我对简单工厂模式的理解

    简单工厂模式就是用一个工厂来生产(实例)有共同属性和方法但又不相同的事物.可以很方便的增加有共同的方法和属性的事物.
个人认为的好处有:
    1,增加有共同的方法和属性的事物,不会修改已有的事物.
    2,结构清晰明了.

    从一个计算器的例子里我们可以看出:
    一个抽象类,加上四个操作运算类,最后一个工厂类.

Operation
    public abstract class Operation
    {
        protected double numberA;
        protected double numberB;

        private double result = 0;

        public Operation()
        {
        }

        public Double NumberA
        {
            set
            {
                this.numberA = value;
            }
        }

        public  Double NumberB
        {
            set
            {
                this.numberB = value;
            }
        }


        public virtual Double Result
        {
            get 
            {
                return result;
            }
        }
    }

Operator
public class OperationAdd:Operation
    {
        public override double Result
        {
            get
            {
                return base.numberA + base.numberB;
            }
        }
    }

    public class OperationSub : Operation
    {
        public override double Result
        {
            get
            {

                return base.numberA - base.numberB;
            }
        }
    }

    public class OperationMul : Operation
    {
        public override double Result
        {
            get
            {
                return base.numberA * base.numberB; 
            }
        }
    }

    public class OperationDiv : Operation
    {
        public override double Result
        {
            get
            {
                if (base.numberB == 0)
                {
                    throw new Exception("The divisor can not zero");
                }
                return base.numberA / base.numberB; 
            }
        }
    }

Factory
 public class OperationFactory
    {
        public static Operation CreateOperationFactory(string operate)
        {
            Operation operation = null;
            switch (operate)
            {
                case "+":
                    operation = new OperationAdd();
                    break;
                case "-":
                    operation = new OperationSub();
                    break;
                case "*":
                    operation = new OperationMul();
                    break;
                case "/":
                    operation = new OperationDiv();
                    break;
            }
            return operation;
        }
    }

 static void Main(string[] args)
        {
            Console.WriteLine("Please enter a  digit A:");
            string strNumberA = Console.ReadLine();
            Console.WriteLine("Please enter a operator(+,-,*,/):");
            string strOperator = Console.ReadLine();
            Console.WriteLine("Please enter a digit B:");
            string strNumberB = Console.ReadLine();
Operation operation = OperationFactory.CreateOperationFactory(strOperator);
operation.NumberA = Double.Parse(strNumberA);
            operation.NumberB = Double.Parse(strNumberB);
Console.WriteLine(string.Format("{0} {1} {2} = {3}", strNumberA, strOperator,strNumberB, operation.Result));
}