我想從XML文件加載數據並顯示在列表框中？

Question

我有三類：

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; 
    } 
} 

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; 
    } 
} 

三是：

public class Teacher:Person 
{ 
    public int RoomNumber 
    { 
     get; 
     set; 
    } 

    public override string ToString() 
    { 
     return base.ToString() + "," + RoomNumber; 
    } 
} 

我還有一個類，其中i只需調用來自PeronDB（Class）的數據

public class Persons 
{ 
    private List<Person> persons = null; 

     public void Fill() 
    { 
     persons = PersonDB.GetPersons(); 
    } 
} 

InPersonDB class如果其Student節點隨後讀取數據（如果其Teacher節點隨後也讀取數據並且如果它Person也讀取數據），則執行此操作。

public class PersonDB 
{ 
    private const string path = @"..\..\Persons.xml"; 

    public static List<Person> GetPersons() 
    { 
     List<Person> persons = new List<Person>(); 

     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; 
    } 
    } 

並在窗體的Load我在做什麼：

 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(); 
    } 

它告訴我在** personList.Changed + =新Persons.ChangeHandler（HandleChange）; **這行錯誤。它顯示我「對象引用未設置爲對象的實例」。這個錯誤。 請大家幫幫我如何解決它？

Answer 1

在某些時候你必須實例personList。

在你的代碼，因爲它仍然是空你不能處理上personList的Changed事件。

所以不是

Persons personList = null; 

也許嘗試

Persons personList = new Persons();