public static IEnumerable<TResult> SelectManay<TSource, TCollection, TResult>(this IEnumerable<TSource> source, Func<TSource, int, IEnumerable<TCollection>> collectionSelector, Func<TSource, TCollection, TResult> resultSelector ) 的基本架構

gistfile1.cs

        public static IEnumerable<TResult> SelectManay<TSource, TCollection, TResult>(this IEnumerable<TSource> source,
            Func<TSource, int, IEnumerable<TCollection>> collectionSelector,
            Func<TSource, TCollection, TResult> resultSelector
            )
        {
            var index = 0;
            foreach (TSource sourceItem in source)
            {
                IEnumerable<TCollection> collection = collectionSelector(sourceItem, index);

                foreach (TCollection collecitonItem in collection)
                {
                    TResult result = resultSelector(sourceItem, collecitonItem);
                    yield return result;
                }
                
                index++;
            }
        }