Refactoring Object instantiation to introduce fake constructors in VisualBasic6
- Problem
- Bruce Mckinney mentions that one of the biggest failures in VisualBasic (actually now VbClassic) up to version 6 is it's lack of constructors. This causes the problem that once you create a new object in VB, it is in an invalid state until you initialize each field, one at a time, unless you have a convention like, say: "always call the 'Init' method after creating a new object.
However, you have no way to impose the use of such convention and therefore you can't prevent users from using invalid instances of your objects.
- Solution
- One way I've found I can cope with that is by making all public classes in an ActiveX DLL to be PublicNotCreatable and introducing one single GlobalMultiuse class which I call CCreator, whose only purpose it's to create and initialize the other public classes in the DLL. Notice that this is not a Factory Object pattern, and hence the different name. (It may help to explain the difference for those who are only somewhat familiar with the Factory pattern.)
How to
:
- Create a new Creator Class in your ActiveX DLL project. I use CCreator for its name as a convention, but you can use what you like.
- Change your new class' instancing property to be GlobalMultiuse. This way you can use it's methods like they're part of VB's global functions.
- Pick one of the other public classes in your DLL and change it's instancing property to PublicNotCreatable.
- Add a new public method to your Creator Class. I use the convention of concatenating "New" & "ClassName" for the creation method names. So if you're refactoring your "CCustomer" class you'd add a
Public Function NewCCustomer(sName as String) As CCustomer
Set NewCCustomer = New CCustomer
With NewCCustomer
.Name = sName
' Add your initialization code here
End With
End Function
method to the class.
- Find and substitute every use of
Set obj = New CCustomer
obj.Name = sValue
with
Set obj = NewCCustomer(sValue)
Since your CCustomer class is now PublicNotCreatable, the compiler will warn you if you miss any occurrence of new.
- Compile and test.
- Repeat for all your other classes.