Calling async method synchronously

Question

I have an async method:

public async Task<string> GenerateCodeAsync()
{
    string code = await GenerateCodeService.GenerateCodeAsync();
    return code;
}

I need to call this method from a synchronous method.

How can I do this without having to duplicate the GenerateCodeAsync method in order for this to work synchronously?

Update

Yet no reasonable solution found.

However, I see that HttpClient already implements this pattern

using (HttpClient client = new HttpClient())
{
    // async
    HttpResponseMessage responseAsync = await client.GetAsync(url);

    // sync
    HttpResponseMessage responseSync = client.GetAsync(url).Result;
}

Solution 1

You can access the Result property of the task, which will cause your thread to block until the result is available:

string code = GenerateCodeAsync().Result;

---

Note: In some cases, this might lead to a deadlock: Your call to Result blocks the main thread, thereby preventing the remainder of the async code to execute. You have the following options to make sure that this doesn't happen:

• Add .ConfigureAwait(false) to your library method or

• explicitly execute your async method in a thread pool thread and wait for it to finish:

    string code = Task.Run(() => GenerateCodeAsync).Result;
  

This does not mean that you should just mindlessly add .ConfigureAwait(false) after all your async calls! For a detailed analysis on why and when you should use .ConfigureAwait(false), see the following blog post:

• .NET Blog: ConfigureAwait FAQ

Solution 2

You should get the awaiter (GetAwaiter()) and end the wait for the completion of the asynchronous task (GetResult()).

string code = GenerateCodeAsync().GetAwaiter().GetResult();

Solution 3

You should be able to get this done using delegates, lambda expression

private void button2_Click(object sender, EventArgs e)
    {

        label1.Text = "waiting....";

        Task<string> sCode = Task.Run(async () =>
        {
            string msg =await GenerateCodeAsync();
            return msg;
        });

        label1.Text += sCode.Result;

    }

    private Task<string> GenerateCodeAsync()
    {
        return Task.Run<string>(() => GenerateCode());
    }

    private string GenerateCode()
    {
        Thread.Sleep(2000);
        return "I m back" ;
    }

Solution 4

I need to call this method from a synchronously method.

It's possible with GenerateCodeAsync().Result or GenerateCodeAsync().Wait(), as the other answer suggests. This would block the current thread until GenerateCodeAsync has completed.

However, your question is tagged with asp.net, and you also left the comment:

I was hoping for a simpler solution, thinking that asp.net handled this much easier than writing so many lines of code

My point is, you should not be blocking on an asynchronous method in ASP.NET. This will reduce the scalability of your web app, and may create a deadlock (when an await continuation inside GenerateCodeAsync is posted to AspNetSynchronizationContext). Using Task.Run(...).Result to offload something to a pool thread and then block will hurt the scalability even more, as it incurs +1 more thread to process a given HTTP request.

ASP.NET has built-in support for asynchronous methods, either through asynchronous controllers (in ASP.NET MVC and Web API) or directly via AsyncManager and PageAsyncTask in classic ASP.NET. You should use it. For more details, check this answer.

Solution 5

Microsoft Identity has extension methods which call async methods synchronously. For example there is GenerateUserIdentityAsync() method and equal CreateIdentity()

If you look at UserManagerExtensions.CreateIdentity() it look like this:

 public static ClaimsIdentity CreateIdentity<TUser, TKey>(this UserManager<TUser, TKey> manager, TUser user,
        string authenticationType)
        where TKey : IEquatable<TKey>
        where TUser : class, IUser<TKey>
    {
        if (manager == null)
        {
            throw new ArgumentNullException("manager");
        }
        return AsyncHelper.RunSync(() => manager.CreateIdentityAsync(user, authenticationType));
    }

Now lets see what AsyncHelper.RunSync does

  public static TResult RunSync<TResult>(Func<Task<TResult>> func)
    {
        var cultureUi = CultureInfo.CurrentUICulture;
        var culture = CultureInfo.CurrentCulture;
        return _myTaskFactory.StartNew(() =>
        {
            Thread.CurrentThread.CurrentCulture = culture;
            Thread.CurrentThread.CurrentUICulture = cultureUi;
            return func();
        }).Unwrap().GetAwaiter().GetResult();
    }

So, this is your wrapper for async method. And please don't read data from Result - it will potentially block your code in ASP.

There is another way - which is suspicious for me, but you can consider it too

  Result r = null;

            YourAsyncMethod()
                .ContinueWith(t =>
                {
                    r = t.Result;
                })
                .Wait();

View more solutions

Comments

[Can Sahin]

possible duplicate of How would I run an async Task<T> method synchronously?

[Catalin]

I was hoping for a simpler solution, thinking that asp.net handled this much easier than writing so many lines of code

[Paulo Morgado]

Why don't just embrace async code? Ideally you'd want more async code, not less.

[Nicholas Petersen]

[Why don't just embrace async code?] Ha, it may be precisely because one is embracing async code that they need this solution as large parts of the project get converted! You cannot rebuild Rome in a day.

[dimzon]

Well, next step. I need to be able to kill thread, running my synchronous method so GenerateCodeAsync must be killed too

[Warty]

The answers given here are incorrect. You can still deadlock on .Result, even if you schedule on the thread pool, because you yourself can be running on the threadpool. It really boils down to mixing async + synchronous code is a really bad idea. If possible, consider making the root async and wrapping your long-running synchronous work either with the long running flag or a TaskCompletionSource and your own worker threads that process a queue and signal completion.

[lissajous]

@NicholasPetersen sometimes 3rd-party library can force you to do this. Example building dynammic messages in WithMessage method out of FluentValidation. There is no async API for this due to library design - WithMessage overloads are static. Other methods of passing dynamic arguments to WithMessage are strange.

[Catalin]

I am overwriting SaveChanges() method of DbContext, and here i am calling the async methods, so unfortunately async controller won't help me in this situation

[noseratio]

@RaraituL, use SaveChangesAsync instead.

[Catalin]

i did used SaveChangesAsync but i needed to enforce the same methods on SaveChanges also

[noseratio]

@RaraituL, in general, you don't mix async and sync code, pick euther model. You can implement both SaveChangesAsync and SaveChanges, just make sure they don't get called both in the same ASP.NET project.

[Catalin]

Not all .NET MVC filters support asynchronous code, for example IAuthorizationFilter, so i cannot use async all the way

[Yinda Yin]

If invoking result risks a deadlock, then when is it safe to get the result? Does every asynchronous call require Task.Run or ConfigureAwait(false)?

[Justin Skiles]

@Noseratio that is an unrealistic goal. There are too many libraries with asynchronous and synchronous code as well as situations in which using only one model is not possible. MVC ActionFilters don't support asynchronous code, for example.

[noseratio]

There is no "main thread" in ASP.NET (unlike a GUI app), but the deadlock is still possible because of how AspNetSynchronizationContext.Post serializes async continuations: Task newTask = _lastScheduledTask.ContinueWith(_ => SafeWrapCallback(action)); _lastScheduledTask = newTask;

[noseratio]

@RobertHarvey: If you have no control over the implementation of the async method you're blocking on, then yes, you should wrap it with Task.Run to stay safe. Or use something like WithNoContext to reduce redundant thread switching.

[Appetere]

This snippet will not compile. The return type from Task.Run is Task. See this MSDN blog for full explanation.

[Faiyaz]

Thanks for pointing out, yes it returns Task type. Replacing "string sCode" to Task<string> or var sCode should resolve it. Adding a full compile code for ease.

[dimzon]

[My point is, you should not be blocking on an asynchronous method in ASP.NET] imaginate you must implement some IMyInterface.MySync() method and this method somehow invoked in ASP.NET. You can't change IMyInterface contract.

[dimzon]

@Noserato, the question is about calling asynchronous method from synchronous. Sometime you can't change API you implementing. Let's say you implementing some synchronous interface from some 3-rd party framework "A" (you can't rewrite framework to asynchronous manner) but 3rd-party library "B" you are trying to use in your implementation has only asynchronous. Also resulting product is also library and can be used anywhere including ASP.NET etc.

[Warty]

NOTE: Calling .Result can still deadlock if the caller is on the thread pool itself. Take a scenario where the Thread Pool is of size 32 and 32 tasks are running and Wait()/Result waiting on a yet-to-be-scheduled 33rd task that wants to run on one of the waiting threads.

[Oliver]

We've run into deadlocks using this solution. Be warned.

[foka]

MSDN Task.GetAwaiter: This method is intended for compiler use rather than for use in application code.

[David Clarke]

What do you consider to be the issue with the second way you have suggested?

[Jonathan Hansen]

I still got the error Dialog popup (against my will), with the buttons 'Switch To' or 'Retry'…. however, the call actually executes and does return with a proper response.

[Theodor Zoulias]

@DavidClarke probably the thread safety issue of accessing a non-volatile variable from multiple threads without a lock.

[Scratte]

Kindly elaborate on your Answer. How is it used? How specifically does it help to answer the Question?

[ToolmakerSteve]

@Warty - to clarify, for the deadlock to happen, the thread pool would have to re-use the same thread that is waiting for the result. This will happen rarely - which is bad, because when it does happen, it won't be obvious what went wrong.

[ToolmakerSteve]

IMHO, the essence of what makes this more likely to succeed, is using Task.Run to wrap the call - thus moving it to the thread pool. However, based on other answer and comments, this does not ensure it will never deadlock - it might simply make the deadlock "rare" - thus even harder to track down what is going wrong.

[ToolmakerSteve]

Despite that Microsoft reference, its Task.Run that makes this work (when running on ASP.NET context thread). Wait is irrelevant - Result does the equivalent when task has not completed. See this SO Q&A.

[Frank Thomas]

As mentioned by others, this can result in random deadlock conditions - totally do not recommend it.

[Frank Thomas]

@ToolmakerSteve I don't agree with you. I looked at this reference and the code between the two is not the same. Pulling the .Result directly from the task can randomly result in deadlocks. I know, I've had the issue. Very hard to troubleshoot when it happens.

[Frank Thomas]

I would have to say that this is the most correct solution when you cannot make the calling method async. The best answer is to make the whole flow async.

[ToolmakerSteve]

@FrankThomas - Looking at the source reference mentioned in stackoverflow.com/a/35948021/199364, Task.Wait tests IsWaitNotificationEnabledOrNotRanToCompletion, then calls InternalWait. Future.Result tests IsWaitNotificationEnabledOrNotRanToCompletion, then calls GetResultCore, which does if (!IsCompleted) InternalWait. I don't see any difference, w.r.t. deadlock potential. Of course its impossible to prove absence of deadlocks, and any change in code changes timing, so its entirely possible to have one approach fail randomly, and the other work .. until it doesn't.

[ToolmakerSteve]

(I mean its impossible to prove absence, unless the absence is provable by design or by static analysis; my point is that you can't determine that Wait doesn't cause though Result does, given the implementation shown.)

[Can Sahin]

@FrankThomas: What alternative do you recommend? (Serious question) I'll gladly update my answer with a solution which does not risk deadlocks.

[Frank Thomas]

@ToolmakerSteve Thanks for the further explanation. I think it really comes down to trying to make the execution flows that need to be async to be async from beginning to end, whenever possible. Thanks again.

[Frank Thomas]

@Heinzi The only solid solution is to make a program flow constantly async from start to finish. If that is not possible. The only solution I found that didn't seem to generate deadlocks is answered below: var task = Task.Run(() => GenerateCodeAsync()); task.Wait(); string code = task.Result;

[Can Sahin]

@FrankThomas: Even there deadlock-freeness is not guaranteed, since Task.Run uses the thread pool (see ToolmakerSteve's comments on this and other answers). In addition, Task.Run executes the code in a different thread, which might cause GenerateCodeAsync to behave differently. It's really a pity that there is no built-in, deadlock-free method to execute async code synchronously.

[Frank Thomas]

@Heinzi I hear you mate. It's forced me to really decide to go async all the way or none of the way.

[Saturn K]

@Oliver, I just came across this article looking for why my application in WebForms was deadlocked when I had the code above. This does indeed cause a deadlock.