Count()(linq扩展名)和List <T> .Count之间有区别吗？

Question

List<string> list = new List<string>() {"a", "b", "c"};
IEnumerable<string> enumerable = list;

int c1 = list.Count;
int c2 = list.Count();
int c3 = enumerable.Count();

最后3个陈述之间在性能和实施方面是否存在差异？将list.Count()表现更差或相同list.Count,如果引用类型IEnumerable<string>是否重要？

Answer 1

让我们看看Reflector:

public static int Count<TSource>(this IEnumerable<TSource> source)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    ICollection<TSource> is2 = source as ICollection<TSource>;
    if (is2 != null)
    {
        return is2.Count;
    }
    ICollection is3 = source as ICollection;
    if (is3 != null)
    {
        return is3.Count;
    }
    int num = 0;
    using (IEnumerator<TSource> enumerator = source.GetEnumerator())
    {
        while (enumerator.MoveNext())
        {
            num++;
        }
    }
    return num;
}

所以,如果你的IEnumerable<T>工具ICollection<T>或者ICollection,它会返回Count属性.