LINQ中的陷阱--TakeWhile&SkipWhile

在用TakeWhile,SkipWhile设置陷阱之前,我们先来看一看他们的兄弟Take和Skip:

public static IEnumerable<T> Take<T>(IEnumerable<T> source, int count)

public static IEnumerable<T> Skip<T>(IEnumerable<T> source, int count)

 这两个操作符从字面上看就能理解其含义.Take将枚举出source中的前count个元素,返回给客户端.而Skip 则恰好相反,将跳过source中的前count个元素,枚举其余元素.LINQ内部实现代码十分简单,不用多分析.不过,他们的兄弟TakeWhile 和SkipWhile确埋下了个陷阱,让我小小的摔了一跤.现在,我就来重新布置这个陷阱: 

考虑如下的数据源:

static List<Customer> customers = new List<Customer> {
new Customer { CustomerID=1,Name="woody1"},
new Customer { CustomerID=2,Name="woody2"},
new Customer { CustomerID=3,Name="woody3"},
new Customer { CustomerID=4,Name="woody1"}
};

　在这个数据源的基础上,我进行了如下操作:

var cs1 = customers.TakeWhile(c => c.Name == "woody1");
var cs2 = customers.TakeWhile(c => c.Name == "woody2");
var cs3 = customers.SkipWhile(c => c.Name == "woody1");
var cs4 = customers.SkipWhile(c => c.Name == "woody2");

好了.现在,你能猜得出来cs1--cs4这四个IEnumerable<Customer>变量中都保存着些什么什么元素吗? 
  正确答案是: 
  cs1 : woody1(CustomerID=1) 
  cs2 : 没有任何元素 
  cs3 : woody2 , woody3 , woody1(CustomerID=4) 
  cs4 : woody1(CustomerID=1),woody2,woody3,woody1(CustomerID=4) 
  Surprise?:)反正我是小小的"惊喜"了一下.OK.研究实现代码吧... 
  TakeWhile在LINQ中实现的思想是:对数据源进行枚举,从第一个枚举得到的元素开始,调用客户端传入的predicate( c.Name == ""woodyN"),如果这个predicate委托返回true的话,则将该元素作为Current元素返回给客户端,并且,继续进行相同的枚举,判断操作.但是,一旦predicate返回false的话,MoveNext()方法将会返回false,枚举就此打住,忽略剩下的所有元素. 
类似的,SkipWhile也对数据源进行枚举,从第一个枚举得到的元素开始,调用客户端的predicate,如果返回true,则跳过该元素,继续进行枚举操作.但是,如果一旦predicate返回为false,则该元素以后的所有元素,都不会再调用predicate,而全部枚举给客户端. 

这两个方法总结为：遍历时，检查predicate条件，只要一遇到返回false，就打住，后面的元素不再去检测，直接返回结果。
  (内部实现代码很简单,不再列出) 
  现在,再回头看看陷阱的正确答案,是不是跑出来了呢?:)最开始,我一直以为是LINQ的一个BUG,还打算上LINQ论坛报BUG,不过,后来细想 Take,Skip,再详细阅读了LINQ的文档后,发现似乎这并不是BUG,这就是这两个操作符的正确逻辑.不过,起这样的名字,出这样的结果,实在让人觉得困惑啊~

 

完整的控制台程序代码如下：

class Customer
    {
        public int CustomerID { get; set; }
        public string Name { get; set; }
    }

class Program
    {
        static void Main(string[] args)
        {
            List<Customer> customers = new List<Customer> {
                new Customer { CustomerID=1,Name="woody1"},
                new Customer { CustomerID=2,Name="woody2"},
                new Customer { CustomerID=3,Name="woody3"},
                new Customer { CustomerID=4,Name="woody1"}
            };      
      
            var cs1 = customers.TakeWhile(c => c.Name == "woody1");
            var cs2 = customers.TakeWhile(c => c.Name == "woody2");
            var cs3 = customers.SkipWhile(c => c.Name == "woody1");
            var cs4 = customers.SkipWhile(c => c.Name == "woody2");

            Console.WriteLine("Result One:TakeWhile(c => c.Name == woody1)");
            foreach (var customer in cs1)
            {
                Console.WriteLine(customer.CustomerID + ":" + customer.Name);
            }
            Console.WriteLine("Result Two:TakeWhile(c => c.Name == woody2)");
            foreach (var customer in cs2)
            {
                Console.WriteLine(customer.CustomerID + ":" + customer.Name);
            }
            Console.WriteLine("Result Three:SkipWhile(c => c.Name == woody1)");
            foreach (var customer in cs3)
            {
                Console.WriteLine(customer.CustomerID + ":" + customer.Name);
            }
            Console.WriteLine("Result Four:SkipWhile(c => c.Name == woody2)");
            foreach (var customer in cs4)
            {
                Console.WriteLine(customer.CustomerID + ":" + customer.Name);
            }
            Console.ReadKey();

        }