Property

Chapter 17 Working with VB .NET

Property

To design a VB .NET property you have to design an attribute with the <<Property>> stereotype, another attribute with the <<PropertyImplementation>> stereotype is automatically created, it is displayed with an underscore sign in the list of attributes. The corresponding getter and setter operations are also automatically created.

You can get rid of the implementation attribute.

If you remove the getter operation, the ReadOnly keyword is automatically generated. If you remove the setter operation, the WriteOnly keyword is automatically generated. If you remove both getter and setter operations, the attribute no longer has the <<Property>> stereotype.

When you define a <<Property>> attribute, the attribute changeability and the getter/setter operations are tightly related as explained in the following table:

Operations	Property attribute changeability
If you keep both getter and setter operations	Property is Changeable
If you remove the setter operation of a changeable property	Property becomes Read-only
If you remove the getter operation of a changeable property	Property becomes Write-only

On the other hand, if you modify the property changeability, operations will reflect this change, for example, if you turn a changeable property into a read-only property, the setter operation is automatically removed.

In the following example, class Button contains a property Caption. The Getter operation has been removed which causes the WriteOnly keyword to appear in the property declaration line:

Public Class Button
   Private captionValue As String
   Public WriteOnly Property Caption() As String
      Set (ByVal Value As String)
         captionValue = value
         Repaint()
         
      End Set
   End Property
End Class

Must override: Set the Must override extended attribute of the property to True to express that the property in a base class must be overridden in a derived class before it can be used
Overridable: Set the Overridable extended attribute of the property to True to express that the property can be overridden in a derived class
Overrides: Set the Overrides extended attribute of the property to True to express that a property overrides a member inherited from a base class
Parameters: Type a value in the value box of the Property parameters extended attribute to specify which value of the property attribute is to be used as parameter. In the following example, class Person contains property attribute ChildAge. The parameter used to sort the property is ChildName:

Public Class Person
   Private _ChildAge As Integer
   
   Private Property ChildAge(ChildName as String) As Integer
      Get
         return _ChildAge
      End Get
      Set (ByVal Value ChildAge As Integer)
         If (_ChildAge <> newChildAge)
            _ChildAge = newChildAge
         End If
      End Set
   End Property
End Class