[DllImport("smsdll.dll",
             EntryPoint = "App_Submit",
             CharSet = CharSet.Ansi,//Unicode,
             CallingConvention = CallingConvention.Cdecl)]//Cdecl
             public static extern int App_Submit(string service_id, string dest_terminal_id, string src_id,string content);

 [DllImport("smsdll.dll",
             EntryPoint = "App_Deliver",
             CharSet = CharSet.Ansi,
             CallingConvention = CallingConvention.StdCall)]
             unsafe public static extern int App_Deliver( StringBuilder Src_Id,  StringBuilder Dest_Id,  StringBuilder Msg_Content);
        

 StringBuilder srdid = new StringBuilder(50);
 StringBuilder destid = new StringBuilder(50);
 StringBuilder msgcontent = new StringBuilder(500);
   ret = App_Deliver(srdid, destid, msgcontent);
  if (ret > 0)
 {

 }
 else
 {   
  }