Inheritance Basics in .NET

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To see how inheritance works in Visual Basic .NET, let s start by defining a simple Person base class:
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Class Person Fields visible from outside the class Public FirstName As String Public LastName As String End Class
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All you need to inherit an Employee class from Person is an Inherits clause immedi ately after the Class statement:
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The Employee class inherits from Person Class Employee Inherits Person End Class
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Object-Oriented Programming
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Or you can use the following syntax to convince your C# colleagues that Visual Basic .NET is a first-class language:
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A more C++-like syntax Class Employee: Inherits Person End Class
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The great thing about inheritance in Visual Basic .NET is that you can inherit from any object, including objects for which you don t have the source code, because all the plumbing code is provided by the .NET Framework. The only exception to this rule occurs when the author of the class you want to derive from has marked the class sealed, which means that no other class can inherit from it. (You ll find more informa tion about sealed classes later in this chapter.) The derived class inherits all the Public and Friend fields, properties, methods, and events of the base class. Inheriting a field can be a problem, though, because a derived class becomes dependent on that field, and the author of the base class can t change the implementation of that field for example, to make it a calculated value without breaking the derived class. For this reason, it s usually preferable that classes meant to work as base classes should include only Private fields. You should always use a prop erty instead of a field to make a piece of data visible outside the class because you can always change the internal implementation of a property without any impact on derived classes. (To save space and code, some of the examples in this section use fields instead of properties: in other words, do as I say, not as I do.) The derived class also inherits all the shared members of the base class. For this reason, all types expose the Equals and ReferenceEquals shared methods that they inherit from System.Object.
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Extending the Derived Class
You can extend the derived class with new fields, properties, and methods simply by adding these new members anywhere in the class block:
Class Employee Inherits Person Two new public fields Public BaseSalary As Single Public HoursWorked As Integer A new private field Private m_HourlySalary As Single A new property Property HourlySalary() As Single Get Return m_HourlySalary End Get Set(ByVal Value As Single) m_HourlySalary = Value
5:
End Set End Property A new method Function Salary() As Single Return BaseSalary + m_HourlySalary * HoursWorked End Function End Class
Inheritance
Using the Derived Class
You can use the new class without even knowing that it derives from another class. However, being aware of the inheritance relationship between two classes helps you write more flexible code. For example, inheritance rules state that you can always assign a derived object to a base class variable. In this case, the rule guarantees that you can always assign an Employee object to a Person variable:
Dim e As New Employee
e.FirstName = Joe"
e.LastName = Doe"
This assignment always works.
Dim p As Person = e
This proves that p points to the Employee object.
Console.WriteLine(p.CompleteName) => Joe Doe
The compiler knows that Person is the base class for Employee, and it therefore knows that all the properties and methods that you can invoke through the p variable are exposed by the Employee object as well. As a result, these calls can never fail. This sort of assignment also works when the derived class inherits from the base class indirectly. Indirect inheritance means that there are intermediate classes along the inheritance path, such as when you have a PartTimeEmployee class that derives from Employee, which in turn derives from Person. A consequence of this rule is that you can assign any object reference to an Object vari able because all .NET classes derive from System.Object either directly or indirectly:
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