Restrict type in a Collection inside a class module

Question

I have a collection inside a class module. I\'d like to restrict the object type that is \"addable\" to this collection, i.e. collection should only ever accept objects of o

Answer 1

This is what I did. I liked Rob van Gelder's example, as pointed to by @jtolle, but why should I be content with making a "custom collection class" that will only accept one specific object type (e.g. People), forever? As @jtolle points out, this is super annoying.

Instead, I generalized the idea and made a new class called UniformCollection that can contain any data type -- as long as all items are of the same type in any given instance of UniformCollection.

I added a private Variant that is a placeholder for the data type that a given instance of UniformCollection can contain.

Private mvarPrototype As Variant

After making an instance of UniformCollection and before using it, it must be initialized by specifying which data type it will contain.

Public Sub Initialize(Prototype As Variant)
    If VarType(Prototype) = vbEmpty Or VarType(Prototype) = vbNull Then
        Err.Raise Number:=ERR__CANT_INITIALIZE, _
            Source:=TypeName(Me), _
            Description:=ErrorDescription(ERR__CANT_INITIALIZE) & _
                TypeName(Prototype)
    End If
    ' Clear anything already in collection.
    Set mUniformCollection = New Collection
    If VarType(Prototype) = vbObject Or VarType(Prototype) = vbDataObject Then
        ' It's an object. Need Set.
        Set mvarPrototype = Prototype
    Else
        ' It's not an object.
        mvarPrototype = Prototype
    End If
    ' Collection will now accept only items of same type as Prototype.
End Sub

The Add method will then only accept new items that are of the same type as Prototype (be it an object or a primitive variable... haven't tested with UDTs yet).

Public Sub Add(NewItem As Variant)
    If VarType(mvarPrototype) = vbEmpty Then
        Err.Raise Number:=ERR__NOT_INITIALIZED, _
            Source:=TypeName(Me), _
            Description:=ErrorDescription(ERR__NOT_INITIALIZED)
    ElseIf Not TypeName(NewItem) = TypeName(mvarPrototype) Then
        Err.Raise Number:=ERR__INVALID_TYPE, _
            Source:=TypeName(Me), _
            Description:=ErrorDescription(ERR__INVALID_TYPE) & _
                TypeName(mvarPrototype) & "."
    Else
        ' Object is of correct type. Accept it.
        ' Do nothing.
    End If

    mUniformCollection.Add NewItem

End Sub

The rest is pretty much the same as in the example (plus some error handling). Too bad RvG didn't go the whole way! Even more too bad that Microsoft didn't include this kind of thing as a built-in feature...