Can t specify the async modifier on the Main method of a console app

Question

I am new to asynchronous programming with the async modifier  I am trying to figure out how to make sure that my Main method of a console application actually runs asynchronously   class Program       static void Main string   args                Bootstrapper bs   new Bootstrapper            var list   bs GetList             public class Bootstrapper        public async Task lt List lt TvChannel gt  gt  GetList                 GetPrograms pro   new GetPrograms             return await pro DownloadTvChannels              I know this is not running asynchronously from  the top   Since it is not possible to specify the async modifier on the Main method  how can I run code within main asynchronously

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When the C  5 CTP was introduced  you certainly could mark Main with async    although it was generally not a good idea to do so  I believe this was changed by the release of VS 2013 to become an error   Unless you ve started any other foreground threads  your program will exit when Main completes  even if it s started some background work   What are you really trying to do  Note that your GetList   method really doesn t need to be async at the moment - it s adding an extra layer for no real reason  It s logically equivalent to  but more complicated than    public Task lt List lt TvChannel gt  gt  GetList         return new GetPrograms   DownloadTvChannels

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In my case I had a list of jobs that I wanted to run in async from my main method  have been using this in production for quite sometime and works fine   static void Main string   args        Task Run async      gt    await Task WhenAll jobslist Select nl   gt  RunMulti nl        GetAwaiter   GetResult      private static async Task RunMulti List lt string gt  joblist        await

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To avoid freezing when you call a function somewhere down the call stack that tries to re-join the current thread  which is stuck in a Wait   you need to do the following   class Program       static void Main string   args                Bootstrapper bs   new Bootstrapper            List lt TvChannel gt  list   Task Run  Func lt Task lt List lt TvChannel gt  gt  gt  bs GetList  Result             the cast is only required to resolve ambiguity

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As you discovered  in VS11 the compiler will disallow an async Main method  This was allowed  but never recommended  in VS2010 with the Async CTP  I have recent blog posts about async await and asynchronous console programs in particular  Here s some background info from the intro post   If  quot await quot  sees that the awaitable has not completed  then it acts asynchronously  It tells the awaitable to run the remainder of the method when it completes  and then returns from the async method  Await will also capture the current context when it passes the remainder of the method to the awaitable  Later on  when the awaitable completes  it will execute the remainder of the async method  within the captured context    Here s why this is a problem in Console programs with an async Main   Remember from our intro post that an async method will return to its caller before it is complete  This works perfectly in UI applications  the method just returns to the UI event loop  and ASP NET applications  the method returns off the thread but keeps the request alive   It doesn t work out so well for Console programs  Main returns to the OS - so your program exits   One solution is to provide your own context - a  quot main loop quot  for your console program that is async-compatible  If you have a machine with the Async CTP  you can use GeneralThreadAffineContext from My Documents Microsoft Visual Studio Async CTP Samples C  Testing  Unit Testing AsyncTestUtilities  Alternatively  you can use AsyncContext from my Nito AsyncEx NuGet package  Here s an example using AsyncContext  GeneralThreadAffineContext has almost identical usage  using Nito AsyncEx  class Program       static void Main string   args                AsyncContext Run      gt  MainAsync args               static async void MainAsync string   args                Bootstrapper bs   new Bootstrapper            var list   await bs GetList             Alternatively  you can just block the main Console thread until your asynchronous work has completed  class Program       static void Main string   args                MainAsync args  GetAwaiter   GetResult               static async Task MainAsync string   args                Bootstrapper bs   new Bootstrapper            var list   await bs GetList             Note the use of GetAwaiter   GetResult    this avoids the AggregateException wrapping that happens if you use Wait   or Result  Update  2017-11-30  As of Visual Studio 2017 Update 3  15 3   the language now supports an async Main - as long as it returns Task or Task lt T gt   So you can now do this  class Program       static async Task Main string   args                Bootstrapper bs   new Bootstrapper            var list   await bs GetList             The semantics appear to be the same as the GetAwaiter   GetResult   style of blocking the main thread  However  there s no language spec for C  7 1 yet  so this is only an assumption

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I ll add an important feature that all of the other answers have overlooked  cancellation  One of the big things in TPL is cancellation support  and console apps have a method of cancellation built in  CTRL C   It s very simple to bind them together  This is how I structure all of my async console apps  static void Main string   args        CancellationTokenSource cts   new CancellationTokenSource             System Console CancelKeyPress     s  e    gt                e Cancel   true          cts Cancel                MainAsync args  cts Token  GetAwaiter GetResult       static async Task MainAsync string   args  CancellationToken token

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For asynchronously calling task from Main  use    Task Run   for  NET 4 5  Task Factory StartNew   for  NET 4 0    May require Microsoft Bcl Async library for async and await keywords    Details  http   blogs msdn com b pfxteam archive 2011 10 24 10229468 aspx

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You can do this without needing external libraries also by doing the following   class Program       static void Main string   args                Bootstrapper bs   new Bootstrapper            var getListTask   bs GetList       returns the Task lt List lt TvChannel gt  gt           Task WaitAll getListTask      block while the task completes          var list   getListTask Result

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On MSDN  the documentation for Task Run Method  Action  provides this example which shows how to run a method asynchronously from main   using System  using System Threading  using System Threading Tasks   public class Example       public static void Main                 ShowThreadInfo  Application             var t   Task Run      gt  ShowThreadInfo  Task              t Wait               static void ShowThreadInfo String s                Console WriteLine   0  Thread ID   1                              s  Thread CurrentThread ManagedThreadId              The example displays the following output           Application thread ID  1          Task thread ID  3   Note this statement that follows the example      The examples show that the asynchronous task executes on a different   thread than the main application thread    So  if instead you want the task to run on the main application thread  see the answer by  StephenCleary   And regarding the thread on which the task runs  also note Stephen s comment on his answer      You can use a simple Wait or Result  and there s nothing wrong   with that  But be aware that there are two important differences  1    all async continuations run on the thread pool rather than the main   thread  and 2  any exceptions are wrapped in an AggregateException     See Exception Handling  Task Parallel Library  for how to incorporate exception handling to deal with an AggregateException      Finally  on MSDN from the documentation for Task Delay Method  TimeSpan   this example shows how to run an asynchronous task that returns a value   using System  using System Threading Tasks   public class Example       public static void Main                 var t   Task Run async delegate                                       await Task Delay TimeSpan FromSeconds 1 5                        return 42                              t Wait            Console WriteLine  Task t Status   0   Result   1                              t Status  t Result              The example displays the following output            Task t Status  RanToCompletion  Result  42   Note that instead of passing a delegate to Task Run  you can instead pass a lambda function like this   var t   Task Run async      gt                        await Task Delay TimeSpan FromSeconds 1 5                return 42

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If you re using C  7 1 or later  go with the nawfal s answer and just change the return type of your Main method to Task or Task lt int gt   If you are not    Have an async Task MainAsync like Johan said  Call its  GetAwaiter   GetResult   to catch the underlying exception like do0g said  Support cancellation like Cory said  A second CTRL C should terminate the process immediately   Thanks binki   Handle OperationCancelledException - return an appropriate error code    The final code looks like   private static int Main string   args        var cts   new CancellationTokenSource        Console CancelKeyPress     s  e    gt                e Cancel    cts IsCancellationRequested          cts Cancel                try               return MainAsync args  cts Token  GetAwaiter   GetResult              catch  OperationCanceledException                return 1223     Cancelled           private static async Task lt int gt  MainAsync string   args  CancellationToken cancellationToken           Your code         return await Task FromResult 0      Success

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Newest version of C  - C  7 1 allows to create async console app  To enable C  7 1 in project  you have to upgrade your VS to at least 15 3  and change C  version to C  7 1 or C  latest minor version  To do this  go to Project properties -  Build -  Advanced -  Language version   After this  following code will work   internal class Program       public static async Task Main string   args

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In Main try changing the call to GetList to   Task Run      gt  bs GetList

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C  7 1  using vs 2017 update 3  introduces async main  You can write      static async Task Main string   args          await           For more details C  7 Series  Part 2  Async Main  Update   You may get a compilation error      Program does not contain a static  Main  method suitable for an entry point   This error is due to that vs2017 3 is configured by default as c 7 0 not c 7 1   You should  explicitly modify  the setting of your project to set c 7 1 features   You can set c 7 1  by two methods   Method 1  Using the project settings window    Open the settings of your project Select the Build tab Click the Advanced button Select the version you want As shown in the following figure      Method2  Modify PropertyGroup of  csproj manually  Add this property        lt LangVersion gt 7 1 lt  LangVersion gt    example        lt PropertyGroup Condition      Configuration    Platform       Debug AnyCPU    gt           lt PlatformTarget gt AnyCPU lt  PlatformTarget gt           lt DebugSymbols gt true lt  DebugSymbols gt           lt DebugType gt full lt  DebugType gt           lt Optimize gt false lt  Optimize gt           lt OutputPath gt bin Debug  lt  OutputPath gt           lt DefineConstants gt DEBUG TRACE lt  DefineConstants gt           lt ErrorReport gt prompt lt  ErrorReport gt           lt WarningLevel gt 4 lt  WarningLevel gt           lt Prefer32Bit gt false lt  Prefer32Bit gt           lt LangVersion gt 7 1 lt  LangVersion gt       lt  PropertyGroup gt

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You can solve this with this simple construct   class Program       static void Main string   args                Task Run async      gt                           Do any async anything you need here without worry            GetAwaiter   GetResult              That will put everything you do out on the ThreadPool where you d want it  so other Tasks you start await don t attempt to rejoin a Thread they shouldn t   and wait until everything s done before closing the Console app  No need for special loops or outside libs   Edit  Incorporate Andrew s solution for uncaught Exceptions

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In C  7 1 you will be able to do a proper async Main  The appropriate signatures for Main method has been extended to   public static Task Main    public static Task lt int gt  Main    public static Task Main string   args   public static Task lt int gt  Main string   args     For e g  you could be doing   static async Task Main string   args        Bootstrapper bs   new Bootstrapper        var list   await bs GetList        At compile time  the async entry point method will be translated to call GetAwaitor   GetResult     Details  https   blogs msdn microsoft com mazhou 2017 05 30 c-7-series-part-2-async-main  EDIT   To enable C  7 1 language features  you need to right-click on the project and click  Properties  then go to the  Build  tab  There  click the advanced button at the bottom     From the language version drop-down menu  select  7 1   or any higher value      The default is  latest major version  which would evaluate  at the time of this writing  to C  7 0  which does not support async main in console apps

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class Program        public static EventHandler AsyncHandler       static void Main string   args                 AsyncHandler   async  sender  eventArgs    gt    await AsyncMain               AsyncHandler  Invoke null  null                private async Task AsyncMain                    Your Async Code

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Haven t needed this much yet  but when I ve used console application for Quick tests and required async I ve just solved it like this   class Program       static void Main string   args                MainAsync args  Wait               static async Task MainAsync string   args                   Code here

[c#] Can't specify the 'async' modifier on the 'Main' method of a console app

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