• 在工程中使用设计模式的目的是为了让代码达到复用性好，扩展性好，可维护性强三个最终目的，与此同时还需要降低类与类之间的耦合性。

• 工厂类C#实践
  一个简单的计算类，加减乘除都继承自基类 operation

using System;
namespace Factory1
{
    //Base 
    public class Operation
    {
        private double _numberA = 0;
        private double _numberB = 0;

        public double numberA
        {
            get { return _numberA;}
            set { _numberA = numberA;}
        }
        public double numberB 
        {
            get { return _numberB;}
            set { _numberB = numberB;}
        }
        public virtual double GetResult()
        {
            double result = 0;
            return result;
        }

    }

    //add
    class OperationAdd:Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = numberA + numberB;
            return result;
        }
    }

    //Sub
    class OperationSub:Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = numberA - numberB;
            return result;
        }
    }

    //Multi
    class OperationMul:Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = numberA * numberB;
            return result;
        }
    }

    //Div
    class OperationDiv:Operation
    {

        public override double GetResult()
        {
            double result = 0;
            if(numberB==0){
                throw new Exception("can not be zero!");
            }
            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;
        }
    }
    class MainClass
    {
        public static void Main (string[] args)
        {
            Operation a;
            a = OperationFactory.createOperate("+");
            a.numberA = 2;
            a.numberB = 3;
            double result = a.GetResult();
        }
    }
}