In sample code below, it will create more than one class object inside the for-each loop. That is not good. Never create a new class instance inside a loop.
 
1. Wrong Code:

a_function()

{
          DataSet ds;
          message_class mc = new message_class();
          ds = mc.message_get_saved_by_message_id(l_old_message_id);
          if (ds.Tables.Count > 0)
          {
                  if (ds.Tables[0].Rows.Count > 0)
                  {
                          //do something here
                  }
          }

          //split message here
          foreach (string s in search_words)
          {
                  if (s.Trim() != "")
                  {
                          DataSet ds2;
                          message_class mc = new message_class();  //this is wrong. It should be left on the outside of the loop. 
                          ds2 = mc.message_membership(int.Parse(s.ToString()));
                          if (ds2.Tables.Count > 0)
                          {
                                  if (ds2.Tables[0].Rows.Count > 0)
                                  {
                                          //some codes here - do sth else
                                  }
                          }
                  }
          }
}


2. Correct code:

 a_function()
 {
          DataSet ds;
          message_class mc = new message_class(); //create a new object once only!
          ds = mc.message_get_saved_by_message_id(l_old_message_id);
          if (ds.Tables.Count > 0)
          {
                  if (ds.Tables[0].Rows.Count > 0)
                  {
                          //do something here
                  }
          }
          //split message

          DataSet ds2;
          foreach (string s in search_words)
          {
                  if (s.Trim() != "")
                  {                        
                          ds2 = mc.message_membership(int.Parse(s.ToString()));
                          if (ds2.Tables.Count > 0)
                          {
                                  if (ds2.Tables[0].Rows.Count > 0)
                                  {
                                          //do something here
                                  }
                          }
                  }
          }
 }