Does the use of async/await create a new thread?

Question

I am new to TPL and I am wondering: How does the asynchronous programming support that is new to C# 5.0 (via the new async and await keywords) relate to the creation of threads?

Specifically, does the use of async/await create a new thread each time that they are used? And if there many nested methods that use async/await, is a new thread created for each of those methods?

Answer 1

In short NO

From Asynchronous Programming with Async and Await : Threads

The async and await keywords don't cause additional threads to be created. Async methods don't require multithreading because an async method doesn't run on its own thread. The method runs on the current synchronization context and uses time on the thread only when the method is active. You can use Task.Run to move CPU-bound work to a background thread, but a background thread doesn't help with a process that's just waiting for results to become available.

Answer 2

According to MSDN : async keyword

An async method runs synchronously until it reaches its first await expression, at which point the method is suspended until the awaited task is complete. In the meantime, control returns to the caller of the method, as the example in the next section shows.

Here is a sample code to check it :

class Program  {     static void Main(string[] args)     {         Program p = new Program();         p.Run();     }      private void Print(string txt)     {         string dateStr = DateTime.Now.ToString("HH:mm:ss.fff");         Console.WriteLine($"{dateStr} Thread #{Thread.CurrentThread.ManagedThreadId}\t{txt}");     }      private void Run()     {         Print("Program Start");         Experiment().Wait();         Print("Program End. Press any key to quit");         Console.Read();     }      private async Task Experiment()     {         Print("Experiment code is synchronous before await");         await Task.Delay(500);         Print("Experiment code is asynchronous after first await");     } } 

And the result :

We see the code of Experiment() method after await executes on another Thread.

But if I replace the Task.Delay by my own code (method SomethingElse) :

   class Program {     static void Main(string[] args)     {         Program p = new Program();         p.Run();     }      private void Print(string txt)     {         string dateStr = DateTime.Now.ToString("HH:mm:ss.fff");         Console.WriteLine($"{dateStr} Thread #{Thread.CurrentThread.ManagedThreadId}\t{txt}");     }      private void Run()     {         Print("Program Start");         Experiment().Wait();         Print("Program End. Press any key to quit");         Console.Read();     }      private async Task Experiment()     {         Print("Experiment code is synchronous before await");         await SomethingElse();         Print("Experiment code is asynchronous after first await");     }      private Task SomethingElse()     {         Print("Experiment code is asynchronous after first await");         Thread.Sleep(500);         return (Task.CompletedTask);     } } 

I notice the thread remains the same !

In conclusion, I'll say async/await code could use another thread, but only if the thread is created by another code, not by async/await.

In this case, I think Task.Delay created the thread, so I can conclude async/await does not create a new Thread like said by @Adriaan Stander.