工厂方法模式

开放封闭原则：是说软件实体（类，模块，函数等等）应该可以扩展，但是不可修改。

核心

定义一个用于创建对象的接口，让子类决定实例化哪一个类。

好处

工厂方法模式是简单工厂模式的衍生，解决了许多简单工厂模式的问题。首先完全实现‘开放封闭原则’，实现了可扩展。其次更复杂的层次结构，可以应用于产品结果复杂的场合。工厂方法模式对简单工厂模式进行了抽象。有一个抽象的Factory类（工厂的工厂），这个类将不再负责具体的产品生产，而是只制定一些规范，具体的生产工作由其子类去完成。

例子

情景是：简单计算器，我们输入2个数和一个符号得到运算结果。

首先使用简单工厂模式来实现

简单工厂UML类图

//客户端代码
class Program
{
    static void Main(string[] args)
    {
        try
        {
            Console.Write("请输入数字A：");
            string strNumberA = Console.ReadLine();
            Console.Write("请选择运算符（+，-，*，/）：");
            string strOperate = Console.ReadLine();
            Console.Write("请输入数字B：");
            string strNumberB = Console.ReadLine();
            string strResult = "";

            Operation oper;
            //由工厂来决定生成哪个运算子类
            oper = OperationFactory.createOperate(strOperate);
            oper.NunberA = Convert.ToDouble(strNumberA);
            oper.NunberB = Convert.ToDouble(strNumberB);
            strResult = Convert.ToString(oper.GetResult());

            Console.WriteLine("结果是:" + strResult);
            Console.ReadLine();

        }
        catch (Exception ex)
        {
            Console.WriteLine("输入非法字符。请输入数字" + ex.Message);
        }

    }
}
//运算父类
public class Operation
{
    private double _numberA = 0;
    private double _numberB = 0;

    public double NunberA { get; set; }
    public double NunberB { get; set; }

    public virtual double GetResult()
    {
        double result = 0;
        return result;
    }
}
//运算子类
class OperationAdd : Operation
{
    public override double GetResult()
    {
        double result = 0;
        result = NunberA + NunberB;
        return result;
    }
}

class OperationSub : Operation
{
    public override double GetResult()
    {
        double result = 0;
        result = NunberA - NunberB;
        return result;
    }
}

class OperationMult : Operation
{
    public override double GetResult()
    {
        double result = 0;
        result = NunberA * NunberB;
        return result;
    }
}

class OperationDiv : Operation
{
    public override double GetResult()
    {
        double result = 0;
        if (NunberB == 0)
            throw new Exception("除数不能为0!");
        result = NunberA / NunberB;
        return result;
    }
}
//简单工厂类
public class OperationFactory
{
    public static Operation createOperate(string Operate)
    {
        Operation oper = null;
        switch (Operate)
        {
            case "+":
                oper = new OperationAdd();
                break;
            case "-":
                oper = new OperationSub();
                break;
            case "*":
                oper = new OperationMult();
                break;
            case "/":
                oper = new OperationDiv();
                break;
        }
        return oper;
    }
}

然后我们用工厂方法模式实现：我们省略了运算子类和父类，只集中处理不同部分：

工厂方法模式UML类图

//工厂接口
    interface IFactory
    {
        Operation CreateOperation();
    }

    //接口实现类
    class AddFactory : IFactory
    {
        public Operation CreateOperation()
        {
            return new OperationAdd();
        }
    }
    class SubFactory : IFactory
    {
        public Operation CreateOperation()
        {
            return new OperationSub();
        }
    }
    class MultFactory : IFactory
    {
        public Operation CreateOperation()
        {
            return new OperationMult();
        }
    }
    class DivFactory : IFactory
    {
        public Operation CreateOperation()
        {
            return new OperationDiv();
        }
    }
//客户端
static void Main(string[] args)
{
    try
    {
        Console.Write("请输入数字A：");
        string strNumberA = Console.ReadLine();
        Console.Write("请选择运算符（+，-，*，/）：");
        string strOperate = Console.ReadLine();
        Console.Write("请输入数字B：");
        string strNumberB = Console.ReadLine();
        string strResult = "";
                
        switch (strOperate)
        {
            case "+":
                IFactory operFactory = new AddFactory();
                Operation oper = operFactory.CreateOperation();
                oper.NunberA = Convert.ToDouble(strNumberA);
                oper.NunberB = Convert.ToDouble(strNumberB);
                strResult = Convert.ToString(oper.GetResult());
                break;
            case "-":
                IFactory operFactory1 = new SubFactory();
                break;
            case "*":
                IFactory operFactory2 = new MultFactory();
                break;
            case "/":
                IFactory operFactory3 = new DivFactory();
                break;
        }
 
        Console.WriteLine("结果是:" + strResult);
        Console.ReadLine();
    }
    catch (Exception ex)
    {
        Console.WriteLine("输入非法字符。请输入数字" + ex.Message);
    }

}

例子来源:Linux公社《大话设计模式》
实例代码在我的代码托管上
简单工厂模式
工厂方法模式