Is Nothing comparison gives type mismatch

Question

I am trying to check if the 'Listivew.Tag property is nothing'.

I used to do the 'Is Nothing' check universally for all scenarios as first check to avoid errors

Can someone explain how to do it in VB 6?

 If Not .lvwLocation.Tag Is Nothing Then
    'COMPANY
    str = str & IIf(Len(.lvwLocation.Tag) > 0, " and u.location_id in " & .lvwLocation.Tag, "")
End If

Gives error 'type-mismatch'

Answer 1

Nothing is a valid value for Object variables, and Is is the way to compare object pointers.

But a VB6 control's Tag property is a String, and VB6's String type is not an Object; it's a primitive type. That means a String variable can't be assigned Nothing -- its emptiest possible value is the empty string. (And an Object variable can't be assigned a String value.) For strings just use the same equality/inequality/comparision operators that you use for other primitive (numeric/boolean/date) types:

If .lvwLocation.Tag <> "" Then ...

Answer 2

In VB6 it appears that using Is Nothing to compare Objects works, Every other data type that I tried did not. In .Net Nothing represents the default value of any data type and will work like you expect.

Dim test as Object

If Not test Is Nothing Then
   /////  
End If

Since it appears the data type of th Tag property in VB6 is a string. I would use something like:

If .lvwLocation.Tag <> "" Then      
    /////
End If