带参数SQL语句执行UPDATE操作

XuguConnection conn = new XuguConnection();
conn.ConnectionString = conn_xugu ;
Try
{
    conn.Open();
    XuguCommand cmd = new XuguCommand();
    cmd.Connection = conn;
    string sql_str = "update tp set descri=?,modify_time=?";
    cmd.CommandText = sql_str;

    cmd.Parameters.Add("descri", XuguDbType.VarChar).Value = "测试数据变更";
    string strTime = "2017-12-12 12:11:11";
    DateTime dtTime = DateTime.Parse(strTime);
    cmd.Parameters.Add("modify_time", XuguDbType.DateTime).Value = dtTime;
    int ret = cmd.ExecuteNonQuery();
    return ret;
}
catch (System.Exception ex)
{
    Console.WriteLine(ex.ToString());
    conn.Close();
    return 0;
}