Awaiting multiple Tasks with different results

Question

I have 3 tasks   private async Task lt Cat gt  FeedCat      private async Task lt House gt  SellHouse      private async Task lt Tesla gt  BuyCar        They all need to run before my code can continue and I need the results from each as well  None of the results have anything in common with each other  How do I call and await for the 3 tasks to complete and then get the results

User · Answer

In case you are trying to log all errors make sure you keep Task.WhenAll line in your code, lot of comments suggest that you can remove it and wait for individual tasks. Task.WhenAll is really important for error handling. Without this line you potentially leaving your code open for unobserved exceptions.

var catTask = FeedCat();
var houseTask = SellHouse();
var carTask = BuyCar();

await Task.WhenAll(catTask, houseTask, carTask);

var cat = await catTask;
var house = await houseTask;
var car = await carTask;

Imagine FeedCat throws exception in the following code:

var catTask = FeedCat();
var houseTask = SellHouse();
var carTask = BuyCar();

var cat = await catTask;
var house = await houseTask;
var car = await carTask;

In that case you will never await on houseTask nor carTask. There are 3 possible scenarios here:

SellHouse is already completed successfully when FeedCat failed. In this case you are fine.
SellHouse is not complete and fails with exception at some point. Exception is not observed and will be rethrown on finalizer thread.
SellHouse is not complete and contains awaits inside it. In case your code runs in ASP.NET SellHouse will fail as soon as some of the awaits will completed inside it. This happens because you basically made fire & forget call and synchronization context was lost as soon as FeedCat failed.

Here is error that you will get for case (3):

System.AggregateException: A Task's exception(s) were not observed either by Waiting on the Task or accessing its Exception property. As a result, the unobserved exception was rethrown by the finalizer thread. ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Web.ThreadContext.AssociateWithCurrentThread(Boolean setImpersonationContext)
   at System.Web.HttpApplication.OnThreadEnterPrivate(Boolean setImpersonationContext)
   at System.Web.HttpApplication.System.Web.Util.ISyncContext.Enter()
   at System.Web.Util.SynchronizationHelper.SafeWrapCallback(Action action)
   at System.Threading.Tasks.Task.Execute()
   --- End of inner exception stack trace ---
---> (Inner Exception #0) System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Web.ThreadContext.AssociateWithCurrentThread(Boolean setImpersonationContext)
   at System.Web.HttpApplication.OnThreadEnterPrivate(Boolean setImpersonationContext)
   at System.Web.HttpApplication.System.Web.Util.ISyncContext.Enter()
   at System.Web.Util.SynchronizationHelper.SafeWrapCallback(Action action)
   at System.Threading.Tasks.Task.Execute()<---

For case (2) you will get similar error but with original exception stack trace.

For .NET 4.0 and later you can catch unobserved exceptions using TaskScheduler.UnobservedTaskException. For .NET 4.5 and later unobserved exceptions are swallowed by default for .NET 4.0 unobserved exception will crash your process.

More details here: Task Exception Handling in .NET 4.5

User · Answer

After you use WhenAll  you can pull the results out individually with await   var catTask   FeedCat    var houseTask   SellHouse    var carTask   BuyCar     await Task WhenAll catTask  houseTask  carTask    var cat   await catTask  var house   await houseTask  var car   await carTask    You can also use Task Result  since you know by this point they have all completed successfully   However  I recommend using await because it s clearly correct  while Result can cause problems in other scenarios

User · Answer

Forward Warning  Just a quick headsup to those visiting this and other similar threads looking for a way to parallelize EntityFramework using async await task tool-set  The pattern shown here is sound  however  when it comes to the special snowflake of EF you will not achieve parallel execution unless and until you use a separate  new  db-context-instance inside each and every  Async   call involved   This sort of thing is necessary due to inherent design limitations of ef-db-contexts which forbid running multiple queries in parallel in the same ef-db-context instance     Capitalizing on the answers already given  this is the way to make sure that you collect all values even in the case that one or more of the tasks results in an exception     public async Task lt string gt  Foobar         async Task lt string gt  Awaited Task lt Cat gt  a  Task lt House gt  b  Task lt Tesla gt  c            return DoSomething await a  await b  await c              using  var carTask   BuyCarAsync        using  var catTask   FeedCatAsync        using  var houseTask   SellHouseAsync                  if  carTask Status    TaskStatus RanToCompletion   triple              amp  amp  catTask Status    TaskStatus RanToCompletion   cache              amp  amp  houseTask Status    TaskStatus RanToCompletion      hits             return Task FromResult DoSomething catTask Result  carTask Result  houseTask Result      fast-track                    cat   await catTask          car   await carTask          house   await houseTask            or Task AwaitAll carTask  catTask  houseTask             or await Task WhenAll carTask  catTask  houseTask             it depends on how you like exception handling better          return Awaited catTask  carTask  houseTask             An alternative implementation that has more or less the same performance characteristics could be    public async Task lt string gt  Foobar         using  var carTask   BuyCarAsync        using  var catTask   FeedCatAsync        using  var houseTask   SellHouseAsync                  cat   catTask Status    TaskStatus RanToCompletion   catTask Result    await catTask           car   carTask Status    TaskStatus RanToCompletion   carTask Result    await carTask           house   houseTask Status    TaskStatus RanToCompletion   houseTask Result    await houseTask            return DoSomething cat  car  house

User · Answer

If you re using C  7  you can use a handy wrapper method like this     public static class TaskEx       public static async Task lt  T1  T2  gt  WhenAll lt T1  T2 gt  Task lt T1 gt  task1  Task lt T2 gt  task2                return  await task1  await task2                to enable convenient syntax like this when you want to wait on multiple tasks with different return types  You d have to make multiple overloads for different numbers of tasks to await  of course   var  someInt  someString    await TaskEx WhenAll GetIntAsync    GetStringAsync       However  see Marc Gravell s answer for some optimizations around ValueTask and already-completed tasks if you intend to turn this example into something real

User · Answer

Just await the three tasks separately  after starting them all  var catTask   FeedCat    var houseTask   SellHouse    var carTask   BuyCar     var cat   await catTask  var house   await houseTask  var car   await carTask

User · Answer

You can store them in tasks  then await them all   var catTask   FeedCat    var houseTask   SellHouse    var carTask   BuyCar     await Task WhenAll catTask  houseTask  carTask    Cat cat   await catTask  House house   await houseTask  Car car   await carTask

User · Answer

Given three tasks - FeedCat    SellHouse   and BuyCar    there are two interesting cases  either they all complete synchronously  for some reason  perhaps caching or an error   or they don t   Let s say we have  from the question   Task lt string gt  DoTheThings         Task lt Cat gt  x   FeedCat        Task lt House gt  y   SellHouse        Task lt Tesla gt  z   BuyCar           what here      Now  a simple approach would be   Task WhenAll x  y  z     but     that isn t convenient for processing the results  we d typically want to await that   async Task lt string gt  DoTheThings         Task lt Cat gt  x   FeedCat        Task lt House gt  y   SellHouse        Task lt Tesla gt  z   BuyCar         await Task WhenAll x  y  z          presumably we want to do something with the results        return DoWhatever x Result  y Result  z Result       but this does lots of overhead and allocates various arrays  including the params Task   array  and lists  internally   It works  but it isn t great IMO  In many ways it is simpler to use an async operation and just await each in turn   async Task lt string gt  DoTheThings         Task lt Cat gt  x   FeedCat        Task lt House gt  y   SellHouse        Task lt Tesla gt  z   BuyCar            do something with the results        return DoWhatever await x  await y  await z       Contrary to some of the comments above  using await instead of Task WhenAll makes no difference to how the tasks run  concurrently  sequentially  etc   At the highest level  Task WhenAll predates good compiler support for async await  and was useful when those things didn t exist  It is also useful when you have an arbitrary array of tasks  rather than 3 discreet tasks   But  we still have the problem that async await generates a lot of compiler noise for the continuation  If it is likely that the tasks might actually complete synchronously  then we can optimize this by building in a synchronous path with an asynchronous fallback   Task lt string gt  DoTheThings         Task lt Cat gt  x   FeedCat        Task lt House gt  y   SellHouse        Task lt Tesla gt  z   BuyCar         if x Status    TaskStatus RanToCompletion  amp  amp         y Status    TaskStatus RanToCompletion  amp  amp         z Status    TaskStatus RanToCompletion          return Task FromResult            DoWhatever a Result  b Result  c Result              we can safely access  Result  as they are known           to be ran-to-completion      return Awaited x  y  z      async Task Awaited Task lt Cat gt  a  Task lt House gt  b  Task lt Tesla gt  c        return DoWhatever await x  await y  await z       This  sync path with async fallback  approach is increasingly common especially in high performance code where synchronous completions are relatively frequent  Note it won t help at all if the completion is always genuinely asynchronous   Additional things that apply here    with recent C   a common pattern is for the async fallback method is commonly implemented as a local function   Task lt string gt  DoTheThings         async Task lt string gt  Awaited Task lt Cat gt  a  Task lt House gt  b  Task lt Tesla gt  c            return DoWhatever await a  await b  await c             Task lt Cat gt  x   FeedCat        Task lt House gt  y   SellHouse        Task lt Tesla gt  z   BuyCar         if x Status    TaskStatus RanToCompletion  amp  amp         y Status    TaskStatus RanToCompletion  amp  amp         z Status    TaskStatus RanToCompletion          return Task FromResult            DoWhatever a Result  b Result  c Result              we can safely access  Result  as they are known           to be ran-to-completion      return Awaited x  y  z      prefer ValueTask lt T gt  to Task lt T gt  if there is a good chance of things ever completely synchronously with many different return values   ValueTask lt string gt  DoTheThings         async ValueTask lt string gt  Awaited ValueTask lt Cat gt  a  Task lt House gt  b  Task lt Tesla gt  c            return DoWhatever await a  await b  await c             ValueTask lt Cat gt  x   FeedCat        ValueTask lt House gt  y   SellHouse        ValueTask lt Tesla gt  z   BuyCar         if x IsCompletedSuccessfully  amp  amp         y IsCompletedSuccessfully  amp  amp         z IsCompletedSuccessfully          return new ValueTask lt string gt             DoWhatever a Result  b Result  c Result              we can safely access  Result  as they are known           to be ran-to-completion      return Awaited x  y  z      if possible  prefer IsCompletedSuccessfully to Status    TaskStatus RanToCompletion  this now exists in  NET Core for Task  and everywhere for ValueTask lt T gt

User · Answer

var dn   await Task WhenAll lt dynamic gt  FeedCat   SellHouse   BuyCar       if you want to access Cat  you do this   var ct    Cat dn 0     This is very simple to do and very useful to use  there is no need to go after a complex solution

User · Answer

Use Task WhenAll and then await the results   var tCat   FeedCat    var tHouse   SellHouse    var tCar   BuyCar    await Task WhenAll tCat  tHouse  tCar   Cat cat   await tCat  House house   await tHouse  Tesla car   await tCar     as they have all definitely finished  you could also use Task Value

User · Answer

You can use Task WhenAll as mentioned  or Task WaitAll  depending on whether you want the thread to wait   Take a look at the link for an explanation of both   WaitAll vs WhenAll

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