简单工厂

using System;
using System.Collections.Generic;

public class Calc
{
    public virtual double DoDalc(double p1,double p2)
    {
        return 0;
    }
}

public class Add:Calc
{
    public override double DoDalc(double p1,double p2)
    {
        return p1+p2;
    }
}
public class Minus:Calc
{
    public override double DoDalc(double p1,double p2)
    {
        return p1-p2;
    }
}
public class  Multiply:Calc
{
    public override double DoDalc(double p1,double p2)
    {
        return p1*p2;
    }
}
public class Devide:Calc
{
    public override double DoDalc(double p1,double p2)
    {
        if(p2>-0.00000000001 &&p2<0.00000000001)
        {
            return 0;
        }
        else
        {
            return p1/p2;
        }
    }
}

public class SimpleCalculator
{
    public static Calc CreateCalculator(string strOperator)
    {
        Calc mycalc=null;
        switch(strOperator)
        {
            case "+":
                mycalc=new Add();
                break;
            case "-":
                mycalc=new Minus();
                break;
            case "*":
                mycalc=new Multiply();
                break;
            case "/":
                mycalc=new Devide();
                break;
            default:
                mycalc=new Calc();
                break;
        }
        return mycalc;
    }
}

public class MyClass
{
    public static void Main()
    {
        Calc mycalc=SimpleCalculator.CreateCalculator("*");
        double myresult=mycalc.DoDalc(1.0,20);
        Console.WriteLine(myresult);
        Console.ReadLine();
    }
}