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Implementation； then you can call Method1（）。
Scenario 5： Implementing an Interface That Allows Overriding
Interface IInterface
Sub Method（）
End Interface
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192 CH AP T E R 7 ■ L E A R N IN G AB OU T CO M P O N E N TS AN D C L AS S H I E R AR C H IE S
Class Implementation
Implements IInterface
Public Overridable Sub Method（） Implements IInterface。Method
Console。WriteLine（〃Implementation。Method〃）
End Sub
End Class
Class ImplementationDerived
Inherits Implementation
Public Overrides Sub Method（）
Console。WriteLine（〃ImplementationDerived。Method〃）
End Sub
End Class
Module Test
Public Sub Run（）
Dim implementation As ImplementationDerived = _
New ImplementationDerived（）
Dim inst As IInterface = implementation
' Calls ImplementationDerived。Method
implementation。Method（）
' Calls ImplementationDerived。Method
inst。Method（）
End Sub
End Module
o Keywords used： Overridable and Overrides。
o By default； when you implement an interface without using the Overridable keyword；
you are saying that the derived class can overload only the method。 In many cases； this
is appropriate； as you will want to override the method and thus will need to use the
Overridable and Overrides keywords。
o This is a mon problem； and most beginning Visual Basic programmers are puzzled
by the overloading behavior。
Scenario 6： An Inheritance Tree That Overrides and Overloads
Class Base
Public Overridable Sub Method（）
Console。WriteLine（〃Base。Method〃）
End Sub
End Class
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CH AP T E R 7 ■ L E AR N IN G AB O U T CO M P O N E N TS AN D C L AS S H I E R AR C HI E S 193
Class Derived1
Inherits Base
Public Overrides Sub Method（）
Console。WriteLine（〃Derived1。Method〃）
End Sub
End Class
Class Derived2
Inherits Derived1
Public Overloads Overridable Sub Method（）
Console。WriteLine（〃Derived2。Method〃）
End Sub
End Class
Class Derived3
Inherits Derived2
Public Overrides Sub Method（）
Console。WriteLine（〃Derived3。Method〃）
End Sub
End Class
Module Test
Public Sub Run（）
Dim derivedCls As Derived3 = new Derived3（）
Dim baseCls As Base = derivedCls
Dim derived2cls As Derived2 = derivedCls
' Calls Derived3。Method
derivedCls。Method（）
' Calls Derived1。Method
baseCls。Method（）
' Calls Derived3。Method
derived2cls。Method（）
End Sub
End Module
o Keywords used： Overridable； Overrides； and Overloads。
o The inheritance hierarchy is saying that Derived1。Method（） overrides the behavior of
Base。Method（）。 Derived2。Method（） overloads the behavior of Derived1。Method（）； while
establishing a new overriding base method。 Derived3。Method（） overrides the behavior
Derived2。Method（）； and not Base。Method（） （very important）。
o When confronted with a plex inheritance hierarchy as illustrated by scenario 6； it is
important that you start at the base class and work up the inheritance hierarchy。
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194 CH AP T E R 7 ■ L E A R N IN G AB OU T CO M P O N E N TS AN D C L AS S H I E R AR C H IE S
More Type…Casting Details
This chapter illustrated some type…casting examples。 In Visual Basic； you have two ways to
type cast：
o A forced type cast； which can also be used on value types
o A type cast that queries if a type cast is possible
Consider this hierarchy：
Class Base
Public Sub Method（）
Console。WriteLine（〃Base。Method〃）
End Sub
End Class
Class Derived
Inherits Base
Public Overloads Sub Method（）
Console。WriteLine（〃Derived。Method〃）
End Sub
End Class
The next step is to instantiate the type Derived and cast the instance to the base type：
Dim derivedCls As Derived = New Derived（）
Dim baseCls As Base = derivedCls
When casting from a derived type to a base class type； a type cast is not necessary and you
can assume it is implied。
A type cast is necessary when you want to cast a base class instance to a derived class instance。
Following is the source code for a forced type cast； assuming the inheritance hierarchy from
the previous cast。
Dim backToDerived As Derived = DirectCast（baseCls； Derived）
The forced cast is the use of the method DirectCase。 The cast is forced because a conver
sion to the desired type will occur； r