void create_xml()
    {
        con.Open();
        string str = "";
        sql = "select * from student";
        SqlCommand cmd = new SqlCommand();
        cmd.Connection = con;
        cmd.CommandText = sql;
        SqlDataReader rd = cmd.ExecuteReader();
        str="";
            while (rd.Read())
            {
                str += "";
                str += rd["name"].ToString();
                str += "";
                str += rd["mark"].ToString();
                str += "";
            }
     str += "";
     StreamWriter  sw = File.CreateText("D:/myxml.xml");
     sw.WriteLine(str);
     sw.Close(); 
     con.Close();
    }