xml读取没问题，但无法回写

Question

我有xml文档包含数据：

<patients>
 <patient>
    <firstname>Patience_name_1</firstname>
    <lastname>Patience_surname_1</lastname>
    <age>20</age>
    <gender>Male</gender>
    <exams>
      <exam>
      <examname>1th Exam of Patient 1</examname>
        <id>1</id>
        <date>2/29/2016 12:18:44</date>
        <comment>**1th Exam completed for patience1</comment>
      </exam>
    </exams>
  </patient>
  <patient>
    <firstname>Patience_name_2</firstname>
    <lastname>Patience_surname_2</lastname>...

我读了文件并更新了exp。 firstname并用新文件名编写（＆＃34; editedList.xml＆＃34;）。

public List<patient> ReadXML()
    {
        List<patient> overview = new List<patient>();
        try
        {
            System.Xml.Serialization.XmlSerializer reader =
                    new System.Xml.Serialization.XmlSerializer(typeof(patients));
            System.IO.StreamReader file = new System.IO.StreamReader("data.xml");
            var data = (patients)reader.Deserialize(file);
            file.Close();
            overview = data.patients_list.ToList();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.InnerException.Message);
        }
        return overview;
    }
    public void WriteXML(string patientName)
    {
        List <patient> patients = this.ReadXML();
        patients.Where(w => w.name == patientName).Select(s =>
        {
            s.name = DataKeeper.selectedPatient.name.ToString();
            s.surname = DataKeeper.selectedPatient.surname.ToString();
            s.gender= DataKeeper.selectedPatient.gender.ToString();
            return s;
        } ).ToList();
        try
        {
            patients p = new patients();
            p.patients_list = patients;
            System.Xml.Serialization.XmlSerializer writer =
                    new System.Xml.Serialization.XmlSerializer(typeof(patients));
            var wfile = new System.IO.StreamWriter(@"editedList.xml");
            writer.Serialize(wfile, patients);//Throw exception
            wfile.Close();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.InnerException.Message);
        }
    }

在阅读中没有问题，在我编辑一个字段后我想写它但上面的行显示了一个异常＆＃34;生成XML文档时出错。＆＃34;信息。

EDITED 堆栈跟踪：

at System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String id)
   at System.Xml.Serialization.XmlSerializer.Serialize(TextWriter textWriter, Object o, XmlSerializerNamespaces namespaces)
   at System.Xml.Serialization.XmlSerializer.Serialize(TextWriter textWriter, Object o)
   at WpfApplication1.DataReader.WriteXML(String patientName) in C:\Users\yener.yilmaz\Documents\Visual Studio 2015\Projects\WpfApplication1\WpfApplication1\DataReader.cs:line 49