async Controller with async Action doesn't work

Question

I have async controller with async action. In the action I call WCF service method in SomeMethodOne (it needs 10 seconds to return result) and then I execute some mathematical operations in SomeMethodTwo (it executes about 6 seconds on my computer). As I understand during waiting on result from WCF service method, my computer should execute SomeMethodTwo but it doesn't and all code executes 10 seconds + 6 seconds = 16 seconds. Why?

public class TestController : AsyncController
{
    public async Task<ActionResult> Index()
    {
        string result =  await SomeMethodOne();

        SomeMethodTwo();

        return View();
    }

    private async Task<string> SomeMethodOne() // it needs 10 seconds to return result from WCF service
    {
        using (Service1Client client = new Service1Client())
        {
            return await client.GetDataAsync(5);
        }
    }

    private void SomeMethodTwo() // it executes about 6 seconds on my computer
    {
        double result = 0;
        for (int i = 0; i < 1000000000; i++)
        {
            result += Math.Sqrt(i);
        }
    }
}

The WCF service which I run locally:

public class Service1 : IService1
{
    public string GetData(int value)
    {
        Thread.Sleep(10000);
        return string.Format("You entered: {0}", value);
    }        
}

async does not mean it will not take 16 seconds, it will take 16 seconds — Ehsan Sajjad
– Ehsan Sajjad, Commented Feb 9, 2016 at 11:45
So where is advantage of using async when we call single WCF method? — user3691221
– user3691221, Commented Feb 9, 2016 at 12:04
async just make sure here that your thread is freed in to the app pool and when operation completes then it picks again thread from pool and start executing next — Ehsan Sajjad
– Ehsan Sajjad, Commented Feb 9, 2016 at 12:06
Could you give me example where using async controller and action has more sense? — user3691221
– user3691221, Commented Feb 9, 2016 at 12:20
AsyncController is deprecated. You only need to use async Task<string> on a method to make it asynchronous. — Panagiotis Kanavos
– Panagiotis Kanavos, Commented Feb 9, 2016 at 13:06

Stephen Cleary · Accepted Answer · 2016-02-09 13:03:57Z

3

Your problem is that you're using await right away:

string result =  await SomeMethodOne();

The await means that your controller action is going to "asynchronously wait" (await) for the result of SomeMethodOne before it continues executing.

If you want to do asynchronous concurrency, then don't await right away. Instead, you can start the asynchronous operation going by calling the method and then await later:

public async Task<ActionResult> Index()
{
  Task<string> firstOperation = SomeMethodOne();

  SomeMethodTwo();

  string result = await firstOperation;

  return View();
}

answered Feb 9, 2016 at 13:03

Stephen Cleary

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5 Comments

Ehsan Sajjad Over a year ago

is'nt this statement is synchronous here ? Task<string> firstOperation = SomeMethodOne(); ?

Stephen Cleary Over a year ago

@EhsanSajjad: Well, you're synchronously starting an asynchronous operation.

Ehsan Sajjad Over a year ago

how await will behave here ?

Ehsan Sajjad Over a year ago

is'nt await line be reached after both methodone and methodtwo are executed completely?

Stephen Cleary Over a year ago

@EhsanSajjad: I have an async intro on my blog that explains the particulars.

Jon Hanna · Accepted Answer · 2016-02-09 13:05:18Z

and then I execute [emphasis mine]

Doing one thing and then doing another thing will take as long as both of them added together.

Doing two things at the same time might be quicker. It might be slower, because of context switching (imagine someone doing lots of "multitasking" and spending more time switching between them than working). It's likely it will be quicker here, if you don't have to get the results from the first action in order to do the second:

public async Task<ActionResult> Index()
{
    Task<string> task =  SomeMethodOne();

    SomeMethodTwo();

    string result = await task;

    return View();
}

Obviously if you needed result before you could call SomeMethodTwo() then this wouldn't be possible. There is still an advantage on awaiting SomeMethodOne() (which should be called SomeMethodOneAsync() if possible to fit with .NET conventions), in that if GetDataAsync() is truly async then the thread that was executing this action method can do something else for some other request to your web application, and another thread will pick up on dealing with this one when the I/O operation has returned data. This doesn't help the performance of the single method involved, but does help overall scalability of all the methods being run on the machine for all the web requests.

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