为何不能最大化

void CTestDlg::OnButton1() 
{
   ZeroMemory(&pi,sizeof(PROCESS_INFORMATION));
   //pi是CTestDlg的数据成员类型是PROCESS_INFORMATION
   STARTUPINFO si;
   ZeroMemory(&si,sizeof(STARTUPINFO));
   si.cb=sizeof(STARTUPINFO);
   BOOL b=CreateProcess(NULL,
                        "NOTEPAD.EXE",
NULL,
NULL,
TRUE,
NULL,
NULL,
NULL,
&si,
&pi);
   if(b)
   {
      ::CloseHandle(pi.hProcess);
      ::CloseHandle(pi.hThread);
   }
}void CTestDlg::OnButton2() 
{
   ::PostThreadMessage(pi.dwThreadId,WM_SYSCOMMAND,SC_MAXIMIZE,0);//为何不能让他最大化？
   //如果换成::PostThreadMessage(pi.dwThreadId,WM_QUIT,0,0);
   //却可以让他关闭。怎样才能让他最大化？}