访问者模式的实现

Question

我尝试实现访问者模式，并对使用is和as关键字有疑问。在使用它们时，是否有很大的性能影响？

 class Program
{
    static void Main(string[] args)
    {
        List<IVisitor> visitors = new List<IVisitor>();
        visitors.AddRange(new List<IVisitor>()
        {
            new PersonVisitor(),
            new AnimalVisitor()
        });

        List<Creature> creatures = new List<Creature>();
        creatures.AddRange(new List<Creature>()
        {
            new Person() { Name = "Frank" },
            new Person() { Name = "Tony" },
            new Person() { Name = "Amy" },
            new Animal() { Name = "Bubbles" },
            new Animal() { Name = "Max" }
        });

        CreatureProcessor creatureProcessor = new CreatureProcessor(creatures, visitors);
        creatureProcessor.Process();

        Console.ReadKey();
    }
}

interface IVisitor
{
    void Visit(IElement element);
}

interface IElement
{
    void Accept(IVisitor visitor);
}

class PersonVisitor : IVisitor
{
    public void Visit(IElement element)
    {
        if (element is Person)
        {
            Creature creature = element as Person;

            Console.WriteLine("{0} is a good {1}", creature.Name, "Person");
        }
    }
}

class AnimalVisitor : IVisitor
{
    public void Visit(IElement element)
    {
        if (element is Animal)
        {
            Creature creature = element as Animal;

            Console.WriteLine("{0} is a good {1}", creature.Name, "Animal");
        }
    }
}

class Person : Creature
{

}

class Animal : Creature
{

}

abstract class Creature : IElement
{
    public string Name { get; set; }

    public void Accept(IVisitor visitor)
    {
        visitor.Visit(this);
    }
}

class CreatureProcessor
{
    private readonly List<IVisitor> visitors;
    private readonly List<Creature> creatures;        

    public CreatureProcessor(List<Creature> creatures, List<IVisitor> visitors)
    {
        this.creatures = creatures;
        this.visitors = visitors;
    }

    public void Process()
    {
        foreach (IVisitor visitor in visitors)
        {
            foreach (Creature creature in creatures)
            {
                creature.Accept(visitor);
            }
        }
    }
}

它的工作方式，我应该怎么想，所以谁愿意批评它呢？

Answer 1

只关注is和as

类PersonVisitor : IVisitor {公共空访问(IElement元素){ if (元素为人){生物=元素为人；Console.WriteLine("{0}是良好的{1}“、creature.Name、”人“)；}}

is基本上与as相同。它正在对所讨论的对象执行一个软广播，一个没有抛出异常的强制转换。不同之处在于，is返回一个bool，说明结果，而as返回成功，返回已抛出的对象，否则返回null。所以你做了两次同样的事。

一个更好、更快的方法会是这样。

class PersonVisitor : IVisitor
{
    public void Visit(IElement element)
    {
        Creature creature = element as Person;
        if (creature != null)
        {
            Console.WriteLine("{0} is a good {1}", creature.Name, "Person");
        }
    }
}