構造器過載

Question

有人可以提出這裏有什麼問題嗎？

public class Student 
{ 
    public List<Class> Classes { get; set; } 

    public Student(List<Class> classes) 
    { 
     this.Classes = classes; 
    } 
    public Student(Class class) 
    { 
     //This does not work 
     //Error: Object reference not set to an instance of an object. 
     this.Classes.Add(class); 
    } 
} 

調用它作爲以下工作

var classes = new List<Classes>(); 
classes.Add(new Class("English", "Elective")); 
classes.Add(new Class("Math", "Core")); 
..more classes to add 
Student student = new Student(classes); 

當我有這樣調用（只有一個類添加）

Student student = new Student(new Class("Masters","Accounts")); 

我得到的錯誤。 謝謝。

Answer 1

您與類超載需要添加類之前初始化類。試試這個：

public class Student 
{ 
    public List<Class> Classes { get; set; } 

    public Student(List<Class> classes) 
    { 
     this.Classes = classes; 
    } 
    public Student(Class class) 
    { 
     this.Classes = new List<Class>(); 
     this.Classes.Add(class); 
    } 
} 

Answer 2

在嘗試向其中添加對象之前，您需要初始化Classes。

Answer 3

在第二構造，你永遠不會初始化Classes

Answer 4

this.Classes未初始化：

public class Student 
{ 
    public List<Class> Classes { get; set; } 

    public Student(List<Class> classes) 
    { 
     this.Classes = classes; 
    } 
    public Student(Class class) 
    { 
     this.Classes = new List<Class>(); 
     this.Classes.Add(class); 
    } 
}