Why isn't this a type mismatch?

Question

I answered this question, apparently to OP's satisfaction, but still find their question puzzling. Their question involved an expression in which a workbook object was being concatenated with a string, triggering Run-time Error '438': Object doesn't support this property or method. You can reproduce this sort of error by simply typing

?"Hello, " & ThisWorkbook

In the Immediate Window.

My question is -- why does this raise that error, instead of a error 13 -- type mismatch? A reasonable guess is that VBA tries to find a default property for a workbook object and that a default property doesn't exist. But, if so, I would expect it to be the following error from Microsoft's list of Visual Basic 6.0 error codes: Automation object doesn't have a default value (Error 443).

It is mostly of academic interest, but if the result of concatenating an object without a default property with a string is always Error 438, and that is the only way of triggering Error 438 rather than possibly another error when concatenating a string with an object, then the following code might be of use:

Function HasDefault(O As Variant) As Boolean
    Dim i As Long
    If Not IsObject(O) Then Exit Function
    On Error Resume Next
    i = Len("Hello, " & O)
    If Err.Number = 438 Then
        HasDefault = False
    Else
        HasDefault = True
    End If
End Function

I've tested this on a variety of objects, and for those I've tested it on it has returned False exactly when _Default doesn't show up as a (hidden) member of the object when viewed in the Object Browser. Nevertheless, I don't quite trust this function and am still puzzled by what is going on here.

Answer 1

VBA will try and convert the expressions on each side of the & operator to a data value. The language spec states that:

• If the value type of the expression’s target variable is a class:
• If the declared type of the target is Variant, runtime error 9
  (Subscript out of range) is raised.
• If the declared type of the target is not Variant, and the target has a public default Property Get or function, the data value’s value is the result of invoking this default member for that target with this argument list. This consumes the argument list.
• Otherwise, runtime error 438 (Object doesn’t support this property or method) is raised.

As regards your function, I'd just use:

callbyname(O, "_Default", VbGet)

which will raise a 438 error as appropriate.