Factory Pattern without a Switch or If/Then

Question

I\'m looking for a simple example of how to implement a factory class, but without the use of a Switch or an If-Then statement. All the examples I can find use one. F

Answer 1

Why overcomplicate things? Here is one simple solution:

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();
    }
}

Here is how to do this without using factory class at all:

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();
    }
}

Answer 2

You could make use of custom attributes and reflection.

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

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

In your factory you could then get all classes that inherit from Position and find the one that has the PositionType attribute with the correct value.

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 3

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]);
    }
}

Used like this:

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

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

Answer 4

How about this (no Dictionary required and note that you will get an syntax error if your try to Create<Position>()):

EDIT - Updated to use an IPosition interface implemented explicitly. Only instances of IPosition can access the member functions (e.g. <implementation of Manager>.Title will not compile).

EDIT #2 Factory.Create should return an IPosition not T when using the interface properly.

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();
    }
}