Disable ROUND function in Excel - vba

Question

As an extention to this question, I'm trying to engineer some controls to prevent users in my lab from using Excel's ROUND function (operating procedures require us to use the round-to-even/banker's rounding method).
I found it rather simple to write the function to get the correct rounding
Function RoundEven(num, precision)
RoundEven = Round(num, precision)
End Function
What I would like to do in addition to this is disable the ROUND function altogether. I tried adding
Function Round()
Round = "Bad Rounding Technique"
End Function
But this overwrote the rounding method in RoundEven, and so any rounding I attempt ends up returning the "Bad Rounding Technique" message.
What's the proper way to assign Round a new local value while still having RoundEven pull VBA's default rounding method.

Answer 1

I don't think you can write your own ROUND function and have it replace the Excel's. I just tried it in 2010. Both mine and Excel's show up in intellisense, but regardless of the one I pick, it evaluates as built-in ROUND.
You could use a change event to change any instance of ROUND to `ROUNDEVEN'
Private Sub Worksheet_Change(ByVal Target As Range)
Dim rCell As Range
For Each rCell In Target.Cells
Application.EnableEvents = False
rCell.Formula = Replace$(rCell.Formula, "ROUND(", "ROUNDEVEN(")
Application.EnableEvents = True
Next rCell
End Sub
This is in the sheet's module if you only care about one sheet. You'd need the ThisWorkbook module or a custom class module with an Application variable declare WithEvents depending on how broad a scope you need.
This will definitely work for simple stuff, but I'm quite sure there are circumstances that will break it.
Another thing you could do is validate the formulas before some critical moment. Instead of real-time fixing the problem, just check every cell in the worksheet prior to it being submitted (or whatever the critical event is). You could use the ThisWorkbook BeforeSave event to prevent the user from saving, for instance, if there are any formulas with ROUND( in them.

Answer 2

I'm not sure this will work, but try using: Application.WorksheetFunction.Round to get the original function. (And hope what you do doesn't override it).
Why don't you simply override the Round function and forget about RoundEven?

Answer 3

Will this work for you?
dim callingFunctionName as string ' we need a global variable to let Round know who is calling it
Function RoundEven(num, precision)
callingFunctionName = "RoundEven"
RoundEven = Round(num, precision)
End Function
'' Disable Round function from callers other than RoundEven (In fact you can have a safelist of functions that can call this function now)
Function Round()
if callingFunctionName = "RoundEven" then
'' allow Round here
else
Round = "Bad Rounding Technique"
end if
End Function