什么是的IEnumerable 计数（）之间的主要区别长度？

What are the key differences between IEnumerable Count() and Length?

推荐答案

通过调用指望的IEnumerable＆LT; T＆GT; 我假设你指的是扩展方法计数在 System.Linq.Enumerable 。 长度不在的IEnumerable℃的方法; T＆GT; 而是在净数组类型的属性，如 INT [] 。

By calling Count on IEnumerable<T> I'm assuming you're referring to the extension method Count on System.Linq.Enumerable. Length is not a method on IEnumerable<T> but rather a property on array types in .Net such as int[].

不同的是性能。该长度属性被保证是一个O（1）操作。的的复杂计数扩展方法有所不同基于对象的运行时类型。它会尝试转换为多种类型的支持O（1）长度查询像的ICollection＆LT; T＆GT; 通过计数属性。如果没有可用的，然后它会枚举所有项目，并计算它们具有的O（N）的复杂性。

The difference is performance. TheLength property is guaranteed to be a O(1) operation. The complexity of the Count extension method differs based on runtime type of the object. It will attempt to cast to several types which support O(1) length lookup like ICollection<T> via a Count property. If none are available then it will enumerate all items and count them which has a complexity of O(N).

例如

int[] list = CreateSomeList();
Console.WriteLine(list.Length);  // O(1)
IEnumerable<int> e1 = list;
Console.WriteLine(e1.Count()); // O(1) 
IEnumerable<int> e2 = list.Where(x => x <> 42);
Console.WriteLine(e2.Count()); // O(N)

值 E2 被实现为一个C＃迭代器不支持O（1）计算，因此该方法计数必须枚举整个集合来确定它有多长。

The value e2 is implemented as a C# iterator which does not support O(1) counting and hence the method Count must enumerate the entire collection to determine how long it is.