以下程序为解析xml，使用XmlTextReader类进行解析，总是报错，请高手看看，怎么回事？总是在while (reader.Read())报错。
            OpenFileDialog xml_data = new OpenFileDialog();
            xml_data.Multiselect = true;
            xml_data.Filter = "*.xml|*.xml";
            if (xml_data.ShowDialog() == DialogResult.OK)
            {
                progressBar1.Value = 0;
                textBox_jd.Text = "进度:" + "0%";
                string filepath;
                NameTable table = new NameTable();
                foreach (string filename in xml_data.FileNames)
                {
                    try
                    {
                        FileInfo xmld = new FileInfo(filename);
                        filepath = xmld.FullName;
                        XmlTextReader reader = new XmlTextReader(filepath,table);
                        while (reader.Read())
                        {
                            if (reader.NodeType == XmlNodeType.Element && reader.LocalName== "sn")
                            {
                                listBox_status.Items.Add(reader.ReadString());
                            }
                        }
                        reader.Close();
                        MessageBox.Show("CC");
                    }
                    catch(Exception)
                    {
                        MessageBox.Show("导入数据异常");
                    }
                }
            }

你的xml文件中定义了外部文档
<!DOCTYPE mdc SYSTEM "MeasDataCollection.dtd">
所以你还需要有一个MeasDataCollection.dtd文件,跟你的xml文件放在同一个目录下