString As Long) As Long
#End If

Private Function GetCommandLine() As String
GetCommandLine = "FAILED TO RETRIEVE" ' fallback value is case of error
On Error GoTo Finally ' suppress errors
   Dim strReturn As String
   #If Win64 Then
    Dim lngPtr As LongPtr
   #Else
    Dim lngPtr As Long
   #End If
   Dim StringLength As Long
   'Get the pointer to the commandline string
   lngPtr = GetCommandLineL
   'get the length of the string (not including the terminating null character):
   StringLength = lstrlenL(lngPtr)
   'initialize our string so it has enough characters including the null character:
   strReturn = String$(StringLength + 1, 0)
   'copy the string we have a pointer to into our new string:
   lstrcpyL strReturn, lngPtr
   'now strip off the null character at the end:
   GetCommandLine = Left$(strReturn, StringLength)
Finally: On Error GoTo 0
End Function

Note the compiler directives to make the code 32/64-bit aware -- this is absolutely essential if the workbook resides on a shared drive in a network with mixed 32/64-bit Excel installation. Otherwise opening the workbook might crash the entire Excel session for random unsuspecting users.

BONUS: When debugging the automation routine I have stumbled upon some wonderful tools you can use to play a practical joke on your peers. Namely you can use Application.OnKey to override the function of a commonly used key (like, a letter or an arrow) to something totally bizarre, and wrap it into Application.OnTime to activate at some (possibly random) point in the future. Put it in your auto macro, like so:

Public num As Integer

Function sov(sekunder As Double) As Double
starting_time = Timer
Do
 DoEvents
Loop Until (Timer - starting_time) >= sekunder
End Function

Sub GetDizzy()
num = num + 1
On Error Resume Next
Select Case (num Mod 5)
Case 2 'move to opposite drection
 ActiveCell.Offset(0, 1).Select 
Case 3 'shake the workbook
 dx = Round(Rnd * 10) - 5: dy = Round(Rnd * 10) - 5
 For i = 1 To 3
  ActiveWindow.SmallScroll toright:=dx, down:=dy: sov (50 / 1000)
  ActiveWindow.SmallScroll toleft:=dx, up:=dy: sov (50 / 1000)
 Next i
Case 4 'nudge the workbook
 ActiveCell.ColumnWidth = ActiveCell.ColumnWidth + Round(Rnd * 15) - 7
Case Else 'normal operation
 ActiveCell.Offset(0, -1).Select 
End Select
If num > 200 Then MsgBox "You are tired and dizzy... Why don't you call it a day and go home?"
On Error GoTo 0
End Sub

Sub Payload()
 Application.OnKey "{LEFT}", "GetDizzy"
End Sub

Private Sub Workbook_Open()
Application.OnTime Now + TimeValue("8:00:00"), "Payload"
End Sub

and watch your colleagues of choice get delighted at the emphatic computer offering them to go home at the end of the working day because they are tired and have worked long hours and it's late outside (I've borrowed the sleep function sov() from here). The code is very hard to detect because it executes totally silently and persists even after the workbook containing has long been closed; the empathy will last until Excel is closed or restarted -- but in most industry workplaces I have seen, this only happens when it crashes.

Your mileage may actually vary with the payload, but do remember not to use it for any real sabotage because this is a surefire way to get fired, if you excuse the tautology again.

No more calculation babysitting!

Let's imagine you work with a computer on a daily basis (which, as it were, is a rather common scenario these days), and let's imagine your work includes "relatively lengthy" computational tasks -- lengthy enough that it is not productive to just sit there staring at the proverbial hourglass cursor (or meditating over the progress bar, or praying over the build output console window that it compiles without errors, or whatever it is). So...

So, after some time waiting (depending on your boredom threshold, about 30 seconds for me, I'd guess under 2 minutes for most people) you decide to multitask and switch to another task (work-related or otherwise). Before long, that other task immerses you and you realize that your lengthy computation was actually done minutes ago and you could have, and should have, resumed your workflow earlier. So in an attempt to avoid doing nothing and increase productivity, you just dumped it down the tubes.

Or consider another scenario. Your computation is now lengthy enough (say, 10-20 minutes) so that you decide to grab a coffee from the kitchen, or grab a quick bite/smoke/chat, or whatever it is. It would be cool if you could get an alert that your task has finished, so you can timely return and resume work without having to check on your workstation multiple times.

Or yet another scenario. You need to run an even lengthier calculation, perhaps a few hours on end, so you leave it running after hours. And you have several of them to run after each other, and you want to run as many as you can before the next working day. So again it would be cool if you can get an alert once your calculation finishes, rather than having to log in and check on the computation  progress repeatedly. More importantly, if your task is aborted early due to some mishap (which happened to me due to my own banana fingers more times than I'd confess), you really want to be alerted at once, rather than after what you thing the task should have taken, had it completed normally.

The first scenario can partially be mitigated by running Windows Task Manager, minimizing it, and noticing when the CPU usage drops, indicating that your task has finished. But you still need to remain vigilant and keep watching that tiny indicator, and with current multicore CPUs, the drop may be from 13% to some 2%, which is not very noticeable visually. Not to mention that the two remaining scenarios cannot be worked around in this way.

Wouldn't it be nice to have an automated monitor which can do this for you?

Yeah it sure would.

So let's see how we can do it in Windows Powershell. You can determine the CPU usage of a process using this function (loosely adapted from here):

function get-excel-CPU ($avgs=1)
{
 $result=0
 for ($i=1; $i -le $avgs; $i++)
 {
  $cpuinfo = Get-WmiObject Win32_PerfFormattedData_PerfProc_Process -filter "Name LIKE '%EXCEL%'"
  $result=$result + ($cpuinfo.PercentProcessorTime |Measure -max).Maximum
  start-sleep -m 150
 }
 $result = $result/$avgs
 $result
}

Note the following:

• I use EXCEL as an example because I use it most often. It is trivial to modify it to work with any other program.
• The function takes into account that there can be multiple instances of your program, and will report the CPU usage of the most CPU intensive process. Usually, this is what you want, because only one of your instances will be doing computations anyway, but you can easily fine-tune it to be more instance-specific.
• The function measures CPU usage several ($avgs) times with a short waiting period in between, and then averages the measurement. This is done because some computations, mostly ones heavily on local or network I/O, will have wildly fluctuating CPU usage, so taking one measurement may trick the script into falsely deciding that the computation has finished. You may need to fine-tune the number of measurements and the wait time between them to reflect your specific computation pattern.

Now that we have a way to automatically determine the CPU usage, we can easily wrap it in a control loop:

$poll = 10
$homethresh = 600
$sensitivity = 5
$fullout = new-timespan -Seconds 86400
$mailout = new-timespan -Seconds $homethresh
$sw = [diagnostics.stopwatch]::StartNew()

$thiscpu = get-excel-cpu 5

if ($thiscpu -lt $sensitivity) 
{
 write-host $thiscpu, ": Excel not running, exiting."
}
else
{
 write-host "Initial CPU is ", $thiscpu
 $cnt=0
 while ($sw.elapsed -lt $fullout)
 {
  start-sleep -s $poll
  $thiscpu = get-excel-cpu 3
  $lock = is-locked
  write-host "Elapsed", $sw.elapsed, " -- CPU is ", $thiscpu, "   ",$lock
  if ($thiscpu -lt $sensitivity) {$cnt++} else {$cnt=0}
  if ($cnt -ge 2)
   {
    write-host "FINISHED!!!!"
     if ($lock -eq "UNLOCKED") {show-splash} else {phone-home}
    return
   }
 }  
 write-host "Timed out!"
}

Note the line if (...) {show-splash} else {phone-home} . There are two ways to alert you that the computation has finished. One is to show you a big splash screen, borrowed from here:

function show-splash 
{
 Add-Type -AssemblyName System.Windows.Forms
 $Form = New-Object system.Windows.Forms.Form
  $Form.Text = "Finished"
  $Form.AutoSize = $True
  $Form.AutoSizeMode = "GrowAndShrink"
  $Form.BackColor = "Lime"
  $Font = New-Object System.Drawing.Font("Arial",96,[System.Drawing.FontStyle]::Bold)
  $Form.Font = $Font
  $Label = New-Object System.Windows.Forms.Label
  $Label.Text = "Calculation finished!"
  $Label.AutoSize = $True
  $Form.Controls.Add($Label)
  $Form.Topmost=$True
  # -- this ensures your splash screen appears on top of other windows!
 $Form.ShowDialog()
}

The other is to simply send you an email that gets pushed to your smartphone or smart watch, borrowed from here (using Outlook rather than Send-MailMessage so that your IT department can safely inspect your outgoing email and won't mistake your script for a trojan):

function phone-home
{
 $Outlook = New-Object -ComObject Outlook.Application
 $Mail = $Outlook.CreateItem(0)
  $Mail.To = "youremailaddress@mailserver.com"
  $Mail.Subject = "Calculation finished"
  $Mail.Body ="Your calculation has finished. If you need to start another one, go for it."
 $Mail.Send()
}

Now, how to choose between the two? You will want the splash screen if you are sitting in front of your screen, and the email otherwise. So we need a way of discriminating between the two. Following this idea, we can use

function is-locked
{
try {
$currentuser = gwmi -Class win32_computersystem | select -ExpandProperty username
$process = get-process logonui -ea silentlycontinue
if($currentuser -and $process){"LOCKED"}else{"UNLOCKED"}
return}
#Always return LOCKED if logged in remotely
catch{"LOCKED";return} }

Finally, here is a BAT-file one-liner wrapper, called ps.bat to run your PowerShell script on systems where execution of random scripts has been disallowed by default (for good reason). We cannot override this default without admin privileges, but we can by pass it temporarily by calling

@powershell -ExecutionPolicy RemoteSigned .\%1.ps1

You can then call your PowerShell script, e.g., poll.ps1, and simply type ps poll in your command prompt to invoke it quickly.

Enjoy!

Memory leak tester in Excel

In many scenarios involving complicated Excel calculations, especially those relying on extensive VBA code and/or custom-made add-ins, there is a danger that memory leaks could be created by programming errors such as careless object operations.

Most often, such memory leaks are discovered post factum, when, left unchecked, they cause slowdowns or crashes when an Excel session runs out of memory. In such a case, correcting the error becomes a major pain. How to spot a memory leak offender in a calculation intensive spreadsheet containing thousands of custom functions? 

To this end, I have written a very simple program that basically evaluates a given formula multiple times and measures how the memory consumption increased. I have used this googled snippet to determine the memory consumption:

Declare Function GetCurrentProcessId Lib "kernel32" () As Long

Function GetMemUsage()
  ' Returns the current Excel.Application memory usage in KB
  Set objSWbemServices = GetObject("winmgmts:")
  GetMemUsage = objSWbemServices.Get( _
    "Win32_Process.Handle='" & _
    GetCurrentProcessId & "'").WorkingSetSize / 1024

  Set objSWbemServices = Nothing ' We don't want to cause memory leaks here :)
    ' We don't want to cause memory leaks here :)  
End Function

and wrote a very simple wrapper: 

Sub Measure_Leak()
 Set here = ActiveSheet.Range("A2")
 template = here.Value
 i1from = here.Offset(0, 1).Value: i1to = here.Offset(0, 2).Value
 i2from = here.Offset(0, 3).Value: i2to = here.Offset(0, 4).Value
 Set there = here.Offset(0, 5)
 
 there.Offset(0, 1).Value = GetMemUsage()
 
 For i1 = i1from To i1to
 For i2 = i2from To i2to
  working = "=" & template
  On Error Resume Next
   working = Replace(working, "$1", i1)
   working = Replace(working, "$2", i2)
  On Error GoTo 0
  there.Formula = working
 Next i2, i1
 
 there.Offset(0, 2).Value = GetMemUsage()
 Set here = Nothing
 Set there = Nothing
 ActiveSheet.Calculate
End Sub

Now I put this on an even simpler spreadsheet which looks like this:

Basically, pressing the Measure button evaluates the template expression substituting $1 and $2 with values spanning two ranges, a total of (i2t-i2f+1)*(i1t-i1f+1) times. An increased memory footprint at the end of the execution means there is a memory leak.

In the screenshot we see that a built-in Excel function does not cause any memory leaks (hurra!)

To test it, let us define a really leaky VBA function using this example:

' Put this into class module Class1
Option Explicit
Private A As New Class2
Private Str As String
Private Sub Class_Initialize()
   Set A.B = Me ' Fool garbage collector
   Str = Space(1024 * 10) ' Allocate lots of memory 
End Sub

'Put this into class module Class2
Option Explicit
Public B As Class1

with two functions that look similar but one is known to be leaky:

Function vbaNoLeak()
 Dim MyObject As Class2
 Set MyObject = New Class2
 Set MyObject = Nothing
End Function

Function vbaLeak()
 Dim MyObject As Class1
 Set MyObject = New Class1
 Set MyObject = Nothing
End Function

...and...

Note that using this method to troubleshoot parts of your VBA macro won't always work because many functions are prohibited inside VBA functions (they abort immediately, basically ensuring that VBA functions have no side effects). But it is very easy to modify the code above to be callable as a procedure from within a VBA macro.