reactive/Ix.NET/Source/System.Interactive.Async/System/Linq/Operators/Throw.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.Tasks;

namespace System.Linq
{
    public static partial class AsyncEnumerableEx
    {
        public static IAsyncEnumerable<TValue> Throw<TValue>(Exception exception)
        {
            if (exception == null)
                throw Error.ArgumentNull(nameof(exception));

#if NO_TASK_FROMEXCEPTION
            var tcs = new TaskCompletionSource<bool>();
            tcs.TrySetException(exception);
            var moveNextThrows = new ValueTask<bool>(tcs.Task);
#else
            var moveNextThrows = new ValueTask<bool>(Task.FromException<bool>(exception));
#endif

            //
            // REVIEW: Honor cancellation using conditional expression in MoveNextAsync?
            //

            return AsyncEnumerable.CreateEnumerable(
                _ => AsyncEnumerator.Create<TValue>(
                    () => moveNextThrows,
                    current: null,
                    dispose: null)
            );
        }
    }
}