传递给 C# 中的属性时对象变为 null

Question

我有一个抽象类 Employee 和其他 2 个扩展它的类（开发人员和经理）。我的问题是，当我创建一个 Manager

Employee man = new Manager(1234567, 30, "Bob", "Pie")

并尝试将其设置在新开发人员的 Manager 字段中时，

Employee codemonkey = new Developer(1234568, 20, "Code", "Monkey", (Manager)man)

我不断收到 ArgumentException ，表明我的 Manager 为空。我做了一些检查，当我尝试使用构造函数中的 Manager 属性设置它时，它不知何故变成了空。任何关于我为什么会收到此错误的建议将不胜感激。蒂亚！

每个类的代码如下：

//Employee Class

public abstract class Employee
{
    string firstName, lastName;
    int id, yearsEmployed;

    //Names must be non-empty
    public string FirstName
    {
        get { return firstName; }
        set
        {
            if (!value.Equals(""))
            {
                firstName = value;
            }
            else
                throw new ArgumentException("name cannot be empty");
        }
    }
    public string LastName
    {
        get { return lastName; }
        set
        {
            if (!value.Equals(""))
            {
                lastName = value;
            }
            else
                throw new ArgumentException("name cannot be empty");
        }
    }
    // IDs must be strings consisting of exactly seven digits.
    public int ID
    {
        get { return id; }
        set
        {
            if (value.ToString().Length == 7)
            {
                id = value;
            }
            else
                throw new ArgumentException("ID must consist of 7 digits");
        }
    }
    // Years employed must always be non-negative.
    public int YearsEmployed
    {
        get { return yearsEmployed; }
        set
        {
            if (value >= 0)
            {
                yearsEmployed = value;
            }
            else
                throw new ArgumentException("Year employed must be non-negative");
        }
    }
    //Constructor
    public Employee(int id, int yearsEmployed,
                    string firstName, string lastName)
    {
        this.FirstName = firstName;
        this.LastName = lastName;
        this.ID = id;
        this.YearsEmployed = yearsEmployed;
    }
    public abstract int GetLevel { get; }
    public abstract string GetTitle { get; }
    public string GetFullTitle { get { return GetTitle + " " + GetLevel; } }
}

//Developer Class:

 public class Developer : Employee
{
    Manager manager;

    //Manager cannot be null
    public Manager Manager
    {
        get { return manager; }
        set
        {
            if (manager != null)
            {
                manager = value;
            }
            else
                throw new ArgumentException("Manager cannot be null");
        }
    }

    //Constructor
    public Developer(int id, int yearsEmployed, string firstName,
                    string lastName, Manager manager)
        : base(id, yearsEmployed, firstName, lastName)
    {
        Console.WriteLine("manager is not null:" + manager != null); //True here
        this.Manager = manager; // manager is null here
    }

    public override int GetLevel
    {
        get { return (this.YearsEmployed + 1) / 3; }
    }

    public override string GetTitle
    {
        get { return "Developer"; }
    }
}

//Manager Class

public class Manager : Employee
{
    //Constructor
    public Manager(int id, int yearsEmployed,
                    string firstName, string lastName)
        : base(id, yearsEmployed, firstName, lastName) { }

    public override int GetLevel
    {
        get { return (YearsEmployed + 1) / 2; }
    }

    public override string GetTitle
    {
        get { return "Manager"; }
    }
}

Answer 1

你不想说：

if (value != null)

而不是

if (manager != null)

Manager 字段将被初始化为 null。 value 关键字表示传递给属性的数据。

Answer 2

更改

 if (manager != null) 

为

 if (value != null) 

Answer 3

您从未设置字段管理器的值，仅设置属性管理器的值，因此在属性中，当您检查管理器的值时，它为空，因为它尚未设置。您可以在构造函数中设置字段管理器：

this.manager=manager

并检查属性中的值

if (value!=null)
{
     manager =value;
}

（无论如何您都需要执行此操作，否则您赋予属性的值永远不会被使用并且永远不会更新）

取决于您是否想要管理器能够被改变。

Answer 4

在 Developer.Manager 的 setter 中更改

if (manager != null)

为

if (value != null)