C＃继承&铸件 [英] C# Inheritance & Casting

问题描述

我有以下代码，我得到以下异常InvalidCastException：无法将'Employee'类型的对象强制转换为'EmployeeProfile'。

I have the following code, I get the following exception "InvalidCastException: Unable to cast object of type 'Employee' to type 'EmployeeProfile'."

    private class Employee
    {
        public string Name { get; private set; }

        public Employee()
        {
            this.Name = "employee";
        }

        public override string ToString()
        {
            return this.Name;
        }
    }

    private class EmployeeProfile : Employee
    {
        public string Profile { get; private set; }

        public EmployeeProfile() : base()
        {
            this.Profile = string.Format("{0}'s profile", this.Name);
        }

        public override string ToString()
        {
            return this.Profile;
        }
    }

    public void RunTest()
    {
        Employee emp = new Employee();
        EmployeeProfile prof = (EmployeeProfile)emp; // InvalidCastException here

        System.Console.WriteLine(emp);
        System.Console.WriteLine(prof);
    }

也许我的大脑已经烧坏，但我认为你可以投下一个子类型它的基本类型？我在这里想念的是什么？也许这是一个假期...谢谢你！

Maybe my brain is burned out, but I thought you can cast a subtype to its base type? What am I missing here? Maybe it is a vacation... thank you!

推荐答案

你可以为它投一个子类型基础类型。但是您要将基类型的实例转换为子类型。

You can cast a subtype to its base type. But you are casting an instance of the base type to the subtype.

EmployeeProfile是一个Employee。不一定相反。

An EmployeeProfile is-an Employee. Not necessarily the other way around.

因此将工作：

EmployeeProfile prof = new EmployeeProfile();
Employee emp = prof;

然而，这个模型充满了糟糕的设计。员工档案不是一种特殊的员工，是吗？员工拥有个人资料更有意义。你是在这里的组合模式。

However, this model reeks of bad design. An employee profile is not a special kind of an employee, is it? It makes more sense for an employee to have a profile. You are after the composition pattern here.

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