关键字
Me关键字在.NET中的使用
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ME 函数大家都很熟了,在VB6中ME只能应用于窗体,但在.NET中就没用这个限制了,快来看看使用方法吧!!!!!!!!
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v 1. 活用Me参考值
1.1认识Me参考值
1.2程序传回Me参考值
v 2. 认深入了解Me参考值
1. 使用Me参考值
1.1 认识Me参考值
类别之程序成员(Procedure Member) 各含一个Me参考变量﹐它永远参考到「目前对象」(Current Object)。目前对象就是正接受并处理讯息之对象。例如﹐
'ex01.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'------------------------------------------------------------------------------
Public Class Fee
Private amount As Decimal
Public Sub New(ByVal amt As Decimal)
Me.amount = amt
End Sub
Public Sub disp()
MessageBox.Show("Amount is " + str( Me.amount ))
End Sub
End Class
'--------------------------------------------------------------------------------
Public Class Form1
Inherits System.WinForms.Form
Public Sub New()
MyBase.New()
Form1 = Me
'This call is required by the Win Form Designer.
InitializeComponent()
'TODO: Add any initialization after the InitializeComponent() call
End Sub
'Form overrides dispose to clean up the component list.
Public Overrides Sub Dispose()
MyBase.Dispose()
components.Dispose()
End Sub
#Region " Windows Form Designer generated code "
.......
#End Region
Protected Sub Form1_Click(ByVal sender As Object,
ByVal e As System.EventArgs)
Dim a As New Fee(100)
Dim b As New Fee(80)
a.disp()
b.disp()
End Sub
End Class
此程序输出如下﹕Amount is 100
Amount is 80
a 和 b是Fee类别之对象。当计算机执行指令──
a.disp()
a 就是目前对象﹐disp()程序里的Me正参考到对象a。
也可看成：
图1、 Me参考值与目前对象
请注意﹕Me参考到对象a﹐也就是Me与a皆参考到同一个对象。
当计算机执行另一指令── b.disp()时，b 即为目前对象﹐而disp()程序之Me参考正指向对象 b。
由于Me正参考到对象b﹐所以Me与b参考到同一个对象。写程序时﹐宜充分利用Me参考。
1.2 程序传回Me参考值
在应用上﹐程序常传回Me参考值﹐可创造奇妙的效果﹐这种效果也是VB的重要特色。希望您能仔细了解Me指针之使用场合﹐能让您写出完美的OOP程序来﹗现在﹐请看个熟悉的程序──
'ex02.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'------------------------------------------------------------------------------
Public Class Money
Private balance As Decimal
Public Sub New(ByVal amount As Decimal)
balance = amount
End Sub
Public Sub add(ByVal saving As Decimal)
balance = balance + saving
End Sub
Public Sub Display()
MessageBox.Show("Balance is " + str(balance))
End Sub
End Class
'--------------------------------------------------------------------------------
Public Class Form1
Inherits System.WinForms.Form
Public Sub New()
MyBase.New()
Form1 = Me
'This call is required by the Win Form Designer.
InitializeComponent()
'TODO: Add any initialization after the InitializeComponent() call
End Sub
'Form overrides dispose to clean up the component list.
Public Overrides Sub Dispose()
MyBase.Dispose()
components.Dispose()
End Sub
#Region " Windows Form Designer generated code "
.......
#End Region
Protected Sub Form1_Click(ByVal sender As Object,
ByVal e As System.EventArgs)
Dim orange As New Money(100)
orange.add(300)
orange.add(80)
orange.Display()
End Sub
End Class
此程序输出如下﹕Balance is 480
Money 类别的 balance资料﹐记录存款余额。main()内之对象orange接受两讯息──add(300)及add(80) ﹐欲存入两项金额。
指令── orange.add(300)
orange.add(80)
表示﹕先存入 300元再存入80元﹐有先后次序。若上述图形改为──
则更具次序感。于是﹐上述指令相当于──
这种效果﹐不太陌生吧﹗回忆小学时，班长喊着：「起立、敬礼、坐下」，您不是连续接受到三个讯息吗？渐渐地﹐您已能设计出像日常生活这般亲切之对象了。不过﹐俗语说﹕「万丈高楼平地起」﹐还是必须先对Me参考有充分了解才行﹗请看个程序──
'ex03.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'-----------------------------------------------------------------------------------
Public Class Money
Private balance As Decimal
Public Sub New(ByVal amount As Decimal)
balance = amount
End Sub
Public Function add(ByVal saving As Decimal) As Money
balance = balance + saving
add = Me
End Function
Public Sub Display()
MessageBox.Show("Balance is " + str(balance))
End Sub
End Class
'----------------------------------------------------------------------------------
Public Class Form1
Inherits System.WinForms.Form
Public Sub New()
MyBase.New()
Form1 = Me
'This call is required by the Win Form Designer.
InitializeComponent()
'TODO: Add any initialization after the InitializeComponent() call
End Sub
'Form overrides dispose to clean up the component list.
Public Overrides Sub Dispose()
MyBase.Dispose()
components.Dispose()
End Sub
#Region " Windows Form Designer generated code "
.......
#End Region
Protected Sub Form1_Click(ByVal sender As Object,
ByVal e As System.EventArgs)
Dim orange As New Money(100)
orange.add(300).add(80)
orange.Display()
End Sub
End Class
此程序输出如下﹕Balance is 480
由于Me永远参考到目前对象﹐此刻Me正参考到对象orange。
图2、 程序传回目前对象之参考值
oragne对象就是Me所指之对象﹐也可以说Me与orange皆参考到同一个对象。指令──
add = Me
传回目前对象之参考值──即orange对象之参考。add() 程序之定义──
于是add()把目前对象之参考值Me传回Form1_Click()。此刻﹐orange.add(300)之值也是参考值，与orange参考到同一个对象。
于是，Form1_Click()程序之指令──
成为orange对象之别名了。
原来的指令── orange.add(300).add(80)
相当于── orange.add(80)
不过﹐此时orange对象之 balance变量值为400元﹐而非原来的100元了。此orange再接受讯息── add(80)﹐其 balance值增加为480 元。orange接到第 2个讯息── add(80)时﹐计算机再执行add() 程序﹐其再度传回orange的参考值﹐使得整个指令──
又成为orange之别名。因之﹐亦能把disp()讯息接于其后﹐如下﹕
'ex04.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'------------------------------------------------------------------------------------
Public Class Money
Private balance As Decimal
Public Sub New(ByVal amount As Decimal)
balance = amount
End Sub
Public Function add(ByVal saving As Decimal) As Money
balance = balance + saving
add = Me
End Function
Public Sub Display()
MessageBox.Show("Balance is " + str(balance))
End Sub
End Class
'-------------------------------------------------------------------------------------
Public Class Form1
Inherits System.WinForms.Form
Public Sub New()
MyBase.New()
Form1 = Me
'This call is required by the Win Form Designer.
InitializeComponent()
'TODO: Add any initialization after the InitializeComponent() call
End Sub
'Form overrides dispose to clean up the component list.
Public Overrides Sub Dispose()
MyBase.Dispose()
components.Dispose()
End Sub
#Region " Windows Form Designer generated code "
.......
#End Region
Protected Sub Form1_Click( ByVal sender As Object,
ByVal e As System.EventArgs)
Dim orange As New Money(100)
orange.add(300).add(80).Display()
End Sub
End Class
此程序输出如下﹕Balance is 480
orange对象接到第 1个讯息──add(300)﹐计算机就执行add()程序，执行到结尾指令﹐传回Me（即orange对象）参考值。此时Form1_Click()的orange.add(300)就是orange对象之参考值﹐亦即orange.add() 是orange对象之别名﹔则orange和 orange.add(300)重合在一起﹐代表着同一对象──原来的orange对象。
接下来﹐第 2个讯息──add(80)传给orange.add(300) ﹐相当于传给orange对象。再度执行到 add()里的的add = Me指令时﹐又令orange.add(300).add(80) 成为 orange.add(300)之别名﹐即orange之别名﹔于是﹐三者代表同一对象──原来的orange对象。
接下来﹐第3个讯息──Display传给orange.add(300).add(80) ﹐相当于传给orange对象。
于是输出orange对象内的balance值。
以程序传回Me参考值之技巧将应用于许多方面。为了更了解这种方法﹐请看个特殊情形── 程序传回新对象之参考值。此对象不是目前对象，但内容是从目前对象拷贝而来。这不同于传回Me参考值的情形﹐两种用法常令人搞混﹗现在﹐把程序改为──
'ex05.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'------------------------------------------------------------------------------------
Public Class Money
Private balance As Decimal
Public Sub New(ByVal amount As Decimal)
balance = amount
End Sub
Public Function add(ByVal saving As Decimal) As Money
Dim newobj As Money
newobj = New Money( balance + saving )
add = newobj
End Function
Public Sub Display()
MessageBox.Show("Balance is " + str(balance))
End Sub
End Class
'-------------------------------------------------------------------------------------
Public Class Form1
Inherits System.WinForms.Form
Public Sub New()
MyBase.New()
Form1 = Me
'This call is required by the Win Form Designer.
InitializeComponent()
'TODO: Add any initialization after the InitializeComponent() call
End Sub
'Form overrides dispose to clean up the component list.
Public Overrides Sub Dispose()
MyBase.Dispose()
components.Dispose()
End Sub
#Region " Windows Form Designer generated code "
.......
#End Region
Protected Sub Form1_Click( ByVal sender As Object,
ByVal e As System.EventArgs)
Dim orange As New Money(100)
orange.add(300).add(80).Display()
End Sub
End Class
此程序输出如下﹕Balance is 480
当orange对象接到第 1个讯息──add(300)﹐计算机就执行add()程序﹐诞生一个Money类别的新对象﹐把目前对象内容（即orange对象之值）拷贝一份给Form1_Click()。这份拷贝就是orange.add(300)之值。
orange.add(300) 即为拷贝回来的那份对象﹐并非原来的orange对象。当讯息──add(80)传给orange.add(300)所代表的对象时﹐计算机就执行add()函数﹐此时目前对象是orange.add(300) 而非原来的orange。执行时﹐又把目前对象──orange.add(300)内容拷贝一份给新诞生的对象，传回给Form1_Click()程序﹐这份拷贝就是orange.add(300).add(80) 之值。
由于每回执行add()就产生一份新对象（虽然内容相同﹐但占不同的内存空间）﹐其后的讯息皆传给add()所诞生之新对象﹐而非orange对象，所以这些讯息皆不影响原来orange对象之内容。
请注意﹕Display()并未传回对象之参考值﹐则Display()讯息之后不得再接其它讯息了。所以﹐如果Form1_Click()程序改写如下，那就错了──
Protected Sub Form1_Click( ByVal sender As Object,
ByVal e As System.EventArgs)
Dim orange As New Money(100)
orange.add(300).Display().add(80) 'Error!
End Sub
End Class
因Display()不传回对象之参考值﹐则指令──
其后之讯息──add(80) 是错的。如何改正呢﹖很简单﹐只需叫Display()程序传回 Me（目前对象之参考值）或新对象之参考值即可﹐如下﹕
'ex06.bas
Imports System.ComponentModel
Imports System.Drawing
Imports System.WinForms
'-------------------------------------------------------------------------------------
Public Class Money
Private balance As Decimal
Public Sub New(ByVal amount As Decimal)
balance = amount
End Sub
Public Function add(ByVal saving As Decimal) As Money
Dim newobj As Money
newobj = New Money( balance + saving )
add = newobj
End Function
Public Function Display() As Money
MessageBox.Show("Balance is " + str(balance))
Display = Me
End Function
End Class
'-----------------------------------------------------------------------------------
Public Class Form1
Inherits System.WinForms.Form
Public Sub New()
MyBase.New()
Form1 = Me
'This call is required by the Win Form Designer.
InitializeComponent()
'TODO: Add any initialization after the InitializeComponent() call
End Sub
'Form overrides dispose to clean up the component list.
Public Overrides Sub Dispose()
MyBase.Dispose()
components.Dispose()
End Sub
#Region " Windows Form Designer generated code "
........
#End Region
Protected Sub Form1_Click(ByVal sender As Object,
ByVal e As System.EventArgs)
Dim orange As New Money(100)
orange.Display().add(300).Display().add(80).Display()
End Sub
End Class
此程序输出﹕
Balance is 100
Balance is 400
Balance is 480
此程序中﹐orange先接受Display()讯息﹐印出存款额﹔再接受add(300)讯息﹐使存款额提高 300元﹐再接受Display()讯息﹐依序下去。Display()传回来目前对象orange之参考值，add()则传回新诞生对象之参考值。
2. 深入了解Me参考
VB在编译时﹐会自动为程序成员产生Me参考变量﹐并令Me固定参考到目前对象(Current Object)。于此将细谈VB如何产生Me参考变量﹐让您更能深刻了解Me参考变量的特性和角色。首先﹐VB在编译程序成员时﹐会暗中偷加上1 个参考参数──Me﹐成为该程序成员的第1 个参数。例如﹕
Class Person
Private name As String
Priavte age As Integer
Public Sun New(ByVal na As String, ByVal a As Integer)
name = na
age = a
End Sub
Public Sub Display()
Messagebox.Show( name + ", " + str( age ) )
End Sub
End Class
在编译此程序时﹐VB会将偷加上Me参考变量如下﹕
Class Person
Private name As String
Priavte age As Integer
Public Sun New( ByVal Me As Person, ByVal na As String,
ByVal a As Integer)
Me.name = na
Me.age = a
End Sub
Public Sub Display(ByVal Me As Person)
Messagebox.Show( Me.name + ", " + str( Me.age ) )
End Sub
End Class
无论是在New()或Display()程序里﹐Me皆固定参考到目前对象﹐您不能改变Me之值。接下来﹐请看如何令Me参考到目前对象﹖假如有个Form1_Click()程序如下﹕
Sub Form1_Click( .... )
Dim x As New Person("Tom", 26)
x.Display()
End Sub
VB在编译时﹐会把指令──x.Display()转换为﹕
Person_Display(x)
意谓着﹕呼叫Person类别的Display()程序来处理x 对象之内容。在呼叫这程序时﹐就把x参考值传递给Display()程序里的Me参数﹐如下﹕
于是﹐Me就固定指向x对象了﹐而这x 对象就是我们欲处理之对象﹐亦
即就是目前对象了。请再看个例子吧﹗若Person类别改为﹕
Class Person
Private name As String
Priavte age As Integer
Public Sun New(ByVal na As String, ByVal a As Integer)
name = na
age = a
End Sub
Public Sub Print()
Me.Display()
End Sub
Public Sub Display()
Messagebox.Show( name + ", " + str( age ) )
End Sub
End Class
则VB会为这3 个程序成员加上Me参考值。其中Print()程序转换如下﹕
Public Sub Print(ByVal Me As Person)
Person_Display( Me )
End Sub
上述的Me.Display()转换为Person_Display( Me )之后﹐就把这Print()内的Me值传给了Display()内的Me了﹐此时两个程序内的Me皆参考到目前对象了。
以上所提的一般程序成员并不包括共享程序成员(Shared Member Function)。还记得吗﹖共享程序成员的目的是﹕处理有关整个类别的事情﹐而不是用来处理对象之内容。在另一方面﹐Me参考到目前对象﹐一般程序成员经由Me来存取目前对象之内容。既然共享程序成员不需存取某特定对象之值﹐当然就不需要Me参考变量了﹐因之VB并不会为共享程序成员偷加上Me参考变量。简而言之﹐VB的规则是──
「VB编译时﹐并不会为共享程序成员加上Me参考变量﹐所以
共享程序成员里没有Me参考变量﹐也就无法存取对象之内容」
如前面所述﹐在呼叫一般程序成员时﹐必须把目前对象之参考值传递过去给该程序成员。但共享程序成员里并无Me参考﹐所以无法呼叫一般程序成员。可得知VB的规则──
「共享程序成员不能呼叫一般程序成员﹐但可呼叫别的共享程序成员」
反之﹐一般程序成员却可呼叫共享程序成员。例如﹕
Class Person
Private name As String
Priavte age As Integer
Shared plast As Person
Public Sun New(ByVal na As String, ByVal a As Integer)
name = na
age = a
plast = Me
End Sub
Shared Sub DispLast()
plast.Display()
'Display() Error !!
End Sub
Public Sub Print()
DispLast()
End Sub
Public Sub Display()
Messagebox.Show( name + ", " + str( age ) )
End Sub
End Class
Display()是一般程序成员﹐共享程序成员DispLast()不能直接呼叫Display()程序如下﹕
Shared Sub DispLast()
Display() Error !!
End Sub
因为VB会将之转换为──
Shared Sub DispLast()
Person_Display( Me ) Error !!
End Sub
但DispLast()是共享程序成员﹐并无Me参考变量﹐所以错了。至于Print()
呼叫DispLast()﹐VB将之转换为──
Public Sub Print( ByVal Me As Person )
Person_DispLast()
End Sub
虽然Print() 内有Me参考﹐但DispLast()是静态程序成员并不需要Me
﹐Print() 并未把Me传给DispLast()﹐这是对的