函数功能
  该函数将指定的消息发送到一个或多个窗口。此函数为指定的窗口调用窗口程序，直到窗口程序处理完消息再返回。而函数PostMessage不同，将一个消息寄送到一个线程的消息队列后立即返回。
函数原型
  LRESULT SendMessage（HWND hWnd，UINT Msg，WPARAM wParam，LPARAM IParam）；
  参数：
sendmessage返回值  hWnd：其窗口程序将接收消息的窗口的句柄。如果此参数为HWND_BROADCAST，则消息将被发送到系统中所有顶层窗口，包括无效或不可见的非自身拥有的窗口、被覆盖的窗口和弹出式窗口，但消息不被发送到子窗口。
  Msg：指定被发送的消息。
  wParam：指定附加的消息指定信息。
  IParam：指定附加的消息指定信息。
  返回值：返回值指定消息处理的结果，依赖于所发送的消息。
  备注：需要用HWND_BROADCAST通信的应用程序应当使用函数RegisterWindowMessage来为应用程序间的通信取得一个唯一的消息。
  如果指定的窗口是由正在调用的线程创建的，则窗口程序立即作为子程序调用。如果指定的窗口是由不同线程创建的，则系统切换到该线程并调用恰当的窗口程序。线程间的消息只有在线程执行消息检索代码时才被处理。发送线程被阻塞直到接收线程处理完消息为止。
  Windows CE：Windows CE不支持Windows桌面平台支持的所有消息。使用SendMesssge之前，要检查发送的消息是否被支持。
  速查：Windows NT：3.1及以上版本：Windows：95及以上版本；Windows CE：1.0及以上版本；头文件：winuser.h；输入库：user32.lib；Unicode：在Windows NT环境下以Unicode和ANSI方式实现。
  SendMessage函数使用实例
  程序控制拉下或收起组合框的下拉列来
  一般情况下，为了拉下或收起组合框的下拉列表，需要用键盘或鼠标进行操作，而有时我们希望程序运行的某个时刻自动拉出下拉列表（比如在一些演示程序中），为了实现这个目的，我们也只有借助于SendMessage函数，方法是发一个CB_SHOWDROPDOWN(&H14F)消息给组合框。
  在发CB_SHOWDROPDOWN消息时，wParam参数决定了是拉下列表（=True时）还是收起列表（＝False时），lParam无用（设为0）。
举例说明
  #include "windows.h"
  int main(int argc, char* argv[])
  {
  HWND hWnd = ::FindWindow(NULL,"QQ2010");
  if( hWnd != NULL)
  {
  ::SendMessage(hWnd,WM_CLOSE,0,0);
  }
  return 0;
  }
  为说明具体的使用方法，下面提供简单的程序片段。首先在代码模块中做如下声明：
  Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long,ByVal wMsg As Long，ByVal wParam As Long,lParam As Any) As Long
  Const CB_SHOWDROPDOWN=&H14F
  当程序中某处需要拉下组合框Combol的列表时，写如下调用语句：
  SendMessage Combol.hwnd,CB_SHOWDROPDOWN,True,0
  当需要收起组合框Combol的列表时，写如下语句：
  SendMessage Combol.hwnd,CB_SHOWDROPDOWNN,False,0
  The following are the ranges of message numbers.
  Range Meaning
  0 through WM_USER–1 Messages reserved for use by the system.
  WM_USER through 0x7FFF Integer messages for use by private window classes.
  WM_APP through 0xBFFF Messages available for use by applications.
  0xC000 through 0xFFFF String messages for use by applications.
  Greater than 0xFFFF Reserved by the system.
  Message numbers in the first range (0 through WM_USER–1) are defined by the system. Values in this range that are not explicitly defined are reserved by the system.
  Message numbers in the second range (WM_USER through 0x7FFF) can be defined and used by an application to send messages within a private window class. These values cannot be used to define messages that are meaningful throughout an application, because some predefined window classes already define values in this range. For example, predefined control classes such as BUTTON, EDIT, LISTBOX, and COMBOBOX may use these values. Messages in this range should not be sent to other applications unless the applications have been designed to exchange messages and to attach the same meaning to the message numbers.
  Message numbers in the third range (0x8000 through 0xBFFF) are available for application to use as private messages. Message in this range do not conflict with system messages.
  Message numbers in the fourth range (0xC000 through 0xFFFF) are defined at run time w
hen an application calls the RegisterWindowMessage function to retrieve a message number for a string. All applications that register the same string can use the associated message number for exchanging messages. The actual message number, however, is not a constant and cannot be assumed to be the same between different sessions.
  Message numbers in the fifth range (greater than 0xFFFF) are reserved by the system.
  #define WM_NULL 0x0000
  #define WM_CREATE 0x0001
  #define WM_DESTROY 0x0002
  #define WM_MOVE 0x0003
  #define WM_SIZE 0x0005
  #define WM_ACTIVATE 0x0006
  /*
  * WM_ACTIVATE state values
  */
  #define WA_INACTIVE 0
  #define WA_ACTIVE 1
  #define WA_CLICKACTIVE 2
  #define WM_SETFOCUS 0x0007
  #define WM_KILLFOCUS 0x0008
  #define WM_ENABLE 0x000A
  #define WM_SETREDRAW 0x000B
  #define WM_SETTEXT 0x000C
  #define WM_GETTEXT 0x000D
  #define WM_GETTEXTLENGTH 0x000E
  #define WM_PAINT 0x000F
  #define WM_CLOSE 0x0010
  #ifndef _WIN32_WCE
  #define WM_QUERYENDSESSION 0x0011
  #define WM_QUERYOPEN 0x0013
  #define WM_ENDSESSION 0x0016
  #endif
  #define WM_QUIT 0x0012
  #define WM_ERASEBKGND 0x0014
  #define WM_SYSCOLORCHANGE 0x0015
  #define WM_SHOWWINDOW 0x0018
  #define WM_WININICHANGE 0x001A
  #if(WINVER >= 0x0400)
  #define WM_SETTINGCHANGE WM_WININICHANGE
  #endif /* WINVER >= 0x0400 */
  #define WM_DEVMODECHANGE 0x001B

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