Is this a bug in VBA's IsNumeric and CDbl() functions?

Question

Consider the following VBA function:

Function castAndAdd(inputValue As Variant) As Variant

If IsNumeric(inputValue) Then
    castAndAdd = CDbl(inputValue) + 4
Else
    castAndAdd = inputValue
End If

End Function

Calling it from the immediate window gives this output:

?castAndAdd("5,7")
 61 
?castAndAdd("5, 7")
5, 7

Stepping through the "5,7" call, I find that IsNumeric("5,7") returns true. I was thinking that maybe it gives this result because in Europe a comma is used as a decimal separator; this result is odd because I'm in the United States, so my locale should determine that Excel only recognizes a period as a decimal separator, right?

Even if we set aside the Europe/US issue, the bigger problem is that CDbl("5,7") returns 57, so that CDbl("5,7") + 4 returns 61, not 9.7 as I would have expected if the comma is a decimal separator. Is this a bug, or am I just not understanding how to use CDbl()?

Answer 1

The comma is not recognized as decimal, but as thousands separator. The mechanism is not so smart to require that then at least three digits should follow, but essentially it strips any of the thousands separators in interpreting it as a number.

So even CDbl("4,5,,6,7") would yield 4567 as a number. All this is true when the comma is the thousands separator. If, as in some European countries, the point is the thousands separator, then a similar thing will happen with points.