干掉switch语句

goto，可以说是程序界，名声最差的关键字之一了。有人强烈要求，在高级语言中，应该被和谐掉。
如果，你在满是高级业务逻辑的代码里，写上些goto，就好比，你在代码里，嵌了些汇编语言(goto应该待它自己的世界).
其实，我们常用的switch语句中，goto也若隐若现。请参考MSDN关于switch的说明文档。

现在，我分析一下，我们为啥要用switch语句：

先从一个简单的例子入手：

        [Test(Description="switch可以被if等价代替")]
        public void T1()
        {
            int x = 6;

            //if语句
            if (x % 2 == 0)
            {
                Console.WriteLine("偶数");
            }
            else if (x % 2 == 1)
            {
                Console.WriteLine("奇数");
            }
            else
            {
                Console.WriteLine("未知数");
            }

            //等价的switch语句
            switch (x % 2)
            {
                case 0: Console.WriteLine("偶数"); break;
                case 1: Console.WriteLine("奇数"); break;
                default: Console.WriteLine("未知数");
                    break;
            }
        }


可以看出，if与switch几乎没啥区别，纯属个人爱好！

再举个例子：

        [Test(Description="可能用到switch的地方")]
        public void T2()
        {
            int x = 6;

            //if语句分支太多!
            if (x == 0)
            {
                Console.WriteLine("x=0");
            }
            else if (x == 1 || x == 2 || x == 3 || x == 4)
            {
                Console.WriteLine("0<x<5");
            }
            else if (x == 5)
            {
                Console.WriteLine("x=5");
            }
            else if (x == 6 || x == 7 || x == 8 || x == 9)
            {
                Console.WriteLine("5<x<10");
            }
            else
            {
                Console.WriteLine("x<0 or x>9");
            }

            //用switch语句更清晰?
            switch (x)
            {
                case 0: Console.WriteLine("x=0"); break;
                case 1:
                case 2:
                case 3:
                case 4: Console.WriteLine("0<x<5"); break;
                case 5: Console.WriteLine("x=5"); break;
                case 6:
                case 7:
                case 8:
                case 9: Console.WriteLine("5<x<10"); break;
                default: Console.WriteLine("x<0 or x>9");
                    break;
            }
        }


可以看出，遇到这种情况，switch看起来还不错！

现在，我想用用switch的"高级"功能：

        [Test(Description="可以作为糟糕的笔试题")]
        public void T3()
        {
            int x = 6;

            switch (x)
            {
                case 0: Console.WriteLine("x=0"); goto case 0;//while(true)!!!
                case 1:
                case 2:
                case 3:
                case 4: Console.WriteLine("0<x<5"); goto default;
                case 5: Console.WriteLine("x=5"); break;
                case 6:
                case 7:
                case 8:
                case 9: Console.WriteLine("5<x<10"); goto case 1;
                default: Console.WriteLine("x<0 or x>9");
                    break;
            }
        }


你可以在30"之内，"算"出结果吗？

我真正想说的是，除了if...else...与switch外，我们仍然有选择的余地：

        [Test(Description="干掉switch语句")]
        public void T4()
        {
            //我用Dictionary来组织这些规则，可以自定义更高级的类
            var rules = new Dictionary<Predicate<int>, Action>
            {
                {x => x==0,         ()=>Console.WriteLine("x=0")},
                {x => 0<x&&x<5,     ()=>Console.WriteLine("0<x<5")},
                {x => x==5,         ()=>Console.WriteLine("x=5")},
                {x => 5<x&&x<10,    ()=>Console.WriteLine("5<x<10")},
                {x => x<0&&9<x,     ()=>Console.WriteLine("x<0 or x>9")},
                {x => true,         ()=>Console.WriteLine("默认")}
            };

            int y = 6;
            //找出所有满足要求的规则
            rules.Where(x => x.Key(y)).each(x => x.Value());
            //只找出一条满足要求的规则
            rules.First(x => x.Key(y)).Value();
        }


用Predicate/Action就解决了问题，感觉还好吗？


注：

在面向对象编程中，switch还涉及到很多重构，可以参考http://www.refactoring.com/catalog/index.html

附录：

代码 
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using NUnit.Framework;

namespace KSharp
{
    [TestFixture]
    public class TestSwitch
    {
        [Test(Description="switch可以被if代替")]
        public void T1()
        {
            int x = 6;

            //if语句
            if (x % 2 == 0)
            {
                Console.WriteLine("偶数");
            }
            else if (x % 2 == 1)
            {
                Console.WriteLine("奇数");
            }
            else
            {
                Console.WriteLine("未知数");
            }

            //等价的switch语句
            switch (x % 2)
            {
                case 0: Console.WriteLine("偶数"); break;
                case 1: Console.WriteLine("奇数"); break;
                default: Console.WriteLine("未知数");
                    break;
            }
        }

        [Test(Description="可能用到switch的地方")]
        public void T2()
        {
            int x = 6;

            //if语句分支太多!
            if (x == 0)
            {
                Console.WriteLine("x=0");
            }
            else if (x == 1 || x == 2 || x == 3 || x == 4)
            {
                Console.WriteLine("0<x<5");
            }
            else if (x == 5)
            {
                Console.WriteLine("x=5");
            }
            else if (x == 6 || x == 7 || x == 8 || x == 9)
            {
                Console.WriteLine("5<x<10");
            }
            else
            {
                Console.WriteLine("x<0 or x>9");
            }

            //用switch语句更清晰?
            switch (x)
            {
                case 0: Console.WriteLine("x=0"); break;
                case 1:
                case 2:
                case 3:
                case 4: Console.WriteLine("0<x<5"); break;
                case 5: Console.WriteLine("x=5"); break;
                case 6:
                case 7:
                case 8:
                case 9: Console.WriteLine("5<x<10"); break;
                default: Console.WriteLine("x<0 or x>9");
                    break;
            }
        }

        [Test(Description="可以作为笔试题")]
        public void T3()
        {
            int x = 6;

            switch (x)
            {
                case 0: Console.WriteLine("x=0"); goto case 0;//while(true)!!!
                case 1:
                case 2:
                case 3:
                case 4: Console.WriteLine("0<x<5"); goto default;
                case 5: Console.WriteLine("x=5"); break;
                case 6:
                case 7:
                case 8:
                case 9: Console.WriteLine("5<x<10"); goto case 1;
                default: Console.WriteLine("x<0 or x>9");
                    break;
            }
        }

        [Test(Description="干掉switch语句")]
        public void T4()
        {
            //我用Dictionary来组织这些规则，可以自定义更高级的类
            var rules = new Dictionary<Predicate<int>, Action>
            {
                {x => x==0,         ()=>Console.WriteLine("x=0")},
                {x => 0<x&&x<5,     ()=>Console.WriteLine("0<x<5")},
                {x => x==5,         ()=>Console.WriteLine("x=5")},
                {x => 5<x&&x<10,    ()=>Console.WriteLine("5<x<10")},
                {x => x<0&&9<x,     ()=>Console.WriteLine("x<0 or x>9")},
                {x => true,         ()=>Console.WriteLine("默认")}
            };

            int y = 6;
            //找出所有满足要求的规则
            rules.Where(x => x.Key(y)).each(x => x.Value());
            //只找出一条满足要求的规则
            rules.First(x => x.Key(y)).Value();
        }

        [Test]
        public void T5()
        {
            //TODO:实现一般的业务规则类，其中可包括优先级等
        }
    }
}

 原文地址：http://www.cnblogs.com/k-sharp/archive/2010/07/01/1769125.html

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原文地址：https://www.cnblogs.com/poissonnotes/p/1769227.html