ASP.NET MVC 4 asyncronous database call: never returns the HTTP response

Question

I tried to make an async database call to SQL Server and use it along with new ASP.NET MVC 4 async features but it strangely does not return at all. I debugged the code, works nicely but somehow the HTTP request hangs open forever.

Here is what I did:

This is the database call method:

public async Task<IEnumerable<Car>> GetCarsAsync() {

    var connectionString = ConfigurationManager.ConnectionStrings["CarGalleryConnStr"].ConnectionString;
    var asyncConnectionString = new SqlConnectionStringBuilder(connectionString) {
        AsynchronousProcessing = true
    }.ToString();

    using (var conn = new SqlConnection(asyncConnectionString)) {
        using (var cmd = new SqlCommand()) {

            cmd.Connection = conn;
            cmd.CommandText = selectStatement;
            cmd.CommandType = CommandType.Text;

            conn.Open();

            using (var reader = await cmd.ExecuteReaderAsync()) {

                return reader.Select(r => carBuilder(r)).ToList();
            }
        }
    }
}

Here, Select method on SqlDataReader is an extension method which I implemented. carBuilder private method just returns back an instance of Car class.

This is the controller class:

public class HomeController : Controller {

    private readonly GalleryContext ctx = new GalleryContext();

    public async Task<ViewResult> IndexAsync() {

        return View("Index", await ctx.GetCarsAsync());
    }
}

Any idea what I am missing?

Also see stackoverflow.com/q/8478691/27535 and stackoverflow.com/q/1733027/27535 — gbn
– gbn, Commented Apr 3, 2012 at 8:37
@gbn so, do you think the implementation is wrong or pointless? Can u also have a look at this: dba.stackexchange.com/questions/15991/… — tugberk
– tugberk, Commented Apr 3, 2012 at 8:47

nemesv · Accepted Answer · 2012-04-02 21:19:50Z

3

Your controller still needs to derive from AsyncController (see Exercise 4: Using Asynchronous Controllers), but now you need to write less code to achieve the same result with Task<T>.

So this should work:

public class HomeController : AsyncController {

    private readonly GalleryContext ctx = new GalleryContext();

    public async Task<ViewResult> IndexAsync() {

        return View("Index", await ctx.GetCarsAsync());
    }
}

answered Apr 2, 2012 at 21:19

nemesv

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4 Comments

tugberk Over a year ago

I don't think it has to. I have several other Task-based async actions which consumes web services asynchronously and the controller for those derived from Controller class. Let me try, though.

tugberk Over a year ago

ah, bummer! It worked but it is strange because as I said, I have several other Task-based async actions which consumes web services asynchronously and the controller for those derived from Controller class.

nemesv Over a year ago

I don't know what is causing the difference... but according to the sample and the other articles online, you should derive from AsyncController.

tugberk Over a year ago

Hi, please see the @marcind's answer. That is the solution.

marcind · Accepted Answer · 2012-04-04 19:51:29Z

2

There is a bug in MVC 4 Beta that causes a hang when you return an async Task that completes really quickly.

The workaround to it is to add the following as the first line of your action method:

await Task.Yield();

Could you see if that fixes it?

More info here: http://aspnetwebstack.codeplex.com/workitem/22

edited Apr 4, 2012 at 19:51

answered Apr 2, 2012 at 21:37

marcind

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