没有开关或If/Then的工厂模式

Question

我正在寻找一个如何实现工厂类的简单示例,但不使用Switch或If-Then语句.我能找到的所有例子都使用一个.例如,如何修改这个简单示例(如下所示),以便实际工厂不依赖于Switch？在我看来,这个例子违反了开/关原则.我希望能够添加具体的类('经理','文员','程序员'等),而无需修改工厂类.

谢谢!

class Program
{
    abstract class Position
    {
        public abstract string Title { get; }
    }

    class Manager : Position
    {
        public override string Title
        {
            get  { return "Manager"; }
        }
    }

    class Clerk : Position
    {
        public override string Title
        {
            get { return "Clerk"; }
        }
    }

    class Programmer : Position
    {
        public override string Title
        {
            get { return "Programmer"; }
        }
    }

    static class Factory
    {
        public static Position Get(int id)
        {
            switch (id)
            {
                case 0: return new Manager();
                case 1: return new Clerk();
                case 2: return new Programmer();
                default: return new Programmer();
            }
        }
    }

    static void Main(string[] args)
    {
        for (int i = 0; i <= 2; i++)
        {
            var position = Factory.Get(i);
            Console.WriteLine("Where id = {0}, position = {1} ", i, position.Title);
        }
        Console.ReadLine();
    }
}

更新:

哇!感谢大家!我学到了很多东西.在收到所有反馈后,我混合了一些答案并提出了这个问题.我愿意进一步讨论更好的方法.

class Program
{

    public interface IPosition
    {
        string Title { get; }
    }

    class Manager : IPosition
    {
        public string Title
        {
            get { return "Manager"; }
        }
    }

    class Clerk : IPosition
    {
        public string Title
        {
            get { return "Clerk"; }
        }
    }

    class Programmer : IPosition
    {
        public string Title
        {
            get { return "Programmer"; }
        }
    }

static class PositionFactory
{
    public static T Create<T>() where T : IPosition, new()
    {
        return new T();
    }
}


static void Main(string[] args)
    {

        IPosition position0 = PositionFactory.Create<Manager>();
        Console.WriteLine("0: " + position0.Title);

        IPosition position1 = PositionFactory.Create<Clerk>();
        Console.WriteLine("1: " + position1.Title);

        IPosition position2 = PositionFactory.Create<Programmer>();
        Console.WriteLine("1: " + position2.Title);

        Console.ReadLine();
    }
}

另一个编辑:

也可以使用未知类型创建接口的实例:

static class PositionFactory
{
   public static IPosition Create(string positionName)
    {       
        Type type = Type.GetType(positionName);
        return (IPosition)Activator.CreateInstance(type);
    }
}

然后可以如下调用:

IPosition position = PositionFactory.Create("Manager");
Console.WriteLine(position.Title);

Answer 1

这个怎么样(不需要字典,请注意,如果你尝试,你会得到一个语法错误Create<Position>()):

编辑 - 更新为使用明确实现的IPosition接口.只有IPosition的实例才能访问成员函数(例如,<implementation of Manager>.Title不会编译).

编辑#2 Factory.Create在正确使用界面时应该返回一个IP而不是T.

using System;
using System.Collections.Generic;

class Program
{
    interface IPosition
    {
        string Title { get; }
        bool RequestVacation();
    }

    class Manager : IPosition
    {
         string IPosition.Title
        {
            get { return "Manager"; }
        }

        bool IPosition.RequestVacation()
        {
            return true;
        }
    }

    class Clerk : IPosition
    {
        int m_VacationDaysRemaining = 1;

        string IPosition.Title
        {
            get { return "Clerk"; }
        }

        bool IPosition.RequestVacation()
        {
            if (m_VacationDaysRemaining <= 0)
            {
                return false;
            }
            else
            {
                m_VacationDaysRemaining--;
                return true;
            }
        }
    }

    class Programmer : IPosition
    {
        string IPosition.Title
        {
            get { return "Programmer"; }
        }

        bool IPosition.RequestVacation()
        {
            return false;
        }
    }

    static class Factory
    {
        public static IPosition Create<T>() where T : IPosition, new ()
        {
            return new T();
        }
    }

    static void Main(string[] args)
    {
        List<IPosition> positions = new List<IPosition>(3);
        positions.Add(Factory.Create<Manager>());
        positions.Add(Factory.Create<Clerk>());
        positions.Add(Factory.Create<Programmer>());

        foreach (IPosition p in positions) { Console.WriteLine(p.Title);  }
        Console.WriteLine();

        Random rnd = new Random(0);
        for (int i = 0; i < 10; i++)
        {
            int index = rnd.Next(3);
            Console.WriteLine("Title: {0}, Request Granted: {1}", positions[index].Title, positions[index].RequestVacation());
        }

        Console.ReadLine();
    }
}

Answer 2

您可以使用自定义属性和反射.

[PositionType(1)]
class Manager : Position
{
    public override string Title
    {
        get
        { return "Manager"; }
    }
}

[PositionType(2)]
class Clerk : Position
{
    public override string Title
    {
        get
        { return "Clerk"; }
    }
}

然后,在您的工厂中,您可以获取所有继承的类,Position并找到具有PositionType正确值的属性的类.

static class Factory
{
    public static Position Get(int id)
    {
        var types = typeof(Position).Assembly.GetTypes()
            .Where(t => !t.IsAbstract && t.IsSubclassOf(typeof(Position)))
            .ToList();

        Position position = null;
        foreach(var type in types)
        {
           type.GetCustomAttributes<PositionTypeAttribute>();

           if(type.PositionId == id)
           {
               position = Activator.CreateInstance(type) as Position;
               break;
           }
        }

        if(position == null)
        {
            var message = $"Could not find a Position to create for id {id}.";
            throw new NotSupportedException(message);
        }

        return position;
    }
}