计算器示例：

　　

1.运算类：

    public class Operation
    {
        public double NumberA { get; set; }
        public double NumberB { get; set; }
        public virtual double GetResult()
        {
　　　　　　 return 0;
        }
    }

2.加减乘除类（以加法类为例）：

    public class AddOperation:Operation
    {
        public override double GetResult()
        {
            return NumberA + NumberB;
        }
    }

3.简单工厂类：

    public class OperationFactory
    {
        public static Operation CreateOperate(string operate)
        {
            Operation operation;
            switch(operate)
            {
                case "+":
                    operation = new AddOperation();
                    break;
                case "-":
                    operation = new SubOperation();
                    break;
                default :
                    operation = null;
                    break;
            }
            return operation;
        }
    }

4.客户端代码：

    class Program
    {
        static void Main(string[] args)
        {
            Operation operation = OperationFactory.CreateOperate("+");
            oper.NumberA = 1;
            oper.NumberB = 2;
            Console.WriteLine(operation.GetResult());
        }
    }