IQueryable - TryFirst (async) -- Replacement for .FirstOrDefaultAsync() -- Is this helpful for a PR?

Hello all,

I am new to this library, I am working with EFCore. In my scenerio I am working with IQueryable<T>.

I wanted a method like .FirstOrDefaultAsync(), because when I used .FirstOrDefaultAsync(), my return object was a pain, and looked like: Result<Maybe<TEntity?>, Error> , the nullable ? was giving me warnings in code.

I saw the methods TryFirst and TryLast - success I thought! But then I realized that these are not async, and do not use IQueryable

I created these methods for async and IQueryable:

    public static async Task<Maybe<T>> TryFirstAsync<T>(
        this IQueryable<T> source,
        CancellationToken cancellationToken = default)
    {
        var firstOrNull = await source.FirstOrDefaultAsync(cancellationToken);

        if (firstOrNull == null)
        {
            return Maybe<T>.None;
        }

        return Maybe<T>.From(firstOrNull);
    }

    public static async Task<Maybe<T>> TryFirstAsync<T>(
        this IQueryable<T> source,
        Func<T, bool> predicate,
        CancellationToken cancellationToken = default)
    {
        var firstOrEmpty = source.AsEnumerable()
            .Where(predicate)
            .Take(1)
            .AsQueryable();

        var firstOrNull = await firstOrEmpty.FirstOrDefaultAsync(cancellationToken);

        if (firstOrNull == null)
        {
            return Maybe<T>.None;
        }

        return Maybe<T>.From(firstOrNull);
    }

---

Note

Note: I realize that we do not use the Async suffix, however, using the suffix made things must easier, because without the suffix, the method would default to the IEnumerable<T> version, when I wanted to use the IQueryable<T> version.

Question

Is this helpful at all?

What should I do to replace IQueryable<T>.FirstOrDefaultAsync() in the future if this code is not helpful?

[vkhorikov]

We can add these 2 extension methods if we don’t have them yet. Async postfix is fine to make it consistent with EF Core.

Regarding your second overload: please use @maxime-poulain 's version. This code:

var firstOrEmpty = source.AsEnumerable()
            .Where(predicate)
            .Take(1)
            .AsQueryable();

doesn’t work because converting the source to IEnumerable with AsEnumerable() will prompt EF Core to load the whole set of items and then do the filtration in-memory, which defeats the purpose of using IQueriable.

[maxime-poulain]

Lucian Wischik, who worked on the compiler for async/await pattern is clear:

I/O bound operations should always be asynchronous.

So everyting you say regarding having API with Task returning method when the input is an IQueryable is right. https://youtu.be/OpOaiz4mNP0?t=466 (The guy himself explains it there if you are interessted).

BTW FirstOrDefaultAsync has an overload that takes an Expression. You can have

        public static async Task<Maybe<T>> TryFirstAsync<T>(
            this IQueryable<T> source,
            Expression<Func<T, bool>> predicate,
            CancellationToken cancellationToken = default)
        {
            var firstOrNull = await source.FirstOrDefaultAsync(predicate, cancellationToken);

            if (firstOrNull == null)
            {
                return Maybe<T>.None;
            }

            return Maybe<T>.From(firstOrNull);
        }
    // Usage : IQuery<string> query = XXX;
    // var item = await query.TryFirstAsync(x => x == "hello");