Empty IQueryable that implements IAsyncEnumerable

Question

I am looking for an empty IQueryable<T> that implements IAsyncEnumerable<T>. My current code does not work because empty Enumerable does not implement IAsyncEnumerable<T>. Thanks for any help or hint.

I have the following design:

var result = Enumerable.Empty<Foo>().AsQueryable();  // Not working!

if (condition1)
{
    IQueryable<Foo> part1 = ....;

    result = result.Concat(part1);
}

if (condition2)
{
    IQueryable<Foo> part2 = ....;

    result = result.Concat(part2);
}

return await result.ToListAsync();

Error message:

The source IQueryable doesn't implement IAsyncEnumerable<Foo>. Only sources that implement IAsyncEnumerable can be used for Entity Framework asynchronous operations.
   at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.AsAsyncEnumerable[TSource](IQueryable`1 source)
   at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.ToListAsync[TSource](IQueryable`1 source, CancellationToken cancellationToken)

@MichaelRandall The `....` I have represented querying from `DbSet`. The result is `IQueryable`. I am trying to join the two `IQueryable` and await on `ToListAsync()`. `result` needs to implement `IAsyncEnumerable` for everything to work out. I think — Node.JS, Feb 18 '20 at 06:13
Maybe this post can give you some ideas: https://stackoverflow.com/questions/59071994/how-do-you-mock-and-iasyncenumerable — Amir Popovich, Feb 18 '20 at 06:41
@AmirPopovich I understand but I wish there was a way not bring moq into the project. — Node.JS, Feb 18 '20 at 06:46
@Node.JS - I'm not near a computer, so I can't really help you now. I've seen some people talking about an `ToAsyncEnumerable` extension method that may help you.. — Amir Popovich, Feb 18 '20 at 06:54

score 5 · Accepted Answer · answered Feb 18 '20 at 09:18

Use the nuget package System.Linq.Async to get the ToAsyncEnumerable() method:

private static async Task<List<Foo>> GetList()
{
    var result = Enumerable.Empty<Foo>().AsQueryable();

    if (true)
    {
        IQueryable<Foo> part1 = new List<Foo> { new Foo() }.AsQueryable();
        result = result.Concat(part1);
    }

    if (true)
    {
        IQueryable<Foo> part2 = new List<Foo> { new Foo(), new Foo() }.AsQueryable();
        result = result.Concat(part2);
    }

    return await result.ToAsyncEnumerable().ToListAsync();
}

score 1 · Answer 2 · answered Feb 18 '20 at 09:00

You can create an empty IAsyncEnumerable as it is described in this SO.

But if I don't think it is an ideal option since it requires referencing an additional package or implementing AsyncEnumerable.Empty on your own.

Alternatively, you could change your code in a way that does not require an "empty" container for your expressions.

For example:

var expressions = new List<IQueryable<Foo>>();

if (condition1)
{
    IQueryable<Foo> part1 = ....;

    expressions.Add(part1);
}

if (condition2)
{
    IQueryable<Foo> part2 = ....;

    expressions.Add(part2);
}

var result = expressions.Aggregate((acc, i) => acc.Concat(i));

return await result.ToListAsync();

Empty IQueryable that implements IAsyncEnumerable

2 Answers2

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