int r = (int)cmd.ExecuteScalar();//注意Object型別需強制轉換為int 

  1.當ExecuteScalar執行的是聚合函式的時候，返回的不可能是null,如果執行的sql語句不是聚合函式返回的有可能是null

  2.using相當於try.......finally,會自動呼叫dispose方法，所以連線物件開啟後conn.open,不用再呼叫conn.colose

        private bool CheckUserInputOldPwd(string oldpwd, int autoid)
        {
            string sql = string.Format("select count(*) from UserLogin where autoid = '{0}' and loginpwd = '{1}'", autoid, oldpwd);//注意學會這種寫法
            string constr = "Data Source = S7LVYWSI0SBPKLU;Initial Catalog = User;Integrated Security = True"; 
            using(SqlConnection con = new SqlConnection(constr))
            {
                using(SqlCommand cmd = new SqlCommand(sql,con))
	            {
		            con.Open();
                    int r = (int)cmd.ExecuteScalar();//注意Object型別需強制轉換為int
                    return r>0;
	            }            
            }
           
        }