c# async when returning LINQ

Question

I've just realised that this code:

    public async Task<List<Review>> GetTitleReviews(int titleID)
    {
        using (var context = new exampleEntities())
        {
            return await context.Reviews.Where(x => x.Title_Id == titleID).ToList();        
        }
    }

...will not work as async methods cannot await LINQ expressions. I've did some searches but only managed to find some overcomplicated solutions.

How should functions which return LINQ expressions be converted to async versions?

You probably wanted to say LINQ extension methods, not "lambdas"? — zerkms
– zerkms, Commented Oct 30, 2014 at 1:55

sa_ddam213 · Accepted Answer · 2014-10-30 01:58:51Z

13

Add the System.Data.Entity namespace and take advantage of the Async extension methods

In this case ToListAsync should do the trick

using System.Data.Entity;

public async Task<List<Review>> GetTitleReviews(int titleID)
{
    using (var context = new exampleEntities())
    {
        return await context.Reviews.Where(x => x.Title_Id == titleID).ToListAsync();        
    }
}

answered Oct 30, 2014 at 1:58

sa_ddam213

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2 Comments

globetrotter Over a year ago

Although both yours and Peter Duniho's answers worked, I'm marking yours as the answer since it provides a pretty much built-in Async-equivalent method of my original code.

sa_ddam213 Over a year ago

There are many Async extension in that namespace that are very handy when working with EntityFramework, FirstOrDefaluAsync and ForEachAsync are some of my everyday uses

Peter Duniho · Accepted Answer · 2014-10-30 01:56:32Z

6

Technically, that method doesn't return a lambda. It returns a List<Review>.

It's true what you posted won't compile. But this would:

public async Task<List<Review>> GetTitleReviews(int titleID)
{
    using (var context = new exampleEntities())
    {
        return await Task.Run(() => context.Reviews.Where(x => x.Title_Id == titleID).ToList());
    }
}

If that doesn't answer your question, maybe you can be more clear about exactly what you're trying to accomplish and why the above doesn't do it.

answered Oct 30, 2014 at 1:56

Peter Duniho

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3 Comments

globetrotter Over a year ago

I've just tried both yours and @sa_ddam213's solutions and both work, but his solution seems more straightforward. How do your solutions differ in the back-end?

Peter Duniho Over a year ago

The biggest difference is that the other solution requires loading EF into the process for just one method. If you already have EF referenced, it's not a bad approach. But I wouldn't want to take a dependency on EF just for that.

Peter Duniho Over a year ago

I should also mention: each scenario's different, but there can be other performance issues using the EF version, as compared with wrapping the entire expression in a Task. Here, we are only assured of deferred execution for the LINQ Where() method. Ideally, the Reviews property would be fast; it might be deferred execution, or precomputed. In general, we're should be able to assume that property getters are fast. But not everyone follows that rule. Using the EF method could expose the code to unwanted latency if the elements of the expression before the ToListAsync() are slow.

Ângelo Polotto · Accepted Answer · 2021-06-21 19:13:49Z

0

I solved this problem creating an Enumerable extension. That was needed for me because Xamarin Forms don't have Linq ToListAsync support. This is my extension:

public static class EnumerableExtension
{
    public static Task<List<T>> ToListAsync<T>(this IEnumerable<T> enumerable)
    {
        return Task.Run(enumerable.ToList);
    }
}

answered Jun 21, 2021 at 19:13

Ângelo Polotto

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2 Comments

Theodor Zoulias Over a year ago

That's a neat, but also controversial idea.

Ângelo Polotto Over a year ago

For me that this problem appeared like that: that's a screen made in Xamarin Forms is making the app stop... Solve it! So, I found the same problem along all the app: synchronously LINQ operations everywhere. That lock the main thread and, consequently, make the Android close the app. Xamarin Forms don't have ToListAsync and repeat my self with Task.Run(()=>...) it's not good in a big code base. My only ways out was: to create an extension or a static method. I choose extension for the flexibility. I never even know that's a controversial ideia... Thanks to share this knowledge!

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