.net - C# async/await strange behavior in console app -

i built async/await demo console app , strange result. code:

class program {     public static void beginlongio(action act)     {         console.writeline("in beginlongio start... {0} {1}", (datetime.now.ticks - ticks) / timespan.tickspermillisecond, thread.currentthread.managedthreadid);         thread.sleep(1000);         act();         console.writeline("in beginlongio end... \t{0} {1}", (datetime.now.ticks - ticks) / timespan.tickspermillisecond, thread.currentthread.managedthreadid);     }      public static int32 endlongio()     {         console.writeline("in endlongio start... \t{0} {1}", (datetime.now.ticks - ticks) / timespan.tickspermillisecond, thread.currentthread.managedthreadid);         thread.sleep(500);         console.writeline("in endlongio end... \t{0} {1}", (datetime.now.ticks - ticks) / timespan.tickspermillisecond, thread.currentthread.managedthreadid);         return 42;     }      public static task<int32> longioasync()     {         console.writeline("in longioasync start... {0} {1}", (datetime.now.ticks - ticks) / timespan.tickspermillisecond, thread.currentthread.managedthreadid);         var tcs = new taskcompletionsource<int32>();         beginlongio(() =>         {             try { tcs.trysetresult(endlongio()); }             catch (exception exc) { tcs.trysetexception(exc); }         });         console.writeline("in longioasync end... \t{0} {1}", (datetime.now.ticks - ticks) / timespan.tickspermillisecond, thread.currentthread.managedthreadid);         return tcs.task;     }      public async static task<int32> doasync()     {         console.writeline("in doasync start... \t{0} {1}", (datetime.now.ticks - ticks) / timespan.tickspermillisecond, thread.currentthread.managedthreadid);         var res = await longioasync();         thread.sleep(1000);         console.writeline("in doasync end... \t{0} {1}", (datetime.now.ticks - ticks) / timespan.tickspermillisecond, thread.currentthread.managedthreadid);         return res;     }      static void main(string[] args)     {         ticks = datetime.now.ticks;         console.writeline("in main start... \t{0} {1}", (datetime.now.ticks - ticks) / timespan.tickspermillisecond, thread.currentthread.managedthreadid);         doasync();         console.writeline("in main exec... \t{0} {1}", (datetime.now.ticks - ticks) / timespan.tickspermillisecond, thread.currentthread.managedthreadid);         thread.sleep(3000);         console.writeline("in main end... \t\t{0} {1}", (datetime.now.ticks - ticks) / timespan.tickspermillisecond, thread.currentthread.managedthreadid);     }      private static int64 ticks; } 

the result bellow:

maybe not understand makes await. thought if execution comes await execution returns caller method , task awaiting runs in thread. in example operations execute in 1 thread , execution doesn't returns caller method after await keyword. truth?

this isn't how async-await works.

marking method async doesn't create background threads. when call async method runs synchronously until asynchronous point , returns caller.

that asynchronous point when await task haven't completed yet. when complete rest of method scheduled executed. task should represent actual asynchronous operation (like i/o, or task.delay).

in code there no asynchronous point, there's no point in calling thread returned. thread goes deeper , deeper , blocks on thread.sleep until these methods completed , doasync returns.

take simple example:

public static void main() {     mainasync().wait(); }  public async task mainasync() {     // calling thread     await task.delay(1000);     // different threadpool thread } 

here have actual asynchronous point (task.delay) calling thread returns main , blocks synchronously on task. after second task.delay task completed , rest of method executed on different threadpool thread.

if instead of task.delay have used thread.sleep run on same calling thread.