c# - 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.