当参数类型是基类时，将Derived类作为参数传递给方法 [英] Passing Derived class as a parameter to a method when the parameter type is base class

问题描述

我是一个新手，并试图理解继承和设计模式的概念。

I am a newbie and trying to understand concepts of inheritance and design patterns.

我遇到了这种模式 http://en.wikipedia.org/wiki/Strategy_pattern 。

我发现它很有趣，想要了解更多。所以我开发了以下程序。

I found it interesting and wanted to learn more. So I developed the following program.

static void Main(string[] args)
{
    Context context;

    // Three contexts following different strategies
    context = new Context(new ConcreteStrategyAdd());
    int resultA = context.executeStrategy(3, 4);

    context = new Context(new ConcreteStrategySubtract());
    int resultB = context.executeStrategy(3, 4);

    context = new Context(new ConcreteStrategyMultiply());
    int resultC = context.executeStrategy(3, 4);

    Console.Read();
}      

abstract class Strategy
{
    public abstract int execute(int a, int b);

    public void Test()
    {
        Console.Write("tttt");
    }
}

class ConcreteStrategyAdd : Strategy
{
    public override int execute(int a, int b)
    {
        Console.WriteLine("Called ConcreteStrategyAdd's execute()");
        return a + b;  // Do an addition with a and b
    }
}

class ConcreteStrategySubtract : Strategy
{

    public override int execute(int a, int b)
    {
        Console.WriteLine("Called ConcreteStrategySubtract's execute()");
        return a - b;  // Do a subtraction with a and b
    }
}

class ConcreteStrategyMultiply : Strategy
{
    public override int execute(int a, int b)
    {
        Console.WriteLine("Called ConcreteStrategyMultiply's execute()");
        return a * b;   // Do a multiplication with a and b
    }
}

class Context
{
    private Strategy strategy;

    // Constructor
    public Context(Strategy strategy)
    {
        this.strategy = strategy;
    }

    public int executeStrategy(int a, int b)
    {
        return strategy.execute(a, b);
    }
}

该程序编译正常并且正在运行。但我的问题是，当 Context 构造函数期望基类作为参数时，如何将派生类作为参数传递？铸造是否隐含发生？为什么编译器没有错误？

The program compiles fine and is working. But my question is how is it possible to pass derived class as a parameter when the Context constructor is expecting base class as a parameter? Is the casting happening implicitly? Why does the compiler does not error?

context = new Context(new ConcreteStrategyAdd());  

public Context(Strategy strategy)
{
    this.strategy = strategy;
}

推荐答案

简单地说：

派生类（或子类） 是其基类的实例。

因此，当您将 ConcreteStrategyAdd 的实例传递给构造函数时，您实际上是在传递策略对象输入。

So, when you pass an instance of ConcreteStrategyAdd into the constructor, you are essentially passing a Strategy object in.

不涉及任何转换。类型层次结构允许这种类型的编程。它允许程序员在代码中使用多态性。

There is no casting involved. The type hierarchy allows for this type of programming. It allows programmers to use polymorphism in their code.

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