输出是null不知道为什么

public void second()
{
string strsql = "data source=KNUZ7ZSDSSK5SAJ;initial catalog=myschool;user id=sa;pwd=123";

        SqlConnection con = new SqlConnection(strsql);
        try
        {
            con.Open();
            StringBuilder sb = new StringBuilder();
            sb.AppendLine("select");
            sb.AppendLine("    [studentname]");
            sb.AppendLine("    [studentno]");
            sb.AppendLine("from");
            sb.AppendLine("       [student]");
            SqlCommand com = new SqlCommand(sb.ToString(), con);                           
            SqlDataReader reader= com.ExecuteReader();
            if(reader==null){
                Console.WriteLine("出现异常");                                   
            }
            Console.WriteLine("学号\t姓名");
            reader.Read();
            while(reader.Read())
            {
                sb.AppendFormat("{0}\t{1}", reader["studentname"], reader["studentno"]);
                Console.WriteLine(sb);
                sb.Length = 0;                   
            }
            reader.Close();
        }
        catch (Exception)
        {
            Console.WriteLine("null");
        }
        finally 
        {
            con.Close();         
        }

    }

SQL语句拼接有问题啊，童鞋