我想从XML文件加载数据并显示在列表框中？

我有三个class：

首先是人：

public class Person { public string FirstName { get; set; } public string LastName { get; set; } public string Email { get; set; } public Person() { } public override string ToString() { return FirstName + " " +LastName + "\t" +Email; } } 

第二是：

  public class Student:Person { public Student() { } public double AssessmentGrade { get; set; } public double AssignmentGrade { get; set; } public override string ToString() { return base.ToString() + "," +AssessmentGrade + "," + AssignmentGrade; } } 

第三是：

  public class Teacher:Person { public int RoomNumber { get; set; } public override string ToString() { return base.ToString() + "," + RoomNumber; } } 

我有一个类，我只是从PeronDB调用数据（类）

  public class Persons { private List persons = null; public void Fill() { persons = PersonDB.GetPersons(); } } 

我正在做的InPersonDB类，如果它的Student节点然后读取数据，如果它的教师节点然后也读取数据，如果它是Person，它也读取数据。

  public class PersonDB { private const string path = @"..\..\Persons.xml"; public static List GetPersons() { List persons = new List(); XmlReaderSettings settings = new XmlReaderSettings(); settings.IgnoreWhitespace = true; settings.IgnoreComments = true; XmlReader xmlIn = XmlReader.Create(path, settings); if (xmlIn.ReadToDescendant("Student")) { do { Person person = new Person(); Student student = (Student)person; xmlIn.ReadStartElement("Student"); student.FirstName = xmlIn.ReadElementContentAsString(); student.LastName = xmlIn.ReadElementContentAsString(); student.Email = xmlIn.ReadElementContentAsString(); student.AssessmentGrade = xmlIn.ReadElementContentAsInt(); student.AssignmentGrade = xmlIn.ReadElementContentAsInt(); persons.Add(student); } while (xmlIn.ReadToNextSibling("Student")); } else if (xmlIn.ReadToDescendant("Teacher")) { do { Person person = new Person(); Teacher teacher = (Teacher)person; xmlIn.ReadStartElement("Teacher"); teacher.FirstName = xmlIn.ReadElementContentAsString(); teacher.LastName = xmlIn.ReadElementContentAsString(); teacher.Email = xmlIn.ReadElementContentAsString(); teacher.RoomNumber = xmlIn.ReadElementContentAsInt(); persons.Add(teacher); } while (xmlIn.ReadToNextSibling("Teacher")); } if (xmlIn.ReadToDescendant("Person")) { do { Person person = new Person(); xmlIn.ReadStartElement("Person"); person.FirstName = xmlIn.ReadElementContentAsString(); person.LastName = xmlIn.ReadElementContentAsString(); person.Email = xmlIn.ReadElementContentAsString(); persons.Add(person); } while (xmlIn.ReadToNextSibling("Person")); } xmlIn.Close(); return persons; } } 

在我正在做的表单加载中：

  Persons personList = null; private void FillPersonlstBox() { Person p; listBox1.Items.Clear(); for (int i = 0; i < personList.Count; i++) { p = personList[i]; listBox1.Items.Add(p.ToString()); } } private void Load_Click(object sender, EventArgs e) { personList.Changed += new Persons.ChangeHandler(HandleChange); personList.Fill(); FillPersonlstBox(); } 

它告诉我** personList.Changed + = new Persons.ChangeHandler（HandleChange）; **此行错误。它显示“对象引用未设置为对象的实例”。 这个错误。 请大家帮帮我怎么解决？

Answer 1

在某些时候，你必须实例化personList 。

在您的代码中，您无法处理personList上的Changed事件，因为它仍然为null。

而不是

 Persons personList = null; 

也许试试吧

 Persons personList = new Persons();