Async/Await function hangs when caller has infinite loop

Question

I have read that Unity now supports the async/await feature of the C# language, so I try writing some script and attach it to the GameObject in the scene:

 using UnityEngine;
 using System.Threading.Tasks;
 
 public class TestAsync : MonoBehaviour{
     bool bWhileTrue = true;
 
     void Start(){
         func();
     }
 
     void func(){
         Task task = longRunning();
         while(bWhileTrue){
             Debug.Log("func: In WhileTrue loop");
         }
         Debug.Log("func: After whileTrue loop");
     }
         
     async Task longRunning(){
         await Task.Delay(3000);
         Debug.Log("Async: After delay");
         bWhileTrue = false;
     }
 }

What I expect is that
1) from Start(), func() will be called
2) In func(), the (async) longRunning() function is called
3) longRunning() hits the await line, and returns to func(), its caller, while the Task.Delay(3000) is performed asynchronusly.
4) func() enters the infinite loop, and keeps looping because the bWhileTrue is true, printing "func: In whileTrue loop"
5) Eventually, Task.Delay(3000) finishes, and the rest of the longRunning() is performed so that bWhileTrue is set to false.
6) Since bWhileTrue is set to false, func() can exit the infinite while loop, and prints "func: After whileTrue loop"
7) func() returns to Start(), and Start() complete

But when press play, the Editor hangs instead.

I am not sure what I misunderstand because if it is what I think, the Editor should not hang for longer than 3 seconds at maximum. Sorry if I miss anything basic, and that the code seems theoretical, but I am not very familiar with asynchronus programming and want to test how it works in Unity.

As a comparison, this code works fine for C# console application:

 using System;
 using System.Threading.Tasks;
 
 namespace TestAsync {
     class Program {
         static bool bWhileTrue = true;
 
         static void Main(string[] args) {
             func();
         }
 
         static void func(){
             Task task = longRunning();
             while(bWhileTrue) {
                 Console.WriteLine("func: In WhileTrue loop");
             }
             Console.WriteLine("func: After whileTrue loop");
         }
         
         static async Task longRunning(){
             await Task.Delay(3000);
             Console.WriteLine("Async: After delay");
             bWhileTrue = false;
         }
     }
 }

Answer 1

Best Answer

Answer by Bunny83 · Jan 21, 2021 at 06:41 PM

Tasks do not necessarily run on a seperate thread. The default synchronisation context in plain C# / .NET uses a threadpool to run the tasks. However Unity uses it's own context which schedules tasks on the main thread, just like coroutines. Otherwise you couldn't interact with the UnityAPI from within your Task as it's not thread safe. Therefore your task does never finish because you block the main thread and you never return the control to Unity so it can never schedule your task. Your example requires that the task runs on a seperate thread which does not happen in Unity. Here's a rather up-to-date blog about Unity and async / await.

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reevthechameleon · Jan 22, 2021 at 02:51 AM 0

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I see. Thanks for the answer. As per your answer, I have adjusted the code in func() so that it can return control back to Unity:

 async void func(){
     Task task = longRunning();
     while(bWhileTrue){
         Debug.Log("func: In WhileTrue loop");
         await Task.Yield();
     }
     Debug.Log("func: After whileTrue loop");
 }

And now it runs fine.
Thanks for the link too! Will check it out in detail later.

Async/Await function hangs when caller has infinite loop

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