ToArray()是否针对数组进行了优化? [英] Is ToArray() optimized for arrays?

问题描述

ReSharper建议将IEnumerable<T>枚举到列表或数组，因为我有可能IEnumerable<T>的多个枚举" .

ReSharper suggested to enumerate an IEnumerable<T> to a list or array since I had "possible multiple enumerations of IEnumerable<T>".

建议的自动代码重构具有一些内置的优化功能，可以在调用ToArray()之前查看IEnumerable<T>是否已经是数组.

The automatic code re-factoring suggested has some optimization built in to see whether IEnumerable<T> already is an array before calling ToArray().

var list = source as T[] ?? source.ToArray();

• 此优化是否已内置于原始LINQ方法中?
• 如果没有，那么这样做的动机是什么?

推荐答案

不，没有这样的优化.如果source是ICollection，则它将被复制到新数组.这是Buffer<T>结构的代码，被Enumerable用于创建数组:

Nope, there is no such optimization. If source is ICollection, then it will be copied to new array. Here is code of Buffer<T> struct, which used by Enumerable to create array:

internal Buffer(IEnumerable<TElement> source)
{    
    TElement[] array = null;
    int length = 0;
    ICollection<TElement> is2 = source as ICollection<TElement>;
    if (is2 != null)
    {
         length = is2.Count;
         if (length > 0)
         {
             array = new TElement[length]; // create new array
             is2.CopyTo(array, 0); // copy items
         }
    }
    else // we don't care, because array is ICollection<TElement>

    this.items = array;
}

这是Enumerable.ToArray()方法:

public static TSource[] ToArray<TSource>(this IEnumerable<TSource> source)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    Buffer<TSource> buffer = new Buffer<TSource>(source);
    return buffer.ToArray(); // returns items
}

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