Error checking for NULL in VBScript

Question

I have the following VBScript in a Classic ASP page:

function getMagicLink(fromWhere, provider)
    dim url 
    url = "magic.asp?fromwhere=" & fromWhere
    If Not provider is Nothing Then ' Error occurs here
        url = url & "&provider=" & provider 
    End if
    getMagicLink = "<a target='_blank' href='" & url & "'>" & number & "</a>"
end function

I keep getting an "Object Required" error messager on the line that says If Not provider Is Nothing Then.

Either the value is NULL, or it's not NULL, so why am I getting this error?

Edit: When I invoke the object, I pass in either NULL, or I pass in a string.

Solution 1

From your code, it looks like provider is a variant or some other variable, and not an object.

Is Nothing is for objects only, yet later you say it's a value that should either be NULL or NOT NULL, which would be handled by IsNull.

Try using:

If Not IsNull(provider) Then 
    url = url & "&provider=" & provider 
End if

Alternately, if that doesn't work, try:

If provider <> "" Then 
    url = url & "&provider=" & provider 
End if

Solution 2

I see lots of confusion in the comments. Null, IsNull() and vbNull are mainly used for database handling and normally not used in VBScript. If it is not explicitly stated in the documentation of the calling object/data, do not use it.

To test if a variable is uninitialized, use IsEmpty(). To test if a variable is uninitialized or contains "", test on "" or Empty. To test if a variable is an object, use IsObject and to see if this object has no reference test on Is Nothing.

In your case, you first want to test if the variable is an object, and then see if that variable is Nothing, because if it isn't an object, you get the "Object Required" error when you test on Nothing.

snippet to mix and match in your code:

If IsObject(provider) Then
    If Not provider Is Nothing Then
        ' Code to handle a NOT empty object / valid reference
    Else
        ' Code to handle an empty object / null reference
    End If
Else
    If IsEmpty(provider) Then
        ' Code to handle a not initialized variable or a variable explicitly set to empty
    ElseIf provider = "" Then
        ' Code to handle an empty variable (but initialized and set to "")
    Else
        ' Code to handle handle a filled variable
    End If
End If

Comments

[Vivian River]

I tried using If Not IsNull(provider) Then, but then the page raises an exception on url = url & "&provider=" & provider line. The error says, "Object variable not set".

[LittleBobbyTables - Au Revoir]

That's a head-scratcher. Are you passing in vbNull to provider, or something else?

[LittleBobbyTables - Au Revoir]

I was always taught Nothing is for objects only, try passing in vbNull instead. Nothing <> Null in VBScript.

[Cheran Shunmugavel]

Use Null, not vbNull. vbNull is a constant that is used with the VarType function.