Private Class Variables
Sat, Jul 2 2011 13:06
Cory Smith has a set of posts on some of the differences between VB6 and VB10.There’s one which I think needs a bit further clarification: Private Class Variables.
In VB7 and later, you can access private class variables from another instance of the class. So let’s say you had a class A with a private field _z, and instances a1 and a2.In this case a2 can read and modify a1._z.
An example of this might be if you have a class that has a private field to track changes; you might expose whether or not the class has changes as a ReadOnly property, but you’d want the backing field as Private so as it can’t be modified from outside the class, e.g:
Private _hasChanges As Boolean
Private _FirstName As String
Public ReadOnly Property HasChanges As Boolean
Public Property FirstName As String
Set(value As String)
If value <> _FirstName Then
_hasChanges = True
_FirstName = value
Public Shared Function Clone(source As Customer) As Customer
Dim cust As New Customer
cust._FirstName = source._FirstName
cust._hasChanges = source._hasChanges
Note the Clone function, where the code has access to the private variable of a second instance. This makes the language a lot more powerful and flexible. Benefits include:
- access to fields in Shared methods
- provide deep clone and equality methods
- Assists with Factory methods
there’s no real downside to this, as only code written in the same class as the field is defined in can modify the field. Code in derived classes would need the field to be declared as Protected, not Private, so Private remains Private to the code you write.
In terms of VB6 to VB7+, this is a non breaking change. It was an addition to the language that makes a lot of sense as the language moved to be more object orientated. So to claim that this is somehow significant impeding change is really just nonsense: zero impact on existing code, and provides greater flexibility when writing new code.