vb.net中用继承实现多态性
大部分面向对象的程序开发系统都是通过继承来实现多态。比如说跳蚤类和狗类都是从动物类继承过来的。为了突出每一种动物走动的特点，则每一种特定动物类都要重载动物类的"Move"方法。
多态性的问题是因为用户可以需要在还不知道是要对哪种特定动物进行处理的时候，就要调用多种从动物类中派生出来的特定的动物类中的"Move"方法。
在下面的这个TestPolymorphism过程中，用继承来实现多态性：
MustInherit Public Class Amimal '基本类
    MustOverride Public Sub Bite(Byval What As Object)
    MustOverride Public Sub Move(ByRef Distance As Double)
End Class
Public Class Flea
    Inherits Amimal
    Overrides Sub bite(Byval What As Object)
         'Bite something
    End Sub
    Overrides Sub Move(ByRef Distance As Double)
         distance=Distance+1
    End Sub
End Class
 
Public Class Dog
    Inherits Animal
    Overrides Public Sub bite(Byval What As Object)
        'Bite something
    End Sub
    Overrides Sub Move(ByRef Distance As Double)
        distance=Distance+100
    End Sub
End Class
 
Sub TestPolymorphism()
    Dim aDog As New Dog()
    Dim aFlea As New Flea()
    UseAnimal(aFlea) 'Pass a flea object to UseAnimal procedure
    UseAnimal(aDog)  'Pass a Dog object to UseAnimal procedure
End Sub
 
Sub UseAnimal(Byval AnAnimal As Animal)
    Dim distance As Double=0
    'UseAnimal does not care what kind of animal it is using
    'The Move method of both the Flea and the Dog are inherited
    'from the Animal class and can be used interchangeably.
    AnAniml.Move(distance)
    If distance=1 Then
       MessageBox.Show("The animal moved:"&CStr(distance)&_
                      "units,so it must be a Flea.")
    ElseIf distance>1 Then
       MessageBox.Show("The animal moved:"&CStr(distance)&_
                      "units,so it must be a Dog.")
    End IF
End Sub