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.