LINQ的了ToList /的ToArray / ToDictionary性能 [英] Linq ToList/ToArray/ToDictionary performance

问题描述

嗯，我遇到的许多情况下，有一个IEnumerable是不够的。但是我不能确定的上述方法调用的性能。

我真正想问的是：

时了ToList /的ToArray的表现：

1. 在一个O（n）的操作，它复制了IEnumerable到一个新的数组/列表？

2. 如果我叫了LINQ延伸方法的名单上，它有一个O（1）性能，如果我叫了ToList但为O（n），如果调用的ToArray（如果我原来的名单是一个数组相反）？

3. 有些神奇发生，表现为O（1）？

大概意思是O（n），对吧？

解决方案

  

是了ToList / 的ToArray 为O（n）操作的性能，它复制了的IEnumerable 来一个新的数组/列表？

是的。 了ToList 会更有效，因为它并不需要修剪内部缓冲器到合适的长度第一。

  

如果我打电话给名单上的LINQ延伸的方法，它有一个O（1）性能，如果我叫了ToList但为O（n），如果调用的ToArray（如果我原来的名单是一个数组相反）？

没有。对于这两个电话，一个的新的集合始终创建;这就是原始集合的浅表副本。这是更有效地调用了ToList 或的ToArray 任何的ICollection＆LT; T＆GT; 不是一个简单的的IEnumerable＆LT; T＆GT; 不落实的ICollection＆LT; T＆GT; 不过，与集合中的长度是已知的开始。 （这是通过检测执行时，虽然，你并不需要担心编译时类型）

  

大概意思是O（n），对吧？

假设哈希是明智的，这是O（N），是的。基本上，它正是你可能希望它的方式创建一个新的字典。

您可能需要阅读相应的职位，我Edulinq博客系列：

• <一个href="http://$c$cblog.jonskeet.uk/2011/01/01/reimplementing-linq-to-objects-part-20-tolist.aspx"><$c$c>ToList
• <一个href="http://$c$cblog.jonskeet.uk/2011/01/02/reimplementing-linq-to-objects-part-24-toarray.aspx"><$c$c>ToArray
• <一个href="http://$c$cblog.jonskeet.uk/2011/01/02/reimplementing-linq-to-objects-todictionary.aspx"><$c$c>ToDictionary

Well I encounter many situations where having an IEnumerable is not enough. However I'm unsure about the performance of the above method calls.

What I really want to ask is:

Is the performance of ToList/ToArray:

1. an O(n) operation which copies the IEnumerable to a new array/List ?

2. If I called a linq extention method on a list, it has an O(1) performance if I call ToList but O(n) if call ToArray (and the opposite if my original list was an array) ?

3. Some magic happens and the performance is O(1)?

Probably to Dictionary is O(n), right ?

解决方案

Is the performance of ToList/ToArray an O(n) operation which copies the IEnumerable to a new array/List ?

Yes. ToList is slightly more efficient, as it doesn't need to trim the internal buffer to the right length first.

If I called a linq extention method on a list, it has an O(1) performance if I call ToList but O(n) if call ToArray (and the opposite if my original list was an array) ?

No. For both calls, a new collection is always created; that's a shallow copy of the original collection. It's more efficient to call ToList or ToArray on any ICollection<T> than on a simple IEnumerable<T> which doesn't implement ICollection<T> though, as with a collection the length is known to start with. (This is detected at execution time though; you don't need to worry about the compile-time type.)

Probably to Dictionary is O(n), right ?

Assuming the hash is sensible, it's O(N), yes. Basically it creates a new dictionary in exactly the way you'd probably expect it to.

You might want to read the corresponding posts in my Edulinq blog series:

• ToList
• ToArray
• ToDictionary

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