c#中字符串数组和字符串列表有什么区别

Question

I hear on MSDN that an array is faster than a collection.

Can you tell me how string[] is faster then List<string>.

Answer 1

Arrays are a lower level abstraction than collections such as lists. The CLR knows about arrays directly, so there's slightly less work involved in iterating, accessing etc.

However, this should almost never dictate which you actually use. The performance difference will be negligible in most real-world applications. I rarely find it appropriate to use arrays rather than the various generic collection classes, and indeed some consider arrays somewhat harmful . One significant downside is that there's no such thing as an immutable array (other than an empty one)... whereas you can expose read-only collections through an API relatively easily.

Answer 2

The article is from 2004, that means it's about .net 1.1 and there was no generics. Array vs collection performance actually was a problem back then because collection types caused a lot of exta boxing-unboxing operations. But since .net 2.0, where generics was introduced, difference in performance almost gone.

Answer 3

An array is not resizable. This means that when it is created one block of memory is allocated, large enough to hold as many elements as you specify.

A List on the other hand is implicitly resizable. Each time you Add an item, the framework may need to allocate more memory to hold the item you just added. This is an expensive operation, so we end up saying "List is slower than array".

Of course this is a very simplified explanation, but hopefully enough to paint the picture.

Answer 4

An array is the simplest form of collection, so it's faster than other collections. A List (and many other collections) actually uses an array internally to hold its items.

An array is of course also limited by its simplicity. Most notably you can't change the size of an array. If you want a dynamic collection you would use a List.

Answer 5

List<string> is class with a private member that is a string[] . The MSDN documentation states this fact in several places. The List class is basically a wrapper class around an array that gives the array other functionality.

The answer of which is faster all depends on what you are trying to do with the list/array. For accessing and assigning values to elements, the array is probably negligibly faster since the List is an abstraction of the array (as Jon Skeet has said).

If you intend on having a data structure that grows over time (gets more and more elements), performance (ave. speed) wise the List will start to shine. That is because each time you resize an array to add another element it is an O(n) operation. When you add an element to a List (and the list is already at capacity) the list will double itself in size. I won't get into the nitty gritty details, but basically this means that increasing the size of a List is on average a O(log n) operation. Of course this has drawbacks too (you could have almost twice the amount of memory allocated as you really need if you only go a couple items past its last capacity).

Edit: I got a little mixed up in the paragraph above. As Eric has said below, the number of resizes for a List is O(log n), but the actual cost associated with resizing the array is amortized to O(1).