Convert IEnumerable to IAsyncEnumerable

AsyncEnumerableExtensions.cs

    public static class AsyncEnumerableExtensions
    {

        public static IAsyncEnumerable<TResult> SelectAsync<T, TResult>(this IEnumerable<T> enumerable,
            Func<T, Task<TResult>> selector)
        {
            return AsyncEnumerable.CreateEnumerable(() =>
            {
                var enumerator = enumerable.GetEnumerator();
                var current = default(TResult);
                return AsyncEnumerable.CreateEnumerator(async c =>
                    {
                        var moveNext = enumerator.MoveNext();
                        current = moveNext
                            ? await selector(enumerator.Current).ConfigureAwait(false)
                            : default(TResult);
                        return moveNext;
                    },
                    () => current,
                    () => enumerator.Dispose());
            });
        }
    }