上图为UML图，简单工厂模式解决对象的创建问题。

 

一、创建一个Operation基类:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DesignModeDemo
{
    public class Operation
    {

        public double Num1
        {
            get;
            set;
        }

        public double Num2
        {
            get;
            set;
        }

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

二、创建加、减、乘、除的子类，继承基数，重写Operate()方法。实现多态。注意：业务操作类对用户来说，不需要了解其业务实现内容机制，故业务类都是内部类，封装。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DesignModeDemo
{
    class OperationAdd : Operation
    {
        public override double Operate()
        {
            double result = 0d;
            result = Num1 + Num2;
            return result;
        }
    }
}

 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DesignModeDemo
{
    class OperationSubtract:Operation
    {
        public override double Operate()
        {
            double result = 0d;
            result = Num1 - Num2;
            return result;
        }
    }
}

 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DesignModeDemo
{
    class OperationMultiply:Operation
    {
        public override double Operate()
        {
            double result = 0d;
            result = Num1 * Num2;
            return result;
        }
    }
}

 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DesignModeDemo
{
    class OperationDivide:Operation
    {
        public override double Operate()
        {
            double result = 0d;
            if (Num2 != 0)
            {
                result = Num1 / Num2;
            }
            return result;
        }
    }
}

三、简单运算工厂类，根据运算符，实例化指定的业务类。注意这里使用静态方法。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DesignModeDemo
{
    public class OperationFactory
    {
        public static Operation CreateOperation(string operation)
        {
            Operation ope = null;            

            switch (operation)
            { 
                case "+":
                    ope = new OperationAdd();                    
                    break;
                case "-":
                    ope = new OperationSubtract();
                    break;
                case "*":
                    ope = new OperationMultiply();
                    break;
                case "/":
                    ope = new OperationDivide();
                    break;
            }
            return ope;
        }
    }
}

 

            Console.WriteLine("请输入数字A：");
            string num1 = Console.ReadLine();
            Console.WriteLine("请选择+,-,*,/");
            string ope = Console.ReadLine();
            Console.WriteLine("请输入数字B:");
            string num2 = Console.ReadLine();
            Operation operation = OperationFactory.CreateOperation(ope);
            operation.Num1 = double.Parse(num1);
            operation.Num2 = double.Parse(num2);
            Console.WriteLine(string.Format("结果是：{0}", operation.Operate()));