Testing if a string is null - vba

Question

I am pretty new in VBA and I have not yet got used to the syntax completely, so I'm sorry if my question sounds stupid.
I am working with RequisitePro40 and VBA 7.0 in Word 2010. In one of my modules I have the following loop and If conditions:
Dim rqRequirements As ReqPro40.Requirements
Dim rqRequirement As ReqPro40.Requirement
Const eAttrValueLookup_Label = 4
Dim a As Integer
...
For Each vReqKey In rqRequirements
Set rqRequirement = rqRequirements(vReqKey)
If rqRequirement.AttrValue("MyAttreName", eAttrValueLookup_Label).text <> Null Then
a = 1
End If
If rqRequirement.AttrValue("MyAttreName", eAttrValueLookup_Label).text = Null Then
a = 2
End If
Next
In each iteration of the loop, both a = 1 and a = 2 are executed!!
Based on This, the equality and inequality operators are "=" and "<>". Therefore I would expect that either a = 1 or a = 2 execute for a string.
Is there something wrong with my syntax? Or is it a ReqPro related Problem?
I also tried using "Is" and "IsNot" operators but they result in Compiler error: Type mismatch
Can Someone help me with this?
Update: The actual goal is to see if the
rqRequirement.AttrValue("MyAttreName", eAttrValueLookup_Label).text
is Null or not. I added the second if to show the problem that the statement is somehow not working the way I expect it to work.
Replacing "Null" to "vbNullString" did not make any changes.
I also tried the IsNull function as #Slai suggested. the result is pretty much the same:
If IsNull(rqRequirement.AttrValue(att, eAttrValueLookup_Label).text) Then
a = 3
End If
If Not IsNull(rqRequirement.AttrValue(att, eAttrValueLookup_Label).text) Then
a = 4
End If
Both statements a = 3 and a = 4 are true and executed.

Answer 1

VBA doesn't support testing whether a string is "Null". VBA isn't like a .NET language or JavaScript (for example). The basic variable types all have a default value, a String is of zero length ("") from the moment the variable is declared - it has no uninstantiated state. You can also test for vbNullString.
If you test
Dim s as String
Debug.Print s = Null, s <> Null, s = "", s = "a", IsNull(s), s = vbNullString
The return is
Null Null True False False True
So if you're trying to test whether anything has been assigned to a String variable the only things you can do are:
Debug.Print Len(s), s = "", Len(s) = 0, s = vbNullString
Which returns
0 True True True
Note that the slowest of these possibilities is s = "", even though it seems the simplest to remember.

Answer 2

As others have noted, you want to test against the null version of a string, vbNullString, and not against Null specifically. In addition to this, you also need to make sure your object isn't null itself. For example:
Dim rqRequirements As ReqPro40.Requirements
Dim rqRequirement As ReqPro40.Requirement
Const eAttrValueLookup_Label = 4
Dim a As Long ' Avoid Integer since it has a strong habit of causing overflow errors.
...
For Each vReqKey In rqRequirements
Set rqRequirement = rqRequirements(vReqKey)
If Not rqRequirement Is Nothing Then
If rqRequirement.AttrValue("MyAttreName", eAttrValueLookup_Label).text <> vbNullString Then
a = 1
End If
If rqRequirement.AttrValue("MyAttreName", eAttrValueLookup_Label).text = vbNullString Then
a = 2
End If
End If
Next
Now, I haven't worked with this specific object type before, but I am fairly certain that AttrValue("MyAttreName", eAttrValueLookup_Label) is returning some kind of object. If this is the case, then the below pattern would be preferred:
Dim rqRequirements As ReqPro40.Requirements
Dim rqRequirement As ReqPro40.Requirement
Const eAttrValueLookup_Label = 4
Dim a As Long ' Avoid Integer since it has a strong habit of causing overflow errors.
...
For Each vReqKey In rqRequirements
Set rqRequirement = rqRequirements(vReqKey)
If Not rqRequirement Is Nothing Then
Dim Attribute as Object ' Or whatever type it should be
Set Attribute = rq.Requirement.AttrValue("MyAttreName", eAttrValueLookup)
If Not Attribute is Nothing Then
If Attribute.text <> Null Then
a = 1
End If
If Attribute.text = Null Then
a = 2
End If
End If
End If
Next
In this way, we are only ever calling upon the text property of the Attribute if we have actually set the Attribute. This avoids 424 errors down the line.
Finally, if you want to figure out what is happening in the code that is causing both if's to run, do something like this:
Debug.Print "Attribute Text: ", Attribute.Text
This will allow you to see what your code is seeing. You can consider using breakpoints as well.

Answer 3

1) I think you can use vbNullString to test for empty string. Otherwise use "Null" if actual string value.
2) Ensure a is declared as long
If rqRequirement.AttrValue("MyAttreName", eAttrValueLookup_Label).text <> vbNullString Then
a = 1
End If
If rqRequirement.AttrValue("MyAttreName", eAttrValueLookup_Label).text = vbNullString Then
a = 2
Else
a = 3
End If

Answer 4

I landed here looking for an answer to "VBA: How to test if a string is Null"
while this answer may not apply to this particular users situation, it does apply to the subject question.
Dim s As String, s2 As String
s = ""
s2 = vbNullString
Debug.Print StrPtr(s) = 0, StrPtr(s2) = 0
which returns
False True
because vbNullString is a C style NULL pointer for working with COM objects, and so its memory address when returned by the undocumented StrPtr function will always be 0

Answer 5

To ensure mutual exclusivity, ask the question only once.
a = IIf(rqRequirement.AttrValue("MyAttreName", eAttrValueLookup_Label).text = vbNullString , 2, 1)
You can also use an If-Then-Else construct, particularly if you have other actions you want to perform at the same time.
The above code example assumes the ~.text call is correct.