我想知道的扩展方法如何FirstOrDefault的作品？下列哪些算法之一它遵循？

I was wondering on how FirstOrDefault extension method works? Which one of the following algorithms does it follows?

使用：

var arr = new[] {1, 2, 3, 4, 5, 6, 7};
return arr.FirstOrDefault(x => x%2 == 0);

算法1：

for(int i = 0; i < arr.Length; i++)
{
   if(arr[i] % 2 == 0)
     return arr[i];
}
return 0;

算法2：

var list = new List<int>();
for(int i = 0; i < arr.Length; i++)
{
   if(arr[i] % 2 == 0)
     list.Add(arr[i]);
}
return list.Count == 0 ? 0 : list[0];

请问FirstOrDefault算法是足够聪明，选择最佳的一个或它严格遵循这些算法的任何一个？

Does the FirstOrDefault algorithm is smart enough to choose the optimal one or it strictly follow any one of these algorithms?

推荐答案

我看着反射：

public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    IList<TSource> list = source as IList<TSource>;
    if (list != null)
    {
        if (list.Count > 0)
        {
            return list[0];
        }
    }
    else
    {
        using (IEnumerator<TSource> enumerator = source.GetEnumerator())
        {
            if (enumerator.MoveNext())
            {
                return enumerator.Current;
            }
        }
    }
    return default(TSource);
}

它试图用List做，如果集合可以被转换为IList的（并实现Count属性）。否则，它使用枚举。

It tries to do it with a List if the collection can be cast as IList (and implements the Count property). Otherwise it uses the Enumerator.

编辑：另一种方法是用predicate（我现在看到你所谈论的）是不是优化，依靠IEnumerable接口上执行的foreach而不是IList的

The other method with the predicate (which I now see you are talking about) is not as optimised and relies on the IEnumerable interface to perform a foreach rather than IList.

public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    if (predicate == null)
    {
        throw Error.ArgumentNull("predicate");
    }
    foreach (TSource local in source)
    {
        if (predicate(local))
        {
            return local;
        }
    }
    return default(TSource);
}