Question

I am trying to create a function for excel-VBA. This function is used to calculate the index of a certain string in a combobox or listbox. I would like the function to have the following signature:

Public Function MyIndexOf(list As ???, str As String) As Integer

My question is if there is a common superclass to Combobox and Listbox so i can replace ??? with some class.

Maybe there is another way of doing IndexOf. Suggestions are welcome but i want it to be done in a common separate function.

Anyone have a suggestion on how to solve this?

Thanks in advance
/Patrik

Answer 1

There is no inheritance in VBA (or VB 6), so unfortunately there’s no such thing as a superclass.

You do, however, have the benefit of very loose typing. You can declare the variable as a Variant, which is a universal data type that can hold any type of value, including a control. Generally, you should try and avoid using the Variant type if at all possible, but in this case, you have little choice.

So you could declare your function like this:

Public Function MyIndexOf(list As Variant, str As String) As Integer
    ' Do work here...
End Function

Of course, remembering that a Variant data type can really hold any type of value, and that the type is not known until run-time, a good defensive programmer would check to make sure that the specified type is not an Integer or a String or even a control that doesn’t expose the same public members as a ComboBox or ListBox control (thus breaking the code you wrote expecting one that does).

You can determine the actual type of a Variant at runtime using the TypeName function. So you could simply modify the above function to include a guard clause to protect against anything other than a ComboBox or ListBox control being passed in:

Public Function MyIndexOf(list As Variant, str As String) As Integer
    If (Not TypeName(list) = "ComboBox") And (Not TypeName(list) = "ListBox") Then
        MsgBox "Wrong type of list variable was specified."
    Else
        ' Do work here...
    End If
End Function