/// <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);
}