Excel vba: Is it possible to access the value of a cell before the UDF called from that cell executes?

Question

I have a UDF which reads data from other sheets indirectly, adding the same cell across a number of sheets, ie:

Function myFunction( StrArgs As String ) As Long

  ....

End Function

I'm calling this function from cell A1, which right now has a value of 100:

=myFunction( ... )

Calculation is set to manual, and the sheet is refreshed when needed.

Since the arguments StrArgs define the sheets to be queried, I'm including some error-checking inside myFunction in case either the specified sheets don't exist, or there's an error in the specification of StrArgs.

This works fine, however what I'm struggling with is the following: when an error is found within myFunction, I want to return (keep) the existing value of the calling cell, instead of a zero or error value.

What I want to do at the beginning of myFunction is:

existingCellValue = Application.Caller.Text   'or Application.Caller.Value

Then perform the calculations, and when an error is encountered:

myFunction = existingCellValue

However, I find that this returns zero. In the debugger I see that, as soon as myFunction starts to execute, the cell value is already set to zero.

My question is - is there a way to access the existing value of the calling cell, before executing the UDF ?

Function GETNUMBER(Col As String, Row As Integer) As Double
    Dim LookStr As String
    Dim TheAnswer As Double
    Dim CellVal As Variant

On Error GoTo errHandler

    CellVal = Application.Caller.Text

    LookStr = "=" & Col & Row
    TheAnswer = Application.Evaluate(LookStr)

    GETNUMBER = TheAnswer

    On Error GoTo 0

Exit Function

errHandler:
    GETNUMBER = CellVal

End Function

With the above code in a module, I enter the following in my workbook:

Row
1:   | D    | 1    | =GETNUMBER(A1,B1)    | 10
2:   | D    | 2    | =GETNUMBER(A2,B2)    | 20
3:   | D    | 3    | =GETNUMBER(A3,B3)    | 30

This returns the values of 10, 20 and 30 from column D.

Now I change one of the cells in column B to zero, to invoke the errHandler, and return CellVal, stored at the start.

This seems to work, and both Application.Caller.Text and Application.ThisCell.Text give the correct result.

Thanks to both Charles Watson and KazJaw, both of whom answered the question.

Answer 1

There are several possible ways, but they all have disadvantages.
The simplest way is to use Application.Caller.Text but it returns the formatted value rather than the actual value.
See my blog post on the subject for more discussion http://fastexcel.wordpress.com/2012/01/08/writing-efficient-vba-udfs-part-8-getting-the-previously-calculated-value-from-the-calling-cells/

Answer 2

You can do it with Application.ThisCell property . See quite simple example below:

Public Function MultipleAB(a, b)
    Debug.Print Application.ThisCell.Text   'here you get current value
    MultipleAB = a * b                      'here you get new value
End Function

Important!! Application.ThisCell is valid only for UDFs used in cells. When used in other VBA Subs can return errors.

The picture below presents how this solution works (for randomly changed values in cells C1 & C2):