Protected Accessibility

An important thing to keep in mind about inheritance and accessibility is that a derived class does not have access to its base classes' Private members. Private members can be accessed only by the immediate type in which they're defined. Protected members, however, can be accessed within an inheritance hierarchy. The Protected access level restricts access to a member to only the class itself, but it extends access to all derived classes as well. For example:

 Class User   Private SSN As String   Protected Password As String End Class Class Guest   Inherits User   Sub New()     ' Error: SSN is private to User     SSN = "123-45-7890"     ' OK: Password is protected and can be accessed     Password = "password"   End Sub End Class 

The class Guest can access the Password field inherited from its base class because it is Protected . However, it cannot access the SSN field, because it is Private .

When a class accesses a Protected member, the access must take place through an instance of that class or a more derived class. It cannot take place through a base class. For example, the following code is incorrect.

 Class User   Protected Name As String   Private SSN As String   Protected Password As String End Class Class Guest   Inherits User   Shared Sub ChangeName(ByVal u As User, ByVal Name As String)     ' Error: Access to Name in User cannot go through     ' base class User     u.Name = Name   End Sub End Class 

This rule may seem strange , but it is necessary to prevent unexpected access to Protected members. Without the rule, it would be possible to gain access to a Protected member of another type simply by deriving from a common base class.

 Class User   Protected Name As String   Private SSN As String   Protected Password As String End Class Class Administrator   Inherits User End Class Class Guest   Inherits User   Public Sub PrintAdministratorPassword(ByVal u As User)     ' Error: Access to Password in User cannot go through     ' base class User     Console.WriteLine(u.Password)   End Sub End Class 

In this example, Guest cannot access Administrator 's protected field Password ”it can only access the Password field of instances of Guest .

NOTE

Protected and Friend access levels can also be combined ”the Protected Friend access level is the union of the two access levels.