How to handle a class you want to extend which is sealed in the .NET library?

Question

I was reading somewhere about how to handle the issue of wanting to extend a sealed class in the .NET Framework library.

This is often a common and useful task to do

Answer 1

this method may have already been mentioned above by it's formal name, but i don't know it's formal name, so here it is. This example "extends" the TextBox class (example in VB). I believe an advantage of this method is that you do not need to explicitly code or expose built-in members. Hope this is relevant:

VB Class Module "MyTextBox":

public Base as TextBox, CustomProperty as Integer

Private Sub Init(newTextBox as TextBox)
    Set Base = newTextBox
End Sub

public Property Get CustomProperty2() As String
    CustomProperty2 = "Something special"
End Property

To call the code, you might say:

Dim MyBox as New MyTextBox
MyBox.Init MyForm.TextBox3

from here you have access to all built-in members, plus your custom members.

Debug.Print MyBox.Base.Text
MyBox.CustomProperty = 44

For extra polish, you can make Base the default property of the class, and then you can leave out "Base" when you call properties of the Base class. You call Base members like this:

Debug.Print MyBox().Text
MyBox().Text = "Hello World"

VBA Demo