vb6 equivalent to list<someclass>

Question

I want to know if exist a equivalent of (.net)

list<somefixedclass> 

in vb6

I know that exist collection in vb6 but it uses object (variant) instead of a specific object.

thanks.

Answer 1

There is no direct equivalent in VB 6 to the generic List<T> found in VB.NET. However, there is such a thing as a Collection in VB 6, which provides similar functionality. The primary difference is that a VB 6 Collection is not strongly-typed, which means that all objects are stored as Variants in the collection. In some cases, this can be beneficial, because it allows you to store many different types of data in the same collection, and in fact, VB uses this object internally. It's easy enough to use a Collection and up-cast objects as they are retrieved from the class, but there's little you can do. It's not possible to implement strongly-typed collections in the VB runtime.

That being said, there is a workaround you can implement. Similarly to how collections were implemented in early versions of VB.NET before generics were introduced, you can wrap the Collection in a class, where the only access to the internal Collection is through methods that you expose from this class. This design pattern is commonly referred to as a "custom collection".

This does have the benefit of automatically handling casting, and alleviates the consumers of your code from having to remember to mind implementation details like this. It takes care of the (all too likely) possibility that you'll be looping through a collection at run-time that is only supposed to contain one type of object, but accidentally had a second, incompatible type of object added that causes your code to throw an exception. Of course, the disadvantage is that you have to re-implement most of the functionality already provided by the Collection object yourself, in the form of public methods on your custom collection.

Here's an example of how you might go about that:

Public Class CustomerCollection

    ''#Internal collection, exposed by this class
    Private m_Customers As Collection

    Private Sub Class_Initialize()
        ''#Set up the internal collection
        Set m_Customers = New Collection
    End Sub

    Public Sub Add(ByVal cust as Customer, Optional ByVal key as String)
        ''#Add the Customer object to the internal collection
        If IsMissing(key) Then
            m_Customers.Add cust
        Else
            m_Customers.Add cust, key
        End If
    End Sub

    Public Property Get Count() As Integer
        ''#Return the number of objects in the internal collection
        Count = m_Customers.Count
    End Property

    Public Sub Remove(ByVal index As Variant)
        ''#Remove the specified object from the internal collection,
        ''# either by its index or its key
        m_Customers.Remove index
    End Sub

    Public Function Item(ByVal index As Variant) as Customer
        ''#Return the specified object from the internal collection,
        ''# either by its index or its key
        Set Item = m_Customers.Item(index)
    End Function

    Public Sub Clear()
        ''#Removes all objects from the internal collection
        Set m_Customers = New Collection
    End Sub

End Class

Note that in order to set the custom collection's Item property as the collection's default method (like the built-in Collection object), you need to follow these steps in the VB 6 IDE:

1. From the "Tools" menu, click "Procedure Attributes"

2. Select the name of your custom class from the "Name" combo box.

3. When the dialog appears, click the "Advanced" button.

4. Select the "(Default)" item in the "Procedure ID" combo box.

5. Click "OK"

If you'd also like to allow enumeration of your custom class using the For Each syntax (also like the built-in Collection object), you can add a NewEnum function to your custom class:

Public Property Get NewEnum() As IUnknown
    ''#Provides support for enumeration using For Each
    Set NewEnum = m_Customers.[_NewEnum]
End Property

Once you've done that, you need to instruct VB to use this property:

1. As before, open the "Procedure Attributes" dialog from the "Tools" menu

2. Select the name of your custom class from the "Name" combo box.

3. When the dialog appears, click the "Advanced" button.

4. Type the number "-4" in the "Procedure ID" combo box.

5. Click "OK"