Entity Framework Queryable async

The problem seems to be that you have misunderstood how async/await work with Entity Framework.

About Entity Framework

So, let’s look at this code:

public IQueryable<URL> GetAllUrls()
{
    return context.Urls.AsQueryable();
}

and example of it usage:

repo.GetAllUrls().Where(u => <condition>).Take(10).ToList()

What happens there?

1. We are getting IQueryable object (not accessing database yet) using repo.GetAllUrls()
2. We create a new IQueryable object with specified condition using .Where(u => <condition>
3. We create a new IQueryable object with specified paging limit using .Take(10)
4. We retrieve results from database using .ToList(). Our IQueryable object is compiled to sql (like select top 10 * from Urls where <condition>). And database can use indexes, sql server send you only 10 objects from your database (not all billion urls stored in database)

Okay, let’s look at first code:

public async Task<IQueryable<URL>> GetAllUrlsAsync()
{
    var urls = await context.Urls.ToListAsync();
    return urls.AsQueryable();
}

With the same example of usage we got:

1. We are loading in memory all billion urls stored in your database using await context.Urls.ToListAsync();.
2. We got memory overflow. Right way to kill your server

About async/await

Why async/await is preferred to use? Let’s look at this code:

var stuff1 = repo.GetStuff1ForUser(userId);
var stuff2 = repo.GetStuff2ForUser(userId);
return View(new Model(stuff1, stuff2));

What happens here?

1. Starting on line 1 var stuff1 = ...
2. We send request to sql server that we want to get some stuff1 for userId
3. We wait (current thread is blocked)
4. We wait (current thread is blocked)
5. …..
6. Sql server send to us response
7. We move to line 2 var stuff2 = ...
8. We send request to sql server that we want to get some stuff2 for userId
9. We wait (current thread is blocked)
10. And again
11. …..
12. Sql server send to us response
13. We render view

So let’s look to an async version of it:

var stuff1Task = repo.GetStuff1ForUserAsync(userId);
var stuff2Task = repo.GetStuff2ForUserAsync(userId);
await Task.WhenAll(stuff1Task, stuff2Task);
return View(new Model(stuff1Task.Result, stuff2Task.Result));

What happens here?

1. We send request to sql server to get stuff1 (line 1)
2. We send request to sql server to get stuff2 (line 2)
3. We wait for responses from sql server, but current thread isn’t blocked, he can handle queries from another users
4. We render view

Right way to do it

So good code here:

using System.Data.Entity;

public IQueryable<URL> GetAllUrls()
{
   return context.Urls.AsQueryable();
}

public async Task<List<URL>> GetAllUrlsByUser(int userId) {
   return await GetAllUrls().Where(u => u.User.Id == userId).ToListAsync();
}

Note, than you must add using System.Data.Entity in order to use method ToListAsync() for IQueryable.

Note, that if you don’t need filtering and paging and stuff, you don’t need to work with IQueryable. You can just use await context.Urls.ToListAsync() and work with materialized List<Url>.