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

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

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

namespace System.Linq
{
#if REFERENCE_ASSEMBLY
    public static partial class AsyncEnumerableDeprecated
#else
    public static partial class AsyncEnumerable
#endif
    {
        // NOTE: This type of blocking is an anti-pattern. We should never have offered it.
        //       It is being left here for binary compatibility for those who were using it,
        //       and the publicly visible version is marked as Obsolete so we can explain
        //       why it should not be used.

        /// <summary>
        /// Converts an async-enumerable sequence to an enumerable sequence.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence to convert to an enumerable sequence.</param>
        /// <returns>The enumerable sequence containing the elements in the async-enumerable sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> is null.</exception>
        [Obsolete("IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and it does not implement this method because 'sync over async' of this kind is an anti-pattern. Please use a different strategy.")]
        public static IEnumerable<TSource> ToEnumerable<TSource>(this IAsyncEnumerable<TSource> source)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));

            return Core(source);

            static IEnumerable<TSource> Core(IAsyncEnumerable<TSource> source)
            {
                var e = source.GetAsyncEnumerator(default);

                try
                {
                    while (true)
                    {
                        if (!Wait(e.MoveNextAsync()))
                            break;

                        yield return e.Current;
                    }
                }
                finally
                {
                    Wait(e.DisposeAsync());
                }
            }
        }

        // NB: ValueTask and ValueTask<T> do not have to support blocking on a call to GetResult when backed by
        //     an IValueTaskSource or IValueTaskSource<T> implementation. Convert to a Task or Task<T> to do so
        //     in case the task hasn't completed yet.

        private static void Wait(ValueTask task)
        {
            var awaiter = task.GetAwaiter();

            if (!awaiter.IsCompleted)
            {
                task.AsTask().GetAwaiter().GetResult();
                return;
            }

            awaiter.GetResult();
        }

        private static T Wait<T>(ValueTask<T> task)
        {
            var awaiter = task.GetAwaiter();

            if (!awaiter.IsCompleted)
            {
                return task.AsTask().GetAwaiter().GetResult();
            }

            return awaiter.GetResult();
        }
    }
}