没有开关的工厂模式或 If/Then

Question

我正在寻找一个如何实现工厂的简单示例 class，但是没有使用 Switch 或 If-Then 语句。我能找到的所有示例都使用一个。例如，如何修改这个简单的示例（如下），使实际的工厂不依赖于 Switch？在我看来，这个例子违反了 Open/Close 原则。我希望能够添加具体的 classes（'Manager'、'Clerk'、'Programmer' 等）而无需修改工厂 class。

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。在审查了所有反馈后，我混合了一些答案并得出了这个结论。我愿意就更好的方法进行进一步的对话。

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);
    }
}

然后可以这样调用：

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

Answer 1

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，您可以获得所有继承自 Position 的类，并找到具有正确值的 PositionType 属性的那个。

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;
    }
}

Answer 2

public class PositionFactory
{
    private Dictionary<int, Type> _positions;

    public PositionFactory()
    {
        _positions = new Dictionary<int, Type>();
    }

    public void RegisterPosition<PositionType>(int id) where PositionType : Position
    {
        _positions.Add(id, typeof(PositionType));
    }

    public Position Get(int id)
    {
        return (Position) Activator.CreateInstance(_positions[id]);
    }
}

这样使用：

            var factory = new PositionFactory();
            factory.RegisterPosition<Manager>(0);
            factory.RegisterPosition<Clerk>(1);

            Position p = factory.Get(0); //Returns a new Manager instance

Answer 3

这个怎么样（不需要词典，请注意，如果您尝试 Create<Position>() 会出现语法错误）：

EDIT - 更新为使用显式实现的 IPosition 接口。只有 IPosition 的实例可以访问成员函数（例如 <implementation of Manager>.Title 将无法编译）。

编辑 #2 Factory.Create 正确使用界面时应该 return IPosition 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 4

为什么要把事情复杂化？这是一个简单的解决方案：

using System;
using System.Collections.Generic;

class Program
{
    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"; }
        }
    }

    class Factory
    {
        private List<IPosition> positions = new List<IPosition>();
        public Factory()
        {
            positions.Add(new Manager());
            positions.Add(new Clerk());
            positions.Add(new Programmer());
            positions.Add(new Programmer());
        }

        public IPosition GetPositions(int id)
        {
            return positions[id];
        }
    }

    static void Main(string[] args)
    {
        Factory factory = new Factory();

        for (int i = 0; i <= 2; i++)
        {
            var position = factory.GetPositions(i);
            Console.WriteLine("Where id = {0}, position = {1} ", i, position.Title);
        }
        Console.ReadLine();
    }
}

下面是完全不使用工厂 class 的方法：

using System;
using System.Collections.Generic;

class Program
{
    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 void Main(string[] args)
    {
        List<IPosition> positions = new List<IPosition> { new Manager(), new Clerk(), new Programmer(), new Programmer() };

        for (int i = 0; i <= 2; i++)
        {
            var position = positions[i];
            Console.WriteLine("Where id = {0}, position = {1} ", i, position.Title);
        }
        Console.ReadLine();
    }
}

没有开关的工厂模式或 If/Then

Factory Pattern without a Switch or If/Then

c#

factory

factory-pattern