c# - 在不违反开闭原则的情况下重构工厂

Question

我根据文章设计了两个版本的工厂类

http://www.oodesign.com/factory-pattern.html

public abstract class Employee
{
    public string Name { get; set; }
    protected string Role { get; set; }
    public abstract string GetRole();
}

public class Manager : Employee
{
    public Manager()
    {
        Role = "MGR";
    }
    public override string GetRole()
    {
        return this.Role;
    }
}

版本 1: 简单，违反开闭原则

每次添加新的具体类时都需要更改 SimpleEmployeeFactory

public class SimpleEmployeeFactory
{
    public static Employee GetEmployee(int typeId)
    {
        switch (typeId)
        {
            case 1:
                return new Manager();
            case 2:
                return new TechnicalLead();
            default:
                return null; //if the id doesn't have any 
        }
    }
}

版本 2:

重构工厂，在我们使用工厂调用之前仍然需要一个具体类的创建

public abstract class Employee
{
    public string Name { get; set; }
    protected string Role { get; set; }
    public abstract string GetRole();      
    public abstract Employee createEmployee();
} 

public class ChiefTechnologyOfficer : Employee
{
    public ChiefTechnologyOfficer()
    {
        this.Role = "CTO";
    }
    static ChiefTechnologyOfficer()
    {            
        RefactoredFactory.Instance.registerEmployee(5, new ChiefTechnologyOfficer());
    }
    public override string GetRole()
    {            
        return this.Role;
    }        
    public override Employee createEmployee()
    {
        return new ChiefTechnologyOfficer();
    }
}

工厂

class RefactoredFactory
{
    private static readonly RefactoredFactory instance = new RefactoredFactory();

    static RefactoredFactory()
    {
    }

    private RefactoredFactory()
    {
    }

    public static RefactoredFactory Instance
    {
        get
        {
            return instance;
        }
    }

    private Dictionary<int, Employee> registeredEmployees = new Dictionary<int, Employee>();

    public void registerEmployee(int typeId, Employee employeeInst)
    {
        registeredEmployees.Add(typeId, employeeInst);
    }

    public Employee createEmployee(int typeId)
    { 
        return ((Employee)registeredEmployees[typeId]).createEmployee();
    }
}

客户端

 Employee emp = SimpleEmployeeFactory.GetEmployee(1);
 Activator.CreateInstance(typeof(ChiefTechnologyOfficer)); //Avoid
 Employee empFNoR = RefactoredFactory.Instance.createEmployee(5);

您可以看到 Activator.CreateInstance(typeof(ChiefTechnologyOfficer)) 调用使具体类向工厂注册自己。否则我们无法检索对象

有没有一种方法可以在不违反 OCP 原则的情况下创建工厂类并且不创建像我在 RefactoredFactory 类中使​​用的对象那样的对象？

Answer 1

看起来 typeId 受到了Feature Envy 的困扰。相反，定义一个多态类型来捕获类型；例如一个接口(interface):

public interface IEmployeeType
{
    Employee Create()
}

现在您可以定义例如ManagerType 和 TechnicalLeadType 等。示例:

public class ManagerType : IEmployeeType
{
    public Employee Create()
    {
        return new Manager();
    }
}

这本质上是一个 Abstract Factory ，它的优点是当您需要创建一个新的子类型时，您总是可以创建一个新的实现。

如果您处于系统的边界，并且必须将原始值(如整数)转换为多态值，您可以使用 Role Hint patterns 之一- 特别是 Metadata , Role Interface ，或(我最喜欢的)Partial Type Name .

客户端

给定一个 IEmployeeType 实例 employeeType，客户端会简单地去:

Employee emp = employeeType.Create();