Run two async tasks in parallel and collect results in NET 4 5

Question

I ve been trying for a while to get something I thought would be simple working with  NET 4 5  I want to fire off two long running tasks at same time and collect the     results in in the best C  4 5  RTM  way  The following works but I don t like it because    I want Sleep to be an async method so it can await other methods It just looks clumsy with Task Run   I don t think this is even using any new language features at all    Working code   public static void Go         Console WriteLine  Starting         var task1   Task Run      gt  Sleep 5000            var task2   Task Run      gt  Sleep 3000         int totalSlept   task1 Result   task2 Result       Console WriteLine  Slept for a total of     totalSlept     ms       private static int Sleep int ms        Console WriteLine  Sleeping for     ms       Thread Sleep ms       Console WriteLine  Sleeping for     ms     FINISHED        return ms      Non working code   Update  This actually works and is the correct way to do it  the only problem is the Thread Sleep  This code doesn t work because the call to Sleep 5000  immediately starts the task running so Sleep 1000  doesn t run until it has completed  This is true even though Sleep is async and I m not using await or calling  Result too soon   I thought maybe there is a way to get a non-running Task lt T gt  by calling an async method so I could then call Start   on the two tasks  but I can t figure out how to get a Task lt T gt  from calling an async method   public static void Go         Console WriteLine  Starting         var task1   Sleep 5000         blocks     var task2   Sleep 1000        int totalSlept   task1 Result   task2 Result       Console WriteLine  Slept for     totalSlept     ms       private static async Task lt int gt  Sleep int ms        Console WriteLine  Sleeping for     ms       Thread Sleep ms       return ms

User · Answer

While your Sleep method is async  Thread Sleep is not   The whole idea of async is to reuse a single thread  not to start multiple threads   Because you ve blocked using a synchronous call to Thread Sleep  it s not going to work   I m assuming that Thread Sleep is a simplification of what you actually want to do   Can your actual implementation be coded as async methods   If you do need to run multiple synchronous blocking calls  look elsewhere I think

User · Answer

async Task lt int gt  LongTask1              return 0      async Task lt int gt  LongTask2              return 1               Task lt int gt  t1   LongTask1       Task lt int gt  t2   LongTask2       await Task WhenAll t1 t2        now we have t1 Result and t2 Result

User · Answer

It s weekend now       public async void Go                 Console WriteLine  Start fosterage                var t1   Sleep 5000   Kevin            var t2   Sleep 3000   Jerry            var result   await Task WhenAll t1  t2            Console WriteLine   My precious spare time last for only  result Max   ms            Console WriteLine  Press any key and take same beer               Console ReadKey               private static async Task lt int gt  Sleep int ms  string name                    Console WriteLine    name  going to sleep for  ms ms                   await Task Delay ms               Console WriteLine    name  waked up after  ms ms                  return ms

User · Answer

To answer this point       I want Sleep to be an async method so it can await other methods   you can maybe rewrite the Sleep function like this   private static async Task lt int gt  Sleep int ms        Console WriteLine  Sleeping for     ms       var task   Task Run      gt  Thread Sleep ms        await task      Console WriteLine  Sleeping for     ms    END        return ms     static void Main string   args        Console WriteLine  Starting         var task1   Sleep 2000       var task2   Sleep 1000        int totalSlept   task1 Result  task2 Result       Console WriteLine  Slept for     totalSlept     ms        Console ReadKey        running this code will output     Starting Sleeping for 2000 Sleeping for 1000   one second later   Sleeping for 1000END   one second later   Sleeping for 2000END Slept for 3000 ms

User · Answer

You should use Task Delay instead of Sleep for async programming and then use Task WhenAll to combine the task results  The tasks would run in parallel   public class Program               static void Main string   args                        Go                      public static void Go                         GoAsync                Console ReadLine                      public static async void GoAsync                          Console WriteLine  Starting                 var task1   Sleep 5000               var task2   Sleep 3000                int   result   await Task WhenAll task1  task2                Console WriteLine  Slept for a total of     result Sum       ms                        private async static Task lt int gt  Sleep int ms                        Console WriteLine  Sleeping for  0  at  1    ms  Environment TickCount               await Task Delay ms               Console WriteLine  Sleeping for  0  finished at  1    ms  Environment TickCount               return ms

User · Answer

This article helped explain a lot of things  It s in FAQ style   Async Await FAQ  This part explains why Thread Sleep runs on the same original thread - leading to my initial confusion      Does the    async    keyword cause the invocation of a method to queue to   the ThreadPool  To create a new thread  To launch a rocket ship to   Mars       No  No  And no  See the previous questions  The    async    keyword   indicates to the compiler that    await    may be used inside of the   method  such that the method may suspend at an await point and have   its execution resumed asynchronously when the awaited instance   completes  This is why the compiler issues a warning if there are no      awaits    inside of a method marked as    async

[c#] Run two async tasks in parallel and collect results in .NET 4.5

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