简单工厂模式

C#代码
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

/*
*    简单工厂
*    封装业务，使业务逻辑和界面逻辑分开，让它们之间的耦合度下降
*    Operation类及其子类，都实现了可复用。
*    简单工厂模式的最大优点在于工厂类中包含了必要的逻辑依赖，根据客户端的选择条件动态实例化相关的类，对于客户端来说，去除了与具体产品的依赖。
*    但是，每当要增加功能时，都要修改OperationFactory中的switch语句，这样就违背了开放-封闭原则。
*/
namespace Calculater
{
    public class Operation{
        private double _numberA = 0;
        private double _numberB = 0;

        public double NumberA
        {
            get { return _numberA; }
            set { _numberA = value;}
        }
        public double NumberB
        {
            get { return _numberB; }
            set { _numberB = value; }
        }
        public virtual double GetResult()
        {
            double result = 0;
            return result;
        }
    }
    class OperationAdd : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumberA + NumberB;
            return result;
        }
    }
    class OperationSub : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumberA - NumberB;
            return result;
        }
    }
    class OperationMul : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumberA * NumberB;
            return result;
        }
    }
    class OperationDiv : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            if (NumberB == 0)
                throw new Exception("除数不能为0.");
            result = NumberA / NumberB;
            return result;
        }
    }
    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 OperationMul();
                    break;
                case "/":
                    oper = new OperationDiv();
                    break;
            }
            return oper;
        }
    }
    //Client
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                Operation oper;
                oper = Oper

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