类的派生与继承（相关vb.net教程)
上面mydate类中，如果要增加时、分、秒的处理又该如何呢？此时只需在前面的基础上增加三个成员m_hour、 m_minute 、m_second及其处理程序。这样便产生一个派生类mydatetime。总的操作步骤为：
1、在派生类mydatetime中加入语句：inherits mydate，以示派生关系。
2、同时，派生类的构造函数new()和new(y,m,d,h,mi,s)中分别加入mybase.new()和mybase.new(y,m,d)
3、原类的show函数与派生类的show函数前中分别加入：overridable与overrides。
程序见，mydate的派生类mydatetime，程序如下：
Public Class mydate
    Public m_year, m_month, m_day As Integer
    Public Sub New()
        m_year = Microsoft.VisualBasic.Year(Now)
        m_month = Microsoft.VisualBasic.Month(Now)
        m_day = Microsoft.VisualBasic.Day(Now)
    End Sub
    Public Sub New(ByVal y As Integer, ByVal m As Integer, ByVal d As Integer)
        m_year = y : m_month = m : m_day = d
    End Sub
    Public Overridable Sub show()
          Dim s As String
        s = Str(m_year) + "-" + Str(m_month) + "-" + Str(m_day)
        MsgBox(s, MsgBoxStyle.OkOnly, "日期")
    End Sub
End Class
Public Class mydatetime
    Inherits mydate
    Private m_hour, m_minute, m_second
    Public Overrides Sub show()
        Dim s As String
        s = Str(m_year) + "-" + Str(m_month) + "-" + Str(m_day)
        s = s + Str(m_hour) + ":" + Str(m_minute) + ":" + Str(m_second)
        MsgBox(s, MsgBoxStyle.OkOnly, "时间日期")
    End Sub
    Public Sub setsystemtime()
        m_hour = Microsoft.VisualBasic.Hour(Now)
        m_minute = Microsoft.VisualBasic.Minute(Now)
        m_second = Microsoft.VisualBasic.Second(Now)
    End Sub
    Public Sub New()
        MyBase.New()
        setsystemtime()
    End Sub
    Public Sub New(ByVal y As Integer, ByVal m As Integer, ByVal d As Integer, ByVal h As Integer, ByVal mi As Integer, ByVal s As Integer)
        MyBase.New(y, m, d)
        m_hour = h : m_minute = mi : m_second = s
    End Sub
End Class
测试类为：
Public Class Form1
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim a As New mydatetime
        Dim b As New mydatetime(2008, 5, 1, 11, 30, 40)
        a.show()
        b.show()
    End Sub
End Class