Visual Basic 1002005 [A Developers Notebook] [Electronic resources] نسخه متنی

2.12. Safeguard Properties with Split Accessibility

Most
properties consist of a property
get procedure (which allows you to retrieve the property
value) and a property set procedure (which
allows you to set a new value for the property). In previous versions
of Visual Basic, the declared access level of both procedures needed
to be the same. In VB 2005, you can protect a property by assigning
to the set procedure a lower access level than you give to the get
procedure.

Note: In the past, there was no way to create a property that
everyone could read but only your application could update. VB 2005
finally loosens the rules and gives you more flexibility.

2.12.1. How do I do that?

VB recognizes three levels of accessibility. Arranged from most to
least permissive, these are:

Public (available to all classes in all
assemblies)

Friend (available to all code in all the
classes in the current assembly)

Private (only available to code in the same
class)

Imagine you are creating a DLL component that's
going to be used by another application. You might decide to create a
property called Status that the client application
needs to read, and so you declare the property
Public:

Public Class ComponetClass
Private _Status As Integer
Public Property Status( ) As Integer
Get
Return _Status
End Get
Set(ByVal value As Integer)
_Status = value
End Set
End Property
End Class

The problem here is that the access level assigned to the
Status property allows the client to change it,
which doesn't make sense. You could make
Status a read-only property (in other words, omit
the property set procedure altogether), but that
wouldn't allow other classes that are part of your
applications and located in your component assembly to change it.

The solution is to give the property set procedure the
Friend accessibility level.
Here's what the code should look like, with the only
change highlighted:

Public Property Status( ) As Integer
Get
Return _Status
End Get
 Friend Set(ByVal value As Integer)
_Status = value
End Set
End Property

2.12.2. What about...

...read-only and
write-only properties? Split accessibility doesn't
help you if you need to make a read-only property (such as a
calculated value) or a write-only value (such as a password that
shouldn't remain accessible). To create a read-only
property, add the ReadOnly keyword to the property
declaration (right after the accessibility keyword), and remove the
property set procedure. To create a write-only property, remove the
property get procedure and add the WriteOnly
keyword. These keywords are nothing
newthey've been available since Visual Basic
.NET 1.0.