reactive/Ix.NET/Source/System.Interactive.Async/ToAsyncEnumerable.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;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

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

            return CreateEnumerable(
                () =>
                {
                    var e = source.GetEnumerator();

                    return CreateEnumerator(
                        ct => Task.Run(() =>
                                       {
                                           var res = false;
                                           try
                                           {
                                               res = e.MoveNext();
                                           }
                                           finally
                                           {
                                               if (!res)
                                                   e.Dispose();
                                           }
                                           return res;
                                       }, ct),
                        () => e.Current,
                        () => e.Dispose()
                    );
                });
        }

        public static IAsyncEnumerable<TSource> ToAsyncEnumerable<TSource>(this Task<TSource> task)
        {
            if (task == null)
                throw new ArgumentNullException(nameof(task));

            return CreateEnumerable(
                () =>
                {
                    var called = 0;

                    var value = default(TSource);
                    return CreateEnumerator(
                        async ct =>
                        {
                            if (Interlocked.CompareExchange(ref called, 1, 0) == 0)
                            {
                                value = await task.ConfigureAwait(false);
                                return true;
                            }
                            return false;
                        },
                        () => value,
                        () => { });
                });
        }

        public static IEnumerable<TSource> ToEnumerable<TSource>(this IAsyncEnumerable<TSource> source)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));

            return ToEnumerable_(source);
        }

        private static IEnumerable<TSource> ToEnumerable_<TSource>(IAsyncEnumerable<TSource> source)
        {
            using (var e = source.GetEnumerator())
            {
                while (true)
                {
                    if (!e.MoveNext(CancellationToken.None)
                          .Result)
                        break;
                    var c = e.Current;
                    yield return c;
                }
            }
        }
    }
}