ToArrayAsync() throws "The source IQueryable doesn't implement IAsyncEnumerable"

Question

I have a MVC project on ASP.NET Core, my problem is connected with IQueryable and asynchronous. I wrote the following method for search in IQueryable<T> :

private IQueryable<InternalOrderInfo> WhereSearchTokens(IQueryable<InternalOrderInfo> query, SearchToken[] searchTokens)
{
    if (searchTokens.Length == 0)
    {
        return query;
    }
    var results = new List<InternalOrderInfo>();
    foreach (var searchToken in searchTokens)
    {
        //search logic, intermediate results are being added to `results` using `AddRange()`
    }

    return results.Count != 0 ? results.Distinct().AsQueryable() : query;
}

I call this in method ExecuteAsync() :

public async Task<GetAllInternalOrderInfoResponse> ExecuteAsync(GetAllInternalOrderInfoRequest request)
{
    //rest of the code
    if (searchTokens != null && searchTokens.Any())
    {
        allInternalOrderInfo = WhereSearchTokens(allInternalOrderInfo, searchTokens);
    }
    var orders = await allInternalOrderInfo.Skip(offset).Take(limit).ToArrayAsync();
    //rest of the code
}

When I test this I get an InvalidOperationException on line where I call ToArrayAsync()

The source IQueryable doesn't implement IAsyncEnumerable. Only sources that implement IAsyncEnumerable can be used for Entity Framework asynchronous operations.

I had changed ToArrayAsync() to ToListAsync() but nothing have changed. I have searched this problem for a while, but resolved questions are connected mostly with DbContext and entity creating. EntityFramework is not installed for this project and it's better not to do it because of application architecture. Hope someone has any ideas what to do in my situation.

Answer 1

If you are not going to change your design - you have several options:

1) Change AsQueryable to another method which returns IQueryable which also implements IDbAsyncEnumerable . For example you can extend EnumerableQuery (which is returned by AsQueryable ):

public class AsyncEnumerableQuery<T> : EnumerableQuery<T>, IDbAsyncEnumerable<T> {
    public AsyncEnumerableQuery(IEnumerable<T> enumerable) : base(enumerable) {
    }

    public AsyncEnumerableQuery(Expression expression) : base(expression) {
    }

    public IDbAsyncEnumerator<T> GetAsyncEnumerator() {
        return new InMemoryDbAsyncEnumerator<T>(((IEnumerable<T>) this).GetEnumerator());
    }

    IDbAsyncEnumerator IDbAsyncEnumerable.GetAsyncEnumerator() {
        return GetAsyncEnumerator();
    }

    private class InMemoryDbAsyncEnumerator<T> : IDbAsyncEnumerator<T> {
        private readonly IEnumerator<T> _enumerator;

        public InMemoryDbAsyncEnumerator(IEnumerator<T> enumerator) {
            _enumerator = enumerator;
        }

        public void Dispose() {
        }

        public Task<bool> MoveNextAsync(CancellationToken cancellationToken) {
            return Task.FromResult(_enumerator.MoveNext());
        }

        public T Current => _enumerator.Current;

        object IDbAsyncEnumerator.Current => Current;
    }
}

Then you change

results.Distinct().AsQueryable()

to

new AsyncEnumerableQuery<InternalOrderInfo>(results.Distinct())

And later, ToArrayAsync will not throw exception any more (obviously you can create your own extension method like AsQueryable ).

2) Change ToArrayAsync part:

public static class EfExtensions {
    public static Task<TSource[]> ToArrayAsyncSafe<TSource>(this IQueryable<TSource> source) {
        if (source == null)
            throw new ArgumentNullException(nameof(source));
        if (!(source is IDbAsyncEnumerable<TSource>))
            return Task.FromResult(source.ToArray());
        return source.ToArrayAsync();
    }
}

And use ToArrayAsyncSafe instead of ToArrayAsync , which will fallback to synchronous enumeration in case IQueryable is not IDbAsyncEnumerable . In your case this only happens when query is really in-memory list and not query, so async execution does not make sense anyway.

Answer 2

For EF Core:

public static class QueryableExtensions
{
    public static IQueryable<T> AsAsyncQueryable<T>(this IEnumerable<T> input)
    {
        return new NotInDbSet<T>( input );
    }

}

public class NotInDbSet< T > : IQueryable<T>, IAsyncEnumerable< T >, IEnumerable< T >, IEnumerable
{
    private readonly List< T > _innerCollection;
    public NotInDbSet( IEnumerable< T > innerCollection )
    {
        _innerCollection = innerCollection.ToList();
    }


    public IAsyncEnumerator< T > GetAsyncEnumerator( CancellationToken cancellationToken = new CancellationToken() )
    {
        return new AsyncEnumerator( GetEnumerator() );
    }

    public IEnumerator< T > GetEnumerator()
    {
        return _innerCollection.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    public class AsyncEnumerator : IAsyncEnumerator< T >
    {
        private readonly IEnumerator< T > _enumerator;
        public AsyncEnumerator( IEnumerator< T > enumerator )
        {
            _enumerator = enumerator;
        }

        public ValueTask DisposeAsync()
        {
            return new ValueTask();
        }

        public ValueTask< bool > MoveNextAsync()
        {
            return new ValueTask< bool >( _enumerator.MoveNext() );
        }

        public T Current => _enumerator.Current;
    }

    public Type ElementType => typeof( T );
    public Expression Expression => Expression.Empty();
    public IQueryProvider Provider => new EnumerableQuery<T>( Expression );
}

Answer 3

I found I had to do a bit more work to get things to work nicely:

namespace TestDoubles
{
    using Microsoft.EntityFrameworkCore.Query.Internal;
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Linq.Expressions;
    using System.Threading;
    using System.Threading.Tasks;

    public static class AsyncQueryable
    {
        /// <summary>
        /// Returns the input typed as IQueryable that can be queried asynchronously
        /// </summary>
        /// <typeparam name="TEntity">The item type</typeparam>
        /// <param name="source">The input</param>
        public static IQueryable<TEntity> AsAsyncQueryable<TEntity>(this IEnumerable<TEntity> source)
            => new AsyncQueryable<TEntity>(source ?? throw new ArgumentNullException(nameof(source)));
    }

    public class AsyncQueryable<TEntity> : EnumerableQuery<TEntity>, IAsyncEnumerable<TEntity>, IQueryable<TEntity>
    {
        public AsyncQueryable(IEnumerable<TEntity> enumerable) : base(enumerable) { }
        public AsyncQueryable(Expression expression) : base(expression) { }
        public IAsyncEnumerator<TEntity> GetEnumerator() => new AsyncEnumerator(this.AsEnumerable().GetEnumerator());
        public IAsyncEnumerator<TEntity> GetAsyncEnumerator(CancellationToken cancellationToken = default) => new AsyncEnumerator(this.AsEnumerable().GetEnumerator());
        IQueryProvider IQueryable.Provider => new AsyncQueryProvider(this);

        class AsyncEnumerator : IAsyncEnumerator<TEntity>
        {
            private readonly IEnumerator<TEntity> inner;
            public AsyncEnumerator(IEnumerator<TEntity> inner) => this.inner = inner;
            public void Dispose() => inner.Dispose();
            public TEntity Current => inner.Current;
            public ValueTask<bool> MoveNextAsync() => new ValueTask<bool>(inner.MoveNext());
#pragma warning disable CS1998 // Nothing to await
            public async ValueTask DisposeAsync() => inner.Dispose();
#pragma warning restore CS1998
        }

        class AsyncQueryProvider : IAsyncQueryProvider
        {
            private readonly IQueryProvider inner;
            internal AsyncQueryProvider(IQueryProvider inner) => this.inner = inner;
            public IQueryable CreateQuery(Expression expression) => new AsyncQueryable<TEntity>(expression);
            public IQueryable<TElement> CreateQuery<TElement>(Expression expression) => new AsyncQueryable<TElement>(expression);
            public object Execute(Expression expression) => inner.Execute(expression);
            public TResult Execute<TResult>(Expression expression) => inner.Execute<TResult>(expression);
            public IAsyncEnumerable<TResult> ExecuteAsync<TResult>(Expression expression) => new AsyncQueryable<TResult>(expression);
            TResult IAsyncQueryProvider.ExecuteAsync<TResult>(Expression expression, CancellationToken cancellationToken) => Execute<TResult>(expression);
        }
    }
}

This enables me to write tests like this:

    [TestCase("", 3, 5)]
    [TestCase("100", 2, 4)]
    public async Task GetOrderStatusCounts_ReturnsCorrectNumberOfRecords(string query, int expectedCount, int expectedStatusProductionCount)
    {
        // omitted CreateOrder helper function

        const int productionStatus = 6;
        const int firstOtherStatus = 5;
        const int otherOtherStatus = 7;

        var items = new[]
        {
            CreateOrder(1, "100000", firstOtherStatus, 1),
            CreateOrder(2, "100000", firstOtherStatus, 4),
            CreateOrder(3, "100000", productionStatus, 4),
            CreateOrder(4, "100001", productionStatus, 4),
            CreateOrder(5, "100100", productionStatus, 4),
            CreateOrder(6, "200000", otherOtherStatus, 4),
            CreateOrder(7, "200001", productionStatus, 4),
            CreateOrder(8, "200100", productionStatus, 4)
        }.AsAsyncQueryable(); // this is where the magic happens

        var mocker = new AutoMocker();

        // IRepository implementation is also generic and calls DBCntext
        // for easier testing
        mocker.GetMock<IRepository<Order>>() 
            .Setup(m => m.BaseQuery()
            .Returns(items); 
            // the base query is extended in the system under test.
            // that's the behavior I'm testing here

        var sut = mocker.CreateInstance<OrderService>();

        var counts = await sut.GetOrderStatusCountsAsync(4, query);

        counts.Should().HaveCount(expectedCount);
        counts[OrderStatus.Production].Should().Be(expectedStatusProductionCount);
    }

Answer 4

The AsQueryable() will not transform the result list into an Entity Framework IQueryable . And as the error states, the IQueryable that are used with ToArrayAsync() should implement IAsyncEnumerable , which is not what AsQueryable will return.

You can read more about the uses of AsQueryable on enumerables here .

Answer 5

As noted by @Titian Cernicova-Dragomir the exception means that List<InternalOrderInfo> doesn't implement IAsyncEnumerable

But here is a logical/design error. If your method works with IQueryable and returns IQueryable it should work with it as with IQueryable and not as with IEnumarable that assumes that collection is in a memory of app. You really need to read more about the difference between IQueryable and IEnumarable and what you should return from the method. A good point to start is to read answers here and here

So, since you already fetched results from db in WhereSearchTokens method or even before, there is no reason to do asynchronous request to db which is would be done by ToArrayAsync and return IQueryable .

You have two options here:

1) If your collection of InternalOrderInfo is fetched from db into memory before WhereSearchTokens make your all actions in synchronous mode ie call ToArray instead of ToArrayAsync , and return IEnumerable instead of Taks<IQueryable> from both WhereSearchTokens and ExecuteAsync .

2) If your collection of InternalOrderInfo is fetched inside WhereSearchTokens and you want to do the async request to db you need to call async EF API only somewhere in //search logic, intermediate results are being added to results using AddRange() and again return Taks<IEnumerable> istead of Taks<IQueryable> from WhereSearchTokens

Answer 6

For EFCore

Bit late to the party but to others looking to resolve this type of problem, one of possible solution is to change code to use Task.FromResult() method in this way:

var result= await allInternalOrderInfo.Skip(offset).Take(limit);
var orders = await Task.FromResult(result.ToArray());

Answer 7

I wrote an ICollection extension AsAsyncQueryable that I use in my tests

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

namespace Whatevaaaaaaaa
{
    public static class ICollectionExtensions
    {
        public static IQueryable<T> AsAsyncQueryable<T>(this ICollection<T> source) =>
            new AsyncQueryable<T>(source.AsQueryable());
    }

    internal class AsyncQueryable<T> : IAsyncEnumerable<T>, IQueryable<T>
    {
        private IQueryable<T> Source;

        public AsyncQueryable(IQueryable<T> source)
        {
            Source = source;
        }

        public Type ElementType => typeof(T);

        public Expression Expression => Source.Expression;

        public IQueryProvider Provider => new AsyncQueryProvider<T>(Source.Provider);

        public IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken cancellationToken = default)
        {
            return new AsyncEnumeratorWrapper<T>(Source.GetEnumerator());
        }

        public IEnumerator<T> GetEnumerator() => Source.GetEnumerator();
        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
    }

    internal class AsyncQueryProvider<T> : IQueryProvider
    {
        private readonly IQueryProvider Source;

        public AsyncQueryProvider(IQueryProvider source)
        {
            Source = source;
        }

        public IQueryable CreateQuery(Expression expression) =>
            Source.CreateQuery(expression);

        public IQueryable<TElement> CreateQuery<TElement>(Expression expression) =>
            new AsyncQueryable<TElement>(Source.CreateQuery<TElement>(expression));

        public object Execute(Expression expression) => Execute<T>(expression);

        public TResult Execute<TResult>(Expression expression) =>
            Source.Execute<TResult>(expression);
    }



    internal class AsyncEnumeratorWrapper<T> : IAsyncEnumerator<T>
    {
        private readonly IEnumerator<T> Source;

        public AsyncEnumeratorWrapper(IEnumerator<T> source)
        {
            Source = source;
        }

        public T Current => Source.Current;

        public ValueTask DisposeAsync()
        {
            return new ValueTask(Task.CompletedTask);
        }

        public ValueTask<bool> MoveNextAsync()
        {
            return new ValueTask<bool>(Source.MoveNext());
        }
    }
}

Answer 8

It's better to implement collection with IAsyncEnumerable<T> and IQueryable<T> rather than create your own ToListAsync extensions.

You can't apply your extensions in libraries.

For EF Core 5 and above check this implementation and tests .

Short version:

public sealed class FixedQuery<T> : IAsyncEnumerable<T>, IQueryable<T>
{
    public static readonly IQueryable<T> Empty = Create(ArraySegment<T>.Empty);

    public static IQueryable<T> Create(params T[] items)
    {
        return Create((IEnumerable<T>)items);
    }

    public static IQueryable<T> Create(IEnumerable<T> items)
    {
        return new FixedQuery<T>(items ?? ArraySegment<T>.Empty).AsQueryable();
    }

    private readonly IQueryable<T> _items;

    private FixedQuery(IEnumerable<T> items)
    {
        _items = (items ?? throw new ArgumentNullException(nameof(items))).AsQueryable();
    }

    #pragma warning disable CS1998
    public async IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken cancellationToken = default)
    #pragma warning restore CS1998
    {
        foreach (var item in _items)
        {
            yield return item;
        }
    }

    public IEnumerator<T> GetEnumerator()
    {
        return _items.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    public Type ElementType => _items.ElementType;
    public Expression Expression => _items.Expression;
    public IQueryProvider Provider => _items.Provider;
}