reactive/Ix.NET/Source/System.Linq.Async/System/Linq/Operators/First.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the Apache 2.0 License.
// See the LICENSE file in the project root for more information. 

using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace System.Linq
{
    public static partial class AsyncEnumerable
    {
        public static Task<TSource> First<TSource>(this IAsyncEnumerable<TSource> source)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));

            return First(source, CancellationToken.None);
        }

        public static Task<TSource> First<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));

            return FirstCore(source, cancellationToken);
        }

        public static Task<TSource> First<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (predicate == null)
                throw new ArgumentNullException(nameof(predicate));

            return First(source, predicate, CancellationToken.None);
        }

        public static Task<TSource> First<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (predicate == null)
                throw new ArgumentNullException(nameof(predicate));

            return source.Where(predicate).First(cancellationToken);
        }

        public static Task<TSource> First<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<bool>> predicate)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (predicate == null)
                throw new ArgumentNullException(nameof(predicate));

            return First(source, predicate, CancellationToken.None);
        }

        public static Task<TSource> First<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, Task<bool>> predicate, CancellationToken cancellationToken)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (predicate == null)
                throw new ArgumentNullException(nameof(predicate));

            return source.Where(predicate).First(cancellationToken);
        }

        private static async Task<TSource> FirstCore<TSource>(IAsyncEnumerable<TSource> source, CancellationToken cancellationToken)
        {
            if (source is IList<TSource> list)
            {
                if (list.Count > 0)
                {
                    return list[0];
                }
            }
            else if (source is IAsyncPartition<TSource> p)
            {
                var first = await p.TryGetFirst().ConfigureAwait(false);

                if (first.HasValue)
                {
                    return first.Value;
                }
            }
            else
            {
                var e = source.GetAsyncEnumerator();

                try
                {
                    if (await e.MoveNextAsync(cancellationToken).ConfigureAwait(false))
                    {
                        return e.Current;
                    }
                }
                finally
                {
                    await e.DisposeAsync().ConfigureAwait(false);
                }
            }

            throw new InvalidOperationException(Strings.NO_ELEMENTS);
        }
    }
}