C# 继承不符合我的预期

Question

我正在研究头脑优先的设计模式，但我正在用 C# 而不是 java 编写示例。我编写了装饰器模式代码，但我遇到了一些我不知道如何修复的奇怪错误。第 1 行按预期运行，描述为 "Dark Roast"，但是第二行将描述更改为 "unknown beverage" 而不是预期的 "Dark Roast, Mocha." 如何解决此问题？

主程序：

beverage = new DarkRoast();
beverage = new Mocha(beverage);
beverage = new Mocha(beverage);
Console.WriteLine(beverage.Description + " $" + beverage.Cost);

装饰器代码：

namespace DecoratorPattern
{
    public abstract class Beverage
    {
        public abstract double Cost { get; }

        public string Description { get; set; } = "unknown beverage";
    }

    public class DarkRoast : Beverage
    {
        public DarkRoast()
        {
            Description = "Dark Roast";
        }

        public override double Cost => .99;
    }

    public abstract class CondimentDecorator : Beverage
    {
        public abstract string Description { get; }
    }

    public class Mocha : CondimentDecorator
    {
        Beverage beverage;

        public Mocha(Beverage beverage)
        {
            this.beverage = beverage;
        }

        public override string Description
        {
            get { return beverage.Description + ", Mocha"; }
        }

        public override double Cost => .20 + beverage.Cost;
    }
}

Answer 1

您没有在 Beverage class 中将 Description 属性标记为 virtual。因此，如果您的变量 beverage 是 Beverage，则 Description 属性将始终从那里读取。该值将始终为默认值未知。

只需从 CondimentDecorator class 中删除 abstract 声明并将 virtual 添加到基础 class，然后您的覆盖将起作用。

Answer 2

您将原来的 Description 属性隐藏在 CondimentDecorator 中，如果您更改输入 CondimentDecorator 然后你会得到预期的结果。

这应该打印出您期望的值：

 var result = (CondimentDecorator) beverage;
 Console.WriteLine(result.Description);

您需要将基础更改为抽象或虚拟，并在继承 classes

上覆盖它

public abstract class Beverage
{
    public abstract double Cost { get; }

    public virtual Description { get; set; } = "unknown beverage";
}

此时你不需要额外的装饰器class只需覆盖基础对象中的方法

public class Mocha : Beverage
{
    Beverage beverage;

    public Mocha(Beverage beverage)
    {
        this.beverage = beverage;
    }

    public override string Description
    {
        get { return beverage.Description + ", Mocha"; }
        set { throw InvalidOperationException("Cannot Set Value of Decorator"); }
    }

    public override double Cost => .20 + beverage.Cost;
}

C# 继承不符合我的预期

C# inheritance doesn't behave as I expect it to

c#

inheritance

decorator