数组“下标超出范围”的异常行为

Question

Consider this code in MS Word VBA:

Sub Test()
    Dim x() As Document
    ReDim Preserve x(UBound(x) + 1)
    Set x(UBound(x)) = Application.ActiveDocument
End Sub

If I place the cursor inside and hit F5 to run it, I get an error

subscript out of range

on ReDim Preserve x(UBound(x) + 1) . Specifically, it doesn't like UBound(x) , although it should return -1 based on MSDN (see under "return value").

Now, if I set a break point on ReDim Preserve x(UBound(x) + 1) , run it, and then hover the cursor over UBound(x) , it indicates the same thing:

But I've discovered that if I then hover over the variable x in the following line:

and then go back and again hover over UBound(x) in the line above, I get a different result:

Then, if I hit F5 to run the rest of the code, indeed, I get no error that time.

Furthermore, if I change my code to add IsArray(x) :

Sub Test()
    Dim x() As Document
    IsArray (x) 'This is just here to preclude VBA array "bug"
    ReDim Preserve x(UBound(x) + 1)
    Set x(UBound(x)) = Application.ActiveDocument
End Sub

It also runs fine. What is going on, and how can I get my code to work at runtime without the IsArray(x) hack?

Answer 1

MSDN:

If Array has only one element, UBound returns 0. If Array has no elements, for example if it is a zero-length string, UBound returns -1.

It isn't that the array has no elements, it is un-initialized, or doesn't have dimensions. You shouldn't rely on the value of UBound without first using ReDim on the array at least once, or without the use of error handling.

A better approach is to use error handling:

Dim x() As Document

Dim n As Long
On Error Resume Next
Err.Clear
n = LBound(x)
If Err.Number = 0 Then
    Debug.Print "Array is allocated."
Else
    Debug.Print "Array is not allocated."
End If

See this link for other considerations.

Alternatively, if possible, ensure that the array is dimensioned ( ReDim ) as least once.