linq-Empty

源码：

     public static IEnumerable Empty() {
            return EmptyEnumerable.Instance;
     }

    //
    // We have added some optimization in SZArrayHelper class to cache the enumerator of zero length arrays so  
    // the enumerator will be created once per type.
    // 
    internal class EmptyEnumerable
    {
        public static readonly TElement[] Instance = new TElement[0];
    }

 public static IEnumerable DefaultIfEmpty(this IEnumerable source) {
            return DefaultIfEmpty(source, default(TSource));
        }
 
        public static IEnumerable DefaultIfEmpty(this IEnumerable source, TSource defaultValue) {
            if (source == null) throw Error.ArgumentNull("source");
            return DefaultIfEmptyIterator(source, defaultValue);
        }