How to safely call an async method in C# without await

Question

I have an async method which returns no data:

public async Task MyAsyncMethod()
{
    // do some stuff async, don't return any data
}

I'm calling this from another method which returns some data:

public string GetStringData()
{
    MyAsyncMethod(); // this generates a warning and swallows exceptions
    return "hello world";
}

Calling MyAsyncMethod() without awaiting it causes a "Because this call is not awaited, the current method continues to run before the call is completed" warning in visual studio. On the page for that warning it states:

You should consider suppressing the warning only if you're sure that you don't want to wait for the asynchronous call to complete and that the called method won't raise any exceptions.

I'm sure I don't want to wait for the call to complete; I don't need to or have the time to. But the call might raise exceptions.

I've stumbled into this problem a few times and I'm sure it's a common problem which must have a common solution.

How do I safely call an async method without awaiting the result?

Update:

For people suggesting that I just await the result, this is code that is responding to a web request on our web service (ASP.NET Web API). Awaiting in a UI context keeps the UI thread free, but awaiting in a web request call will wait for the Task to finish before responding to the request, thereby increasing response times with no reason.

Answer 1

If you want to get the exception "asynchronously", you could do:

  MyAsyncMethod().
    ContinueWith(t => Console.WriteLine(t.Exception),
        TaskContinuationOptions.OnlyOnFaulted);

This will allow you to deal with an exception on a thread other than the "main" thread. This means you don't have to "wait" for the call to MyAsyncMethod() from the thread that calls MyAsyncMethod; but, still allows you to do something with an exception--but only if an exception occurs.

Update:

technically, you could do something similar with await:

try
{
    await MyAsyncMethod().ConfigureAwait(false);
}
catch (Exception ex)
{
    Trace.WriteLine(ex);
}

...which would be useful if you needed to specifically use try/catch (or using) but I find the ContinueWith to be a little more explicit because you have to know what ConfigureAwait(false) means.

Answer 2

You should first consider making GetStringData an async method and have it await the task returned from MyAsyncMethod.

If you're absolutely sure that you don't need to handle exceptions from MyAsyncMethod or know when it completes, then you can do this:

public string GetStringData()
{
  var _ = MyAsyncMethod();
  return "hello world";
}

BTW, this is not a "common problem". It's very rare to want to execute some code and not care whether it completes and not care whether it completes successfully.

Update:

Since you're on ASP.NET and wanting to return early, you may find my blog post on the subject useful. However, ASP.NET was not designed for this, and there's no guarantee that your code will run after the response is returned. ASP.NET will do its best to let it run, but it can't guarantee it.

So, this is a fine solution for something simple like tossing an event into a log where it doesn't really matter if you lose a few here and there. It's not a good solution for any kind of business-critical operations. In those situations, you must adopt a more complex architecture, with a persistent way to save the operations (e.g., Azure Queues, MSMQ) and a separate background process (e.g., Azure Worker Role, Win32 Service) to process them.

Answer 3

The answer by Peter Ritchie was what I wanted, and Stephen Cleary's article about returning early in ASP.NET was very helpful.

As a more general problem however (not specific to an ASP.NET context) the following Console application demonstrates the usage and behavior of Peter's answer using Task.ContinueWith(...)

static void Main(string[] args)
{
  try
  {
    // output "hello world" as method returns early
    Console.WriteLine(GetStringData());
  }
  catch
  {
    // Exception is NOT caught here
  }
  Console.ReadLine();
}

public static string GetStringData()
{
  MyAsyncMethod().ContinueWith(OnMyAsyncMethodFailed, TaskContinuationOptions.OnlyOnFaulted);
  return "hello world";
}

public static async Task MyAsyncMethod()
{
  await Task.Run(() => { throw new Exception("thrown on background thread"); });
}

public static void OnMyAsyncMethodFailed(Task task)
{
  Exception ex = task.Exception;
  // Deal with exceptions here however you want
}

GetStringData() returns early without awaiting MyAsyncMethod() and exceptions thrown in MyAsyncMethod() are dealt with in OnMyAsyncMethodFailed(Task task) and not in the try/catch around GetStringData()

Answer 4

I end up with this solution :

public async Task MyAsyncMethod()
{
    // do some stuff async, don't return any data
}

public string GetStringData()
{
    // Run async, no warning, exception are catched
    RunAsync(MyAsyncMethod()); 
    return "hello world";
}

private void RunAsync(Task task)
{
    task.ContinueWith(t =>
    {
        ILog log = ServiceLocator.Current.GetInstance<ILog>();
        log.Error("Unexpected Error", t.Exception);

    }, TaskContinuationOptions.OnlyOnFaulted);
}

Answer 5

This is called fire and forget, and there is an extension for that.

Consumes a task and doesn't do anything with it. Useful for fire-and-forget calls to async methods within async methods.

Install nuget package.

Use:

MyAsyncMethod().Forget();

Answer 6

I'm late to the party here, but there's an awesome library I've been using which I haven't seen referenced in the other answers

https://github.com/brminnick/AsyncAwaitBestPractices

If you need to "Fire And Forget" you call the extension method on the task.

Passing the action onException to the call ensures that you get the best of both worlds - no need to await execution and slow your users down, whilst retaining the ability to handle the exception in a graceful manner.

In your example you would use it like this:

   public string GetStringData()
    {
        MyAsyncMethod().SafeFireAndForget(onException: (exception) =>
                    {
                      //DO STUFF WITH THE EXCEPTION                    
                    }); 
        return "hello world";
    }

It also gives awaitable AsyncCommands implementing ICommand out the box which is great for my MVVM Xamarin solution

Answer 7

I guess the question arises, why would you need to do this? The reason for async in C# 5.0 is so you can await a result. This method is not actually asynchronous, but simply called at a time so as not to interfere too much with the current thread.

Perhaps it may be better to start a thread and leave it to finish on its own.

Answer 8

If you really wants to do this. Just to address "Call an async method in C# without await", you can execute the async method inside a Task.Run. This approach will wait until MyAsyncMethod finish.

public string GetStringData()
{
    Task.Run(()=> MyAsyncMethod()).Result;
    return "hello world";
}

await asynchronously unwraps the Result of your task, whereas just using Result would block until the task had completed.

Answer 9

On technologies with message loops (not sure if ASP is one of them), you can block the loop and process messages until the task is over, and use ContinueWith to unblock the code:

public void WaitForTask(Task task)
{
    DispatcherFrame frame = new DispatcherFrame();
    task.ContinueWith(t => frame.Continue = false));
    Dispatcher.PushFrame(frame);
}

This approach is similar to blocking on ShowDialog and still keeping the UI responsive.

Answer 10

The solution is start the HttpClient into another execution task without sincronization context:

var submit = httpClient.PostAsync(uri, new StringContent(body, Encoding.UTF8,"application/json"));
var t = Task.Run(() => submit.ConfigureAwait(false));
await t.ConfigureAwait(false);

Answer 11

Typically async method returns Task class. If you use Wait() method or Result property and code throws exception - exception type gets wrapped up into AggregateException - then you need to query Exception.InnerException to locate correct exception.

But it's also possible to use .GetAwaiter().GetResult() instead - it will also wait async task, but will not wrap exception.

So here is short example:

public async Task MyMethodAsync()
{
}

public string GetStringData()
{
    MyMethodAsync().GetAwaiter().GetResult();
    return "test";
}

You might want also to be able to return some parameter from async function - that can be achieved by providing extra Action<return type> into async function, for example like this:

public string GetStringData()
{
    return MyMethodWithReturnParameterAsync().GetAwaiter().GetResult();
}

public async Task<String> MyMethodWithReturnParameterAsync()
{
    return "test";
}

Please note that async methods typically have ASync suffix naming, just to be able to avoid collision between sync functions with same name. (E.g. FileStream.ReadAsync) - I have updated function names to follow this recommendation.

Answer 12

Maybe I'm too naive but, couldn't you create an event that is raised when GetStringData() is called and attach an EventHandler that calls and awaits the async method?

Something like:

public event EventHandler FireAsync;

public string GetStringData()
{
   FireAsync?.Invoke(this, EventArgs.Empty);
   return "hello world";
}

public async void HandleFireAsync(object sender, EventArgs e)
{
   await MyAsyncMethod();
}

And somewhere in the code attach and detach from the event:

FireAsync += HandleFireAsync;

(...)

FireAsync -= HandleFireAsync;

Not sure if this might be anti-pattern somehow (if it is please let me know), but it catches the Exceptions and returns quickly from GetStringData().