大话设计模式：第一章简单工厂模式

　　面向对象的三大特性：封装，继承，多态。

本文只是第一章计算器例子的源代码分享：

1，定义一个运算基类，所有的运算都继承自它：

    /// <summary>
    /// 所有运算的基类
    /// </summary>
    class Operation
    {
        public double NumberA;
        public double NumberB;     
        /// <summary>
        /// 计算结果让子类重写
        /// </summary>        
        public virtual double GetResult()
        {
            double result=0d;
            return result;
        }
    }

2，定义所有的运算类，继承自运算基类Operation：

    /// <summary>
    /// 下面的类都继承自Operation,重载了GetResult()
    /// </summary>
    class OperationAdd : Operation
    {
        public override double GetResult()
        {
            return (NumberA + NumberB);     
        }
    }
    class OperationSub : Operation
    {
        public override double GetResult()
        {
            return (NumberA - NumberB);
        }
    }
    class OperationMul : Operation
    {
        public override double GetResult()
        {
            return (NumberA * NumberB);
        }
    }
    class OperationDiv : Operation
    {
        public override double GetResult()
        {
            if (NumberB == 0)
                throw new Exception("除数不能为0");
            return (NumberA / NumberB);
        }
    }

3，定义一个简单工厂类，决定实例化哪一个对象：

    /// <summary>
    /// 简单工厂类，判断创建哪一个实例
    /// </summary>
    class OperationFactory
    {       
        /// <summary>
        /// 根据运算符判断实例化哪个对象,静态方法可以直接调用
        /// </summary>
        /// <param name="opr">运算符</param>
        /// <returns>被实例化的运算对象</returns>
        public static Operation CreateObject(string opr)
        {
            Operation OpeObject = null;
            switch (opr)
            {
                case "+":
                    OpeObject = new OperationAdd();
                    break;
                case "-":
                    OpeObject = new OperationSub();
                    break;
                case "*":
                    OpeObject = new OperationMul();
                    break;
                case "/":
                    OpeObject = new OperationDiv();
                    break;                    
            }
            return OpeObject;
        }        
    }

4，在Main方法中测试：

    class Program
    {
        static void Main(string[] args)
        {
            while (true)
            {
                try
                {
                    Console.WriteLine("请输入两个数(每行一个)：");
                    string NumberA = Console.ReadLine();
                    string NumberB = Console.ReadLine();
                    Console.WriteLine("请输入运算符+,-,*,/");
                    string Opr = Console.ReadLine();

                    Operation ope = OperationFactory.CreateObject(Opr);
                    ope.NumberA = Convert.ToDouble(NumberA);
                    ope.NumberB = Convert.ToDouble(NumberB);

                    string rusult = Convert.ToString(ope.GetResult());
                    Console.WriteLine("运算结果是:{0}", rusult);
                }
                catch (Exception ex)
                {
                    Console.WriteLine(ex.Message);
                }
            }        
        }
    }