简单工厂模式

 Operation运算类

    class Operation
    {
        private double _numberA = 0;
        public double NumberA
        {
            get { return _numberA; }
            set { _numberA = value; }
        }

        private double _numberB = 0;
        public double NumberB
        {
            get { return _numberB; }
            set { _numberB = value; }
        }

        public virtual double GetResult()
        {
            double result = 0;
            return result;
        }
    }

加减乘除类

    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;
        }
    }

工厂类

    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;
        }
    }

客户端代码

            Operation oper = OperationFactory.CreateOperate("/");
            oper.NumberA = 1;
            oper.NumberB = 0;
            double result = oper.GetResult();

总结：

　　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判断，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。但此模式违背了开放-封闭原则。