AsyncEnumerableTests.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;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using FluentAssertions;
using Xunit;

namespace Tests
{
    public class AsyncEnumerableTests
    {
        protected static readonly IAsyncEnumerable<int> Return42 = new[] { 42 }.ToAsyncEnumerable();

        protected async Task<TException> AssertThrowsAsync<TException>(Task t)
            where TException : Exception
        {
            return await Assert.ThrowsAsync<TException>(() => t);
        }

        protected async Task AssertThrowsAsync(Task t, Exception e)
        {
            var ex = await Assert.ThrowsAnyAsync<Exception>(() => t);
            Assert.Same(e, ex);
        }

        protected Task AssertThrowsAsync<T>(ValueTask<T> t, Exception e)
        {
            return AssertThrowsAsync(t.AsTask(), e);
        }

        protected async Task NoNextAsync<T>(IAsyncEnumerator<T> e)
        {
            Assert.False(await e.MoveNextAsync());
        }

        protected async Task HasNextAsync<T>(IAsyncEnumerator<T> e, T value)
        {
            Assert.True(await e.MoveNextAsync());
            Assert.Equal(value, e.Current);
        }

        protected async Task SequenceIdentity<T>(IAsyncEnumerable<T> enumerable)
        {
            var en1 = enumerable.GetAsyncEnumerator();
            var en2 = enumerable.GetAsyncEnumerator();

            Assert.Equal(en1.GetType(), en2.GetType());

            await en1.DisposeAsync();
            await en2.DisposeAsync();

            var res1 = await enumerable.ToListAsync();
            var res2 = await enumerable.ToListAsync();

            res1.Should().BeEquivalentTo(res2);
        }

        protected static IAsyncEnumerable<TValue> Throw<TValue>(Exception exception)
        {
            if (exception == null)
                throw new ArgumentNullException(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

            return AsyncEnumerable.Create(
                _ => AsyncEnumerator.Create<TValue>(
                    () => moveNextThrows,
                    getCurrent: null,
                    disposeAsync: null)
            );
        }
    }
}