简单工厂

        static void Main(string[] args)
        {
            try
            {
                double numa = Convert.ToDouble(Console.ReadLine());
                string oper = Console.ReadLine();
                double numb = Convert.ToDouble(Console.ReadLine());
                double result = 0;
                switch (oper)
                {
                    case "+":
                        result = numa + numb;
                        break;
                    case "-":
                        result = numa - numb;
                        break;
                    case "*":
                        result = numa * numb;
                        break;
                    case "/":
                        if (numb != 0)
                        {
                            result = numa / numb;
                        }
                        else
                        {
                            throw new Exception("除数不能为0");
                        }
                        break;
                    default:
                        break;
                }
                Console.WriteLine(result);
                //var a = new System.Data.DataTable().Compute("1*2*3+5","");
                //Console.WriteLine(a);
                Console.ReadLine();
            }
            catch (Exception ex)
            {
                Console.WriteLine("" + ex.Message);
                Console.ReadLine();
            }
            /*
             * 喝酒唱歌，人生真爽
             * 对酒当歌，人生真爽
             * 对酒当歌，人生几何
             * */
            
            /*
             * 可维护 可复用 可扩展 灵活性好
             * 封装 继承 多态 耦合度降低
             * */
            #region 
            /* 短歌行
             * ⑴对酒当歌，人生几何：拿着酒唱歌，这样能有几回呢？
                ⑵譬如朝露，去日苦多：就像早晨的露水，过去的日子太多。
                ⑶慨当以慷，忧思难忘：应当慷慨高歌，心中的忧思难以忘怀。
                ⑷何以解忧，唯有杜康：只有杜康酒可以解除我的忧思。
                ⑸青青子衿，悠悠我心：衿：衣领；悠悠：长貌，形容思念之深。表示对贤才的思慕。
                ⑹但为君故，沉吟至今：但：只；君：指所思慕的人；沉吟：指低吟《子衿》一诗。
                ⑺呦呦鹿鸣，食野之苹：永《诗经.鹿鸣》成句，这里借来表示招纳贤才的意思。苹：艾蒿。
                ⑻我有嘉宾，鼓瑟吹笙：我用鼓瑟来迎接贤才。
                ⑼明明如月，何时可掇：明月是永不能摘掉的，它的运行也是永不能停止的。
                ⑽忧从中来，不可断绝：我绵绵不断的忧愁就从这里来。
                ⑾越陌度阡，枉用相存：越过田间的小路，问候远道而来的客人。
                ⑿契阔谈宴，心念旧恩：久别重逢，宴饮谈心，诉说往日的情意。
                ⒀月明星稀，乌鹊南飞：明月升起 ，星星闪烁，一群寻巢乌鹊向南飞去。
                ⒁绕树三匝，何枝可依？乌鹊绕树飞了三周却没敛翅，哪里才有它们栖身之所？此二句比喻贤才多多益善。无依比喻贤士身无所托。
                ⒂山不厌高，海不厌深：高山不辞土石才见巍峨，大海不弃涓流才见壮阔。比喻用人要“唯才是举”，多多益善。
                ⒃周公吐哺，天下归心。只有像周公那样礼待贤才（周公见到贤才，吐出口中正在咀嚼的食物，马上接待。《史记》载周公自谓：“一沐三握发，一饭三吐哺，犹恐失天下之贤。”），才能使天下人心都归向我。篇末引用周公自比，说明求贤建业的心思[1]  。
            */
            #endregion
        }

    class Operation
    {
        public static double GetResult(double numa,double numb,string operate)
        {
            double result = 0;
            switch (operate)
            {
                case "+":
                    result = numa + numb;
                    break;
                case "-":
                    result = numa - numb;
                    break;
                case "*":
                    result = numa * numb;
                    break;
                case "/":
                    if (numb==0)
                    {
                        throw new Exception("除数不能为0");
                    }
                    result = numa / numb;
                    break;
                default:
                    break;
            }
            return result;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            while (true)
            {
                try
                {
                    double numa = Convert.ToDouble(Console.ReadLine());
                    string operate = Console.ReadLine();
                    double numb = Convert.ToDouble(Console.ReadLine());
                    double result = Operation.GetResult(numa, numb, operate);
                    Console.WriteLine(result);
                }
                catch (Exception ex)
                {
                    Console.WriteLine("" + ex.Message);
                }
                //Console.ReadLine();
            }
        }
    }

namespace ConsoleApplication3
{
    class Operation
    {
        public double NumA { get; set; }
        public double NumB { get; set; }
        public virtual double GetResult()
        {
            double result = 0;
            return result;
        }
    }

    class OperationAdd : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumA + NumB;
            return result;
        }
    }

    class OperationSub : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumA - NumB;
            return result;
        }
    }

    class OperationMul : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumA * NumB;
            return result;
        }
    }

    class OperationDiv : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            if (NumB==0)
            {
                throw new Exception("除数不能为0");
            }
            else
            {
                result = NumA / NumB;
            }
            return result;
        }
    }

    class OperationFactory
    {
        public static Operation createOpearation(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;
                default:
                    break;
            }
            return oper;
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            while (true)
            {
                try
                {
                    double numa = Convert.ToDouble(Console.ReadLine());
                    string operate = Console.ReadLine();
                    double numb = Convert.ToDouble(Console.ReadLine());
                    Operation oper=OperationFactory.createOpearation(operate);
                    oper.NumA = numa;
                    oper.NumB = numb;
                    double result = oper.GetResult();
                    Console.WriteLine(result);
                }
                catch (Exception ex)
                {
                    Console.WriteLine(ex.Message);
                }
            }
        }
    }
}

namespace ClassLibrary1
{
    public class Operation
    {
        public double NumA { get; set; }
        public double NumB { get; set; }
        public virtual double GetResult()
        {
            double result = 0;
            return result;
        }

        public static string checkNumberInput(string currentNumber,string inputString)
        {
            string result = "";
            if (inputString ==".")
            {
                if (currentNumber.IndexOf(".")<0)
                {
                    if (currentNumber.Length==0)
                    {
                        result = "0" + inputString;
                    }
                    else
                    {
                        result = currentNumber + inputString;
                    }
                }
            }
            else if (currentNumber=="0")
            {
                result = inputString;
            }
            else
            {
                result = currentNumber + inputString;
            }
            return result;
        }
    }

    public class OperationAdd : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumA + NumB;
            return result;
        }
    }

    public class OperationSub : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumA - NumB;
            return result;
        }
    }

    public class OperationMul : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumA * NumB;
            return result;
        }                                       
    }

    public class OperationDiv : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            if (NumB==0)
            {
                throw new Exception("除数不能为0");
            }
            else
            {
                result = NumA / NumB;
            }
            return result;
        }
    }

    public class OperationSqr : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = NumB * NumB;
            return result;
        }
    }

    public class OperationSqrt : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            if (NumB<0)
            {
                throw new Exception("负数不能开平方");
            }
            result = Math.Sqrt(NumB);
            return result;
        }
    }

    public class OperationReverse : Operation
    {
        public override double GetResult()
        {
            double result = 0;
            result = -NumB;
            return result;
        }
    }

    public class OperationFactory
    {
        public static Operation createOperation(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;
                case"sqr":
                    operation = new OperationSqr();
                    break;
                case"sqrt":
                    operation = new OperationSqrt();
                    break;
                case "+/-":
                    operation = new OperationReverse();
                    break;
                default:
                    break;
            }
            return operation;
        }
    }
}

    public partial class Form1 : Form
    {
        bool bOperate = false;
        Operation oper;
        public Form1()
        {
            InitializeComponent();
        }

        private void buttonClear_Click(object sender, EventArgs e)
        {
            txtShow.Text = "";
        }

        private void button0_Click(object sender, EventArgs e)
        {
            if(bOperate)
            {
                txtShow.Text = "";
                bOperate = false;
            }
            string number = ((Button)sender).Text;
            txtShow.Text = Operation.checkNumberInput(txtShow.Text,number);
        }

        private void buttonAdd_Click(object sender, EventArgs e)
        {
            if (txtShow.Text!="")
            {
                oper = OperationFactory.createOperation(((Button)sender).Text);
                oper.NumA = Convert.ToDouble(txtShow.Text);
                bOperate = true;
            }
        }

        private void buttonEqual_Click(object sender, EventArgs e)
        {
            if (txtShow.Text!="")
            {
                if (((Button)sender).Text!="=")
                {
                    oper = OperationFactory.createOperation(((Button)sender).Text);
                }

                oper.NumB=Convert.ToDouble(txtShow.Text);
                txtShow.Text=oper.GetResult().ToString();
                bOperate=true;
            }
        }
    }

以上均参考大话设计模式