Redigeeri

Jagamisviis:

Inheritance Basics (Visual Basic)

  • Artikkel

The Inherits statement is used to declare a new class, called a derived class, based on an existing class, known as a base class. Derived classes inherit, and can extend, the properties, methods, events, fields, and constants defined in the base class. The following section describes some of the rules for inheritance, and the modifiers you can use to change the way classes inherit or are inherited:

  • By default, all classes are inheritable unless marked with the NotInheritable keyword. Classes can inherit from other classes in your project or from classes in other assemblies that your project references.

  • Unlike languages that allow multiple inheritance, Visual Basic allows only single inheritance in classes; that is, derived classes can have only one base class. Although multiple inheritance is not allowed in classes, classes can implement multiple interfaces, which can effectively accomplish the same ends.

  • To prevent exposing restricted items in a base class, the access type of a derived class must be equal to or more restrictive than its base class. For example, a Public class cannot inherit a Friend or a Private class, and a Friend class cannot inherit a Private class.

Inheritance Modifiers

Visual Basic introduces the following class-level statements and modifiers to support inheritance:

  • Inherits statement — Specifies the base class.

  • NotInheritable modifier — Prevents programmers from using the class as a base class.

  • MustInherit modifier — Specifies that the class is intended for use as a base class only. Instances of MustInherit classes cannot be created directly; they can only be created as base class instances of a derived class. (Other programming languages, such as C++ and C#, use the term abstract class to describe such a class.)

Overriding Properties and Methods in Derived Classes

By default, a derived class inherits properties and methods from its base class. If an inherited property or method has to behave differently in the derived class it can be overridden. That is, you can define a new implementation of the method in the derived class. The following modifiers are used to control how properties and methods are overridden:

  • Overridable — Allows a property or method in a class to be overridden in a derived class.

  • Overrides — Overrides an Overridable property or method defined in the base class.

  • NotOverridable — Prevents a property or method from being overridden in an inheriting class. By default, Public methods are NotOverridable.

  • MustOverride — Requires that a derived class override the property or method. When the MustOverride keyword is used, the method definition consists of just the Sub, Function, or Property statement. No other statements are allowed, and specifically there is no End Sub or End Function statement. MustOverride methods must be declared in MustInherit classes.

Suppose you want to define classes to handle payroll. You could define a generic Payroll class that contains a RunPayroll method that calculates payroll for a typical week. You could then use Payroll as a base class for a more specialized BonusPayroll class, which could be used when distributing employee bonuses.

The BonusPayroll class can inherit, and override, the PayEmployee method defined in the base Payroll class.

The following example defines a base class, Payroll, and a derived class, BonusPayroll, which overrides an inherited method, PayEmployee. A procedure, RunPayroll, creates and then passes a Payroll object and a BonusPayroll object to a function, Pay, that executes the PayEmployee method of both objects.

Const BonusRate As Decimal = 1.45D
Const PayRate As Decimal = 14.75D

Class Payroll
    Overridable Function PayEmployee(
        ByVal HoursWorked As Decimal,
        ByVal PayRate As Decimal) As Decimal

        PayEmployee = HoursWorked * PayRate
    End Function
End Class

Class BonusPayroll
    Inherits Payroll
    Overrides Function PayEmployee(
        ByVal HoursWorked As Decimal,
        ByVal PayRate As Decimal) As Decimal

        ' The following code calls the original method in the base 
        ' class, and then modifies the returned value.
        PayEmployee = MyBase.PayEmployee(HoursWorked, PayRate) * BonusRate
    End Function
End Class

Sub RunPayroll()
    Dim PayrollItem As Payroll = New Payroll
    Dim BonusPayrollItem As New BonusPayroll
    Dim HoursWorked As Decimal = 40

    MsgBox("Normal pay is: " &
        PayrollItem.PayEmployee(HoursWorked, PayRate))
    MsgBox("Pay with bonus is: " &
        BonusPayrollItem.PayEmployee(HoursWorked, PayRate))
End Sub

The MyBase Keyword

The MyBase keyword behaves like an object variable that refers to the base class of the current instance of a class. MyBase is frequently used to access base class members that are overridden or shadowed in a derived class. In particular, MyBase.New is used to explicitly call a base class constructor from a derived class constructor.

For example, suppose you are designing a derived class that overrides a method inherited from the base class. The overridden method can call the method in the base class and modify the return value as shown in the following code fragment:

Class DerivedClass
    Inherits BaseClass
    Public Overrides Function CalculateShipping(
        ByVal Dist As Double,
        ByVal Rate As Double) As Double

        ' Call the method in the base class and modify the return value.
        Return MyBase.CalculateShipping(Dist, Rate) * 2
    End Function
End Class

The following list describes restrictions on using MyBase:

  • MyBase refers to the immediate base class and its inherited members. It cannot be used to access Private members in the class.

  • MyBase is a keyword, not a real object. MyBase cannot be assigned to a variable, passed to procedures, or used in an Is comparison.

  • The method that MyBase qualifies does not have to be defined in the immediate base class; it may instead be defined in an indirectly inherited base class. In order for a reference qualified by MyBase to compile correctly, some base class must contain a method matching the name and types of parameters that appear in the call.

  • You cannot use MyBase to call MustOverride base class methods.

  • MyBase cannot be used to qualify itself. Therefore, the following code is not valid:

    MyBase.MyBase.BtnOK_Click()

  • MyBase cannot be used in modules.

  • MyBase cannot be used to access base class members that are marked as Friend if the base class is in a different assembly.

For more information and another example, see How to: Access a Variable Hidden by a Derived Class.

The MyClass Keyword

The MyClass keyword behaves like an object variable that refers to the current instance of a class as originally implemented. MyClass resembles Me, but every method and property call on MyClass is treated as if the method or property were NotOverridable. Therefore, the method or property is not affected by overriding in a derived class.

  • MyClass is a keyword, not a real object. MyClass cannot be assigned to a variable, passed to procedures, or used in an Is comparison.

  • MyClass refers to the containing class and its inherited members.

  • MyClass can be used as a qualifier for Shared members.

  • MyClass cannot be used inside a Shared method, but can be used inside an instance method to access a shared member of a class.

  • MyClass cannot be used in standard modules.

  • MyClass can be used to qualify a method that is defined in a base class and that has no implementation of the method provided in that class. Such a reference has the same meaning as MyBase.Method.

The following example compares Me and MyClass.

Class baseClass
    Public Overridable Sub testMethod()
        MsgBox("Base class string")
    End Sub
    Public Sub useMe()
        ' The following call uses the calling class's method, even if
        ' that method is an override.
        Me.testMethod()
    End Sub
    Public Sub useMyClass()
        ' The following call uses this instance's method and not any
        ' override.
        MyClass.testMethod()
    End Sub
End Class
Class derivedClass : Inherits baseClass
    Public Overrides Sub testMethod()
        MsgBox("Derived class string")
    End Sub
End Class
Class testClasses
    Sub startHere()
        Dim testObj As derivedClass = New derivedClass()
        ' The following call displays "Derived class string".
        testObj.useMe()
        ' The following call displays "Base class string".
        testObj.useMyClass()
    End Sub
End Class

Even though derivedClass overrides testMethod, the MyClass keyword in useMyClass nullifies the effects of overriding, and the compiler resolves the call to the base class version of testMethod.

See also