WINDOWS核心编程22章DIPS程序的有关问题

WINDOWS核心编程22章DIPS程序的问题
我在读<<windows核心编程》22章DIPS程序时遇到的问题：
作者为设置对话框图标定义了一函数：
// Sets the dialog box icons
inline void chSETDLGICONS(HWND hwnd, int idi) {
  SendMessage(hwnd, WM_SETICON, TRUE, (LPARAM)
  LoadIcon((HINSTANCE) GetWindowLong(hwnd, GWL_HINSTANCE),MAKEINTRESOURCE(idi)));
  SendMessage(hwnd, WM_SETICON, FALSE, (LPARAM)
  LoadIcon((HINSTANCE) GetWindowLong(hwnd, GWL_HINSTANCE),MAKEINTRESOURCE(idi)));
}
问题一：为何用两次SendMessage(...)
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又作者设置图标的代码：
BOOL Dlg_OnInitDialog(HWND hwnd, HWND hwndFocus, LPARAM lParam) {
  chSETDLGICONS(hwnd, IDI_DIPS);
  return(TRUE);
}
void Dlg_OnCommand(HWND hwnd, int id, HWND hwndCtl, UINT codeNotify) {
  
  switch (id) {
  case IDC_SAVE:
  case IDC_RESTORE:
  case IDCANCEL:
  EndDialog(hwnd, id);
  break;
  }
}
BOOL WINAPI Dlg_Proc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {

  switch (uMsg) {
  chHANDLE_DLGMSG(hwnd, WM_INITDIALOG, Dlg_OnInitDialog);
  chHANDLE_DLGMSG(hwnd, WM_COMMAND, Dlg_OnCommand);
  }
  return(FALSE);
}
问题二：在Dlg_Proc中并未处理WM_SETICON消息，难道对话框自己处理了?
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问题三：我试图如下设置icon: SetClassLong(hwnd,GCL_HICON,IDI_DIPS);
  可是failed!窗口不是依据窗口类而来吗？设置窗口类的icon不是设置窗口属性吗？

------解决方案--------------------
The DefWindowProc function returns a handle to the previous large or small icon associated with the window, depending on the value of wParam.//由DefWindowProc 处理