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就是只允许启动单一个应用程序的实例, 试图启动第二次时不会产生新的实例,并激活已经启动的实例.
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1 生成txt文件。
DimSaveFileDialog1AsNewSaveFileDialog() '创建一个保存对话框
SaveFileDialog1.Filter ="txt files (*.txt)|*.txt" '设置扩展名
IfSaveFileDialog1.ShowDialog() = System.Windows.Forms.DialogResult.OKThen '如果确定保存
My.Computer.FileSystem.WriteAllText(SaveFileDialog1.Filename, Textbox1.Text,False) '保存文本,False表示不追加文本，直接覆盖其内容
EndIf
原文链接：
1.不正确
Dim abc As myClass，定义一个可以引用myClass类型对象的变量abc。这个abc变量存在于栈上。
abc=New myClass。New myClass创建myClass实例对象，这个对象存在于托管堆上。然后（同时）让abc引用（指向）这个对象。
abc变量在栈上，而对象在托管堆上。
abc储存的是对象的地址，而不是对象本身。
看下面的代码
Dim a As myClass,b As myClass,c As myClass
a=New myClass
b=a
c=a
整个过程中只创建了一个myClass对象，位于托管堆上。
三个myClass类型的变量a、b、c位于栈上，都指向了这一个myClass对象
2.类的成员分 实例成员 和 静态成员
实例成员 必须在实例对象上调用
静态成员 可以直接调用，不需要实例对象。
Delphi代码如下：
procedure TForm1.Button1Click(Sender: TObject);
var
购物总价:Integer;
折扣:Extended;
begin
购物总价:=StrToInt(Edit1.Text);
if 购物总价250 then
begin
折扣:=0;
end
else if 购物总价500 then
begin
折扣:=0.05;
end
else if 购物总价1000 then
begin
折扣:=0.075;
end
else if 购物总价2000 then
begin
折扣:=0.1;
end
{
此段的折扣是多少？
else if 购物总价3000 then
begin
折扣:=0.05;
end
}
else if 购物总价=3000 then
begin
折扣:=0.15;
end;
ShowMessage('您享受的折扣是：'+FloatToStr(折扣)
+' 原价：'+IntToStr(购物总价)
+' 折后总价：'+FloatToStr(购物总价*(1-折扣)));
end;
该实例有很多实现方法,下面介绍一种不同的给你:
首先要在窗体中载入以下控件:progressbar(进度条),label(用来实时显示文本提示),两个timer(分别控制进度条进度的增减),然后用以下代码就能实现:
Public Class Form1
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Timer1.Enabled = True
Timer1.Interval = 500
Timer2.Interval = 500
ProgressBar1.Value = 0
ProgressBar1.Maximum = 100
ProgressBar1.Minimum = 0
End Sub
'上述过程是设置控件的初始值,也可以在属性面板设置
Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
ProgressBar1.Value += 1
Label1.Text = "当前完成" ProgressBar1.Value "%"
If (ProgressBar1.Value = 100) Then
Timer1.Enabled = False
Timer2.Enabled = True
End If
End Sub
'timer1用来控制进度的增加
Private Sub Timer2_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer2.Tick
ProgressBar1.Value -= 1
Label1.Text = "当前完成" ProgressBar1.Value "%"
If (ProgressBar1.Value = 0) Then
Timer1.Enabled = True
Timer2.Enabled = False
End If
End Sub
'time2用来控制进度的减少
End Class
Imports System.Text
Imports System.Runtime.InteropServices
Public Class Form1
' 相关API函数声明,注释掉的这里没用到，但是也比较常用吧，这些函数的功能都能搜到。
Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As IntPtr
Private Declare Function FindWindowEx Lib "user32" Alias "FindWindowExA" (ByVal hWnd1 As IntPtr, ByVal hWnd2 As IntPtr, ByVal lpsz1 As String, ByVal lpsz2 As String) As IntPtr
Private Delegate Function EnumChildProc(ByVal hWnd As IntPtr, ByVal lParam As Integer) As Boolean
Private Declare Function EnumChildWindows Lib "user32.dll" (ByVal hWndParent As IntPtr, ByVal lpEnumFunc As EnumChildProc, ByVal lParam As Integer) As Boolean
Private Declare Auto Function SendMessage Lib "User32.dll" (ByVal hWnd As IntPtr, ByVal Msg As Integer, ByVal wParam As Integer, ByVal lParam As String) As Integer
'Private Declare Function CheckDlgButton Lib "user32" Alias "CheckDLGButtonA" (ByVal hDlg As IntPtr, ByVal nIDButton As IntPtr, ByVal wCheck As Integer) As Integer
Private Declare Function GetClassName Lib "user32" Alias "GetClassNameA" (ByVal hWnd As IntPtr, ByVal lpClassName As StringBuilder, ByVal nMaxCount As Integer) As Integer
'Private Declare Function GetWindowThreadProcessId Lib "user32" Alias "GetWindowThreadProcessId" (ByVal hwnd As IntPtr, ByVal lpdwProcessId As Long) As Integer
Private Declare Auto Function GetWindowTextLength Lib "user32" Alias "GetWindowTextLength" (ByVal hwnd As IntPtr) As Integer
Private Declare Function GetWindowText Lib "user32" Alias "GetWindowTextA" (ByVal hwnd As IntPtr, ByVal lpString As StringBuilder, ByVal cch As Integer) As Integer
' 相关消息定义，也有没用到的
Const WM_SETTEXT = HC
Const WM_GETTEXT = HD
'Const WM_SETFOCUS = H7
'Const WM_KILLFOCUS = H8
'Const WM_CLOSE = H10
'Const WM_SYSCOMMAND = H112
'Const SC_CLOSE = HF060
'Const SC_MINIMIZE = HF020
Const BM_GETCHECK = HF0
Const BM_SETCHECK = HF1
Const BM_GETSTATE = HF2
Const BM_SETSTATE = HF3
Const BM_SETSTYLE = HF4
Const BM_CLICK = HF5
'Const BM_GETIMAGE = HF6
'Const BM_SETIMAGE = HF7
Const BST_UNCHECKED = O0
Const BST_CHECKED = O1
Const BST_INDETERMINATE = O2
' 储存窗口句柄
Dim WindowHandle As IntPtr
' 储存两个（或者多个）编辑框句柄
Dim EditHandle As New List(Of IntPtr)
Dim EditWindowsText As List(Of String)
' 储存复选框句柄
Dim CheckHandle As IntPtr = 0
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Button1_Click(sender, e)
End Sub
' EnumChildWindows 回调函数，该函数名作为API函数EnumChildWindows 的一个参数
' 该函数实现了枚举各个子窗口，找出编辑框属性的功能
Public Function EnumChildProcC(ByVal hwnd As IntPtr, ByVal lParam As Integer) As Boolean
Dim dwWindowClass As StringBuilder = New StringBuilder(100)
' 获得某一个句柄的类名
GetClassName(hwnd, dwWindowClass, 100)
If dwWindowClass.ToString.Contains("EDIT") Or dwWindowClass.ToString.Contains("Edit") Then ' 类名包含EDIT的为编辑框
EditHandle.Add(hwnd) ' 存储该句柄
End If
' 返回 True 一直枚举完
Return True
End Function
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
WindowHandle = FindWindow(vbNullString, "登陆")
If WindowHandle.ToInt32 = 0 Then
MsgBox("未捕获到窗口" + "登陆")
Return
End If
' 枚举所有主窗口的子窗口（控件），枚举时自动调用回调函数，完成编辑框句柄的获取
EnumChildWindows(WindowHandle, AddressOf EnumChildProcC, 0)
' 寻找复选框
CheckHandle = FindWindowEx(WindowHandle, IntPtr.Zero, vbNullString, "记住密码")
Dim str As New StringBuilder
Dim j As Integer = 0
' 对编辑框文本赋值
For j = 0 To EditHandle.Count - 1
SendMessage(EditHandle(j), WM_SETTEXT, 0, "Text")
'GetWindowText(EditHandle(j), str, 20)
'EditWindowsText.Add(Str.ToString)
'Str.Clear()
Next
If EditHandle.Count = 0 Then
MsgBox("未找到输入框！")
End If
If CheckHandle.ToInt32 0 Then
'CheckDlgButton(WindowHandle, id, 1)
' 对复选框进行鼠标单击操作
SendMessage(CheckHandle, BM_CLICK, 0, 0)
'SendMessage(CheckHandle, BM_SETCHECK, True, 0)
End If
End Sub
End Class