Stop VBA Evaluate from calling target function twice

Question

I am having trouble getting VBA's Evaluate() function to only execute once; it seems to always run twice. For instance, consider the trivial example below. If we run the RunEval() subroutine, it will call the EvalTest() function twice. This can be seen by the two different random numbers that get printed in the immediate window. The behavior would be the same if we were calling another subroutine with Evaluate instead of a function. Can someone explain how I can get Evaluate to execute the target function once instead of twice? Thank you.

Sub RunEval()
    Evaluate "EvalTest()"
End Sub

Public Function EvalTest()
    Debug.Print Rnd()
End Function

Answer 1

I did a quick search and found that others have reported similar behavior and other odd bugs with Application.Evaluate (see KB823604 and this). This is probably not high on Microsoft's list to fix since it has been seen at least since Excel 2002. That knowledge base article gives a workaround that may work in your case too - put the expression to evaluate in a worksheet and then get the value from that, like this:

Sub RunEval()
    Dim d As Double

    Range("A1").Formula = "=EvalTest()"
    d = Range("A1").Value
    Range("A1").Clear
    Debug.Print d
End Sub

Public Function EvalTest() As Double
    Dim d As Double
    d = Rnd()
    Debug.Print d
    EvalTest = d + 1
End Function

I modified your example to also return the random value from the function. This prints the value a second time but with the one added so the second print comes from the first subroutine. You could write a support routine to do this for any expression.

Answer 2

I don't know of a way to stop it, but you can at least recognize when it is happening most of the time. That could be useful if your computation is time consuming or has side effects that you don't want to have happen twice and you want to short circuit it.

(EDIT: Charles Williams actually has an answer to your specific quesion. My answer could still be useful when you don't know what data type you might be getting back, or when you expect to get something like an array or a range.)

If you use the Application.Caller property within a routine called as a result of a call to Application.Evaluate, you'll see that one of the calls appears to come from the upper left cell of of the actual range the Evaluate call is made from, and one from cell $A$1 of the sheet that range is on. If you call Application.Evaluate from the immediate window, like you would call your example Sub, one call appears to come from the upper left cell of the currently selected range and one from cell $A$1 of the current worksheet. I'm pretty sure it's the first call that's the $A$1 in both cases. (I'd test that if it matters.)

However, only one value will ever be returned from Application.Evaluate. I'm pretty sure it's the one from the second eval. (I'd test that too.)

Obviously, this won't work with calls made from the actual cell $A$1.

(As for me, I would love to know why the double evaluation happens. I would also love to know why the evaluator is exposed at all. Anyone?)

EDIT: I asked on StackOverflow here: http://stackoverflow.com/questions/2647838/why-does-excel-expose-an-evaluate-method-at-all

I hope this helps, although it doesn't directly answer your question.