How to write new linq provider

Question

I need to write the UT for async method, I am using EF6. Then I have the error: "the source 'iqueryable' doesn't implement 'iasyncenumerable moq".

I search on stackoverflow and found that link How to mock an async repository with Entity Framework Core

But I dont understand the code bellow.

Can anyone help me to understand the code below. Thanks

internal class TestAsyncQueryProvider<TEntity> : IAsyncQueryProvider
{
    private readonly IQueryProvider _inner;

    internal TestAsyncQueryProvider(IQueryProvider inner)
    {
        _inner = inner;
    }

    public IQueryable CreateQuery(Expression expression)
    {
        return new TestAsyncEnumerable<TEntity>(expression);
    }

    public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
    {
        return new TestAsyncEnumerable<TElement>(expression);
    }

    public object Execute(Expression expression)
    {
        return _inner.Execute(expression);
    }

    public TResult Execute<TResult>(Expression expression)
    {
        return _inner.Execute<TResult>(expression);
    }

    public IAsyncEnumerable<TResult> ExecuteAsync<TResult>(Expression expression)
    {
        return new TestAsyncEnumerable<TResult>(expression);
    }

    public Task<TResult> ExecuteAsync<TResult>(Expression expression, CancellationToken     cancellationToken)
    {
        return Task.FromResult(Execute<TResult>(expression));
    }
}

internal class TestAsyncEnumerable<T> : EnumerableQuery<T>, IAsyncEnumerable<T>, IQueryable<T>
{
    public TestAsyncEnumerable(IEnumerable<T> enumerable)
        : base(enumerable)
    { }

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

    public IAsyncEnumerator<T> GetEnumerator()
    {
        return new TestAsyncEnumerator<T>(this.AsEnumerable().GetEnumerator());
    }

    IQueryProvider IQueryable.Provider
    {
        get { return new TestAsyncQueryProvider<T>(this); }
    }
}

internal class TestAsyncEnumerator<T> : IAsyncEnumerator<T>
{
    private readonly IEnumerator<T> _inner;

    public TestAsyncEnumerator(IEnumerator<T> inner)
    {
        _inner = inner;
    }

    public void Dispose()
    {
        _inner.Dispose();
    }

    public T Current
    {
        get
        {
            return _inner.Current;
        }
    }

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

write UT for async method in EF6.

The error appears to be in code that you didn't post here. Unless maybe you're trying to call one of these methods using "Moq" as a generic type. — Robert Harvey, Jun 20 '23 at 13:25
@RobertHarvey I just post my code. can you guide me how to the do changes to run my test . thanks — Peter Nguyen, Jun 22 '23 at 05:29

score 0 · Answer 1 · answered Jun 21 '23 at 04:05

I'm not sure about your specific error message, but to explain the code: When using a mocking framework to substitute something like a repository or a DbSet where the consumer is accessing an IQueryable, your mock will need to return a known set of data.

For instance if I have a repository with a method like:

public IQueryable<Order> GetPendingOrders(DateTime? startDate = null);

and I want to mock that I might have a mock like:

var testOrders = new []
{
   new Order { /* fill in order details... /*},
   new Order { /* fill in order details... /*}
};

var mockRepository = new Mock<OrderRepository>();
mockRepository.Setup(x => x.GetPendingOrders(It.IsAny<DateTime>()))
    .Returns(testOrders.AsQueryable());

... and code under test calling that mocked repository then performing a synchronous call like:

var orders = OrderRepository.GetPendingOrders(startDate).ToList();

... will work just fine. Where it will fail is if the code under test expects to use an asynchronous operation.

var orders = await OrderRepository.GetPendingOrders(startDate).ToListAsync();

Using the mock that passes an Array or a List<Order> will fail as the asynchronous EF operation is expecting something that works as an AsyncEnumerable and EnumerableQuery, leading to that code example you employed.

I'd recently come across a problem where the similar code I had used was failing with Autofac's ProjectTo method, even with a synchronous operation. The culprit was this method:

public IQueryable CreateQuery(Expression expression)
{
    return new TestAsyncEnumerable<TEntity>(expression);
}

The updated example I found looks more like:

public IQueryable CreateQuery(Expression expression)
{
    switch (expression)
    {
        case MethodCallExpression m:
            {
                var resultType = m.Method.ReturnType; // it should be IQueryable<T>
                var tElement = resultType.GetGenericArguments()[0];
                var queryType = typeof(AsyncQueryable<>).MakeGenericType(tElement);
                return (IQueryable)Activator.CreateInstance(queryType, expression);
            }
        default:
            return new TestAsyncEnumerable<TEntity>(expression);
    }
}

The other variance I found from your code and the example I had employed was your code extends IAsyncEnumerable<TEntity> where mine for EF6 extends IDbAsyncEnumerable<TEntity>. The implementation details are otherwise pretty much identical but that may likely be significant when dealing with asynchronous operations via EF as it may be expecting a class that is recognized as IDbAsyncEnumerable, not just IAsyncEnumerable.

How to write new linq provider

1 Answers1