Overriding Properties and Methods
A derived class inherits the properties and methods defined in its base class. This is useful because you can reuse these items when appropriate for the derived class. If the property or method in the base class is marked with the Overridable keyword, you can define a new implementation for the member in the derived class. Use the Overrides keyword to shadow the member by redefining it in the derived class. This is useful when you cannot use the member "as is."
In practice, overridden members are often used to implement polymorphism. For more information, see Polymorphism.
The following rules apply to overriding methods.
You can only override members that are marked with the Overridable keyword in their base class.
By default, properties and methods are NotOverridable.
Overridden members must have the same arguments as the inherited members from the base class.
The new implementation of a member can call the original implementation in the parent class by specifying MyBase before the method name.
Overload, override, and shadow are similar concepts that can be easy to confuse. For more information, see Introduction to Objects in Visual Basic.
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