/// <summary>
        /// Asynchronously returns the first element of a sequence, or a default value if the sequence contains no elements.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements of <paramref name="source"/>.</typeparam>
        /// <param name="source">The <see cref="IAsyncQueryable{T}"/> to return the first element of.</param>
        /// <param name="cancellationToken">The cancellation token.</param>
        /// <returns><c>default(TSource)</c> if <paramref name="source"/> is empty; otherwise, the first element in <paramref name="source"/>.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> is null.</exception>
        public static async Task <TSource> FirstOrDefaultAsync <TSource>(this IAsyncQueryable <TSource> source, CancellationToken cancellationToken = default(CancellationToken))
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            var sourceLimitQuery = source.Take(1);

            if (!await sourceLimitQuery.MoveNextAsync(cancellationToken).ConfigureAwait(false))
            {
                return(default(TSource));
            }

            return(sourceLimitQuery.CurrentPage.FirstOrDefault());
        }
        /// <summary>
        /// Asynchronously returns the number of elements in a sequence.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements of <paramref name="source"/>.</typeparam>
        /// <param name="source">The <see cref="IAsyncQueryable{T}"/> that contains the elements to be counted.</param>
        /// <param name="cancellationToken">The cancellation token.</param>
        /// <returns>The number of elements in the input sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> is null.</exception>
        /// <exception cref="OverflowException">The number of elements in <paramref name="source"/> is larger than <see cref="int.MaxValue"/>.</exception>
        /// <exception cref="System.InvalidOperationException">The underlying provider does not support this operation.</exception>
        public static async Task <long> CountAsync <TSource>(this IAsyncQueryable <TSource> source, CancellationToken cancellationToken = default(CancellationToken))
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            var sourceLimitQuery = source.Take(1);

            if (!await sourceLimitQuery.MoveNextAsync(cancellationToken).ConfigureAwait(false))
            {
                return(0);
            }

            var collection = sourceLimitQuery as CollectionResourceQueryable <TSource>;

            if (collection == null)
            {
                throw new InvalidOperationException("This object is not a supported collection resource.");
            }

            return(collection.Size);
        }