- -
if(null == ds.Tables["student"].Rows[0][0].ToString())


OK?

能不能把你出错地方的那段代码发上来
看着代码比较容易找

string str = "select * from 表 where 列名='" + txtname.Text.Trim() + "'";
            SqlConnection conn = new SqlConnection(ConfigurationManager.AppSettings["ConnStr"]);
            conn.Open();
            SqlCommand comm = new SqlCommand(str, conn);
            SqlDataReader dr = comm.ExecuteReader();
            if (dr.Read())
            {输出有的信息}
             else
              {输出没有的信息}

差不多就是这样的，先取一行一列