Is That String Really Empty?

While testing out some operations on a Variant, I ran across an interesting problem. One of the variables I used was a fixed-length string.

Turns out, this type of variable fails the Len test: it returns the predetermined length of the variable even if you don't set it to any value! I recognize, however, that this may be only a minor problem. Who uses fixed-length string variables that often anyway?

There's a method described here for figuring out if the string is variable- or fixed-length, but you need to do it inside the same function. What if you declared the variable at the module level? Or what if you want a generic function instead, so you can just call a function whenever you need to check it? Unfortunately, that page doesn't offer any solution. What follows is the best I could come up with.

Check if a String is Empty

Part of the problem is the definition of "empty". I define it as a string equal to "". This function determines if a string is empty (according to my criteria) and returns
True

or
False

accordingly.

Function IsStringEmpty(stringToCheck As String) As Boolean

  Dim i As Long
  Dim numberOfBlanks As Long

  ' if len is zero, must be empty
  If Len(stringToCheck) = 0 Then
    IsStringEmpty = True
  Else
    ' check if every character is Chr(0)
    ' if so, assume it is an uninitialized fixed-length string
    For i = 1 To Len(stringToCheck)
      If Mid$(stringToCheck, i, 1) = Chr(0) Then
        numberOfBlanks = numberOfBlanks + 1
      Else  ' a valid char, cannot be empty
        Exit Function
      End If
    Next i

    IsStringEmpty = (numberOfBlanks = Len(stringToCheck))

  End If

End Function

There are four possible conditions:

1. A variable-length string that hasn't been set to any value:
   Len(string) = 0
2. A variable-length string that has been set to some value, ex:
   "Hello World"

   :
   Len(string) > 0

3. A fixed-length string that hasn't been set to any value:
   Len(string) > 0

   !

4. A fixed-length string that has been set to some value, ex:
   "Hello World"

   :
   Len(string) > 0

In other words, variable-length strings behave according to (my) common sense, whereas fixed-length strings always return a length even if they haven't been set to anything.

What I found is that in condition #3, the value of string is actually a series of Chr(0) characters, equal to the fixed-length number.

You can see this behavior in the Locals Window: simply declare a fixed length string inside any procedure, then press F8 to start the procedure. You'll see the string variable automatically initialized with Chr(0) characters.

In only one case is the length of the string actually ever zero: a variable-length string that hasn't been set to any value. In that case, the length is zero and we can exit the function. This satisfies condition #1. Otherwise, we need to loop.

To speed things up, I tried to short circuit the function inside the For loop — if it runs into a character other than Chr(0), the function immediately exits and returns False because the string cannot possibly be empty. This handles condition #2 and #4.

The reason we total the number of Chr(0) values and compare it to the length of the string at the end is to avoid cases where Chr(0) is actually a valid part of the input string. Only cases where the entire string consists of Chr(0) will be caught, which meets condition #3. This of course is an assumption, because somewhere out there I'm sure a bunch of Chr(0) characters is a valid string to someone.

Conclusion

I hoped there was a better way to do this but IsEmpty returns false for both types of strings. I tried "string = Empty" but it works the same as Len: returns
True

for variable length strings and
False

for all other cases. The difference between Len and Empty is the subject of the next post.