设计模式：简单工厂

namespace SimleFactory
{
    public class Operation
    {
        public double NumberA { get; set; }
        public double NumberB { get; set; }
        public virtual double GetResult()
        {
            double result = 0;
            return result;
        }
    }
    public class OperationAdd:Operation
    {
        public override double GetResult()
        {
            return NumberA + NumberB;
        }
    }
    public class OperationSub : Operation
    {
        public override double GetResult()
        {
            return NumberA - NumberB;
        }
    }
    public class OperationMul : Operation
    {
        public override double GetResult()
        {
            return NumberA * NumberB;
        }
    }
    public class OperationDiv : Operation
    {
        public override double GetResult()
        {
            if (NumberB == 0)
                throw new Exception("除数不能为0.");
            return NumberA / NumberB;
        }
    }
    public class OperationFactory
    {
        public static Operation CreateOperation(string operate)
        {
            Operation oper = null;
            switch(operate)
            {
                case "+":
                    oper = new OperationAdd();
                    break;
                case "-":
                    oper = new OperationAdd();
                    break;
                case "*":
                    oper = new OperationAdd();
                    break;
                case "/":
                    oper = new OperationAdd();
                    break;
            }
            return oper;
        }
    }
}

　　测试代码：

            Operation oper = OperationFactory.CreateOperation("+");
            oper.NumberA = 1;
            oper.NumberB = 2;
            double result= oper.GetResult();
            Assert.AreEqual(result, 3);