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<List<TvChannel>> 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?
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.
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<T>
. 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.
I have recent blog posts about async/await and asynchronous console programs in particular. Here's some background info from the intro post:
If "await" 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 "main loop" 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(() => 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
.
You can solve this with this simple construct:
class Program
{
static void Main(string[] args)
{
Task.Run(async () =>
{
// 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.
Wait()
with GetAwaiter().GetResult()
you'll avoid the AggregateException
wrapper when things throw.
async main
is being introduced in C# 7.1, as of this writing.
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<List<TvChannel>>
Task.WaitAll(getListTask); // block while the task completes
var list = getListTask.Result;
}
}
getListTask.Result
is also a blocking call and so the above code could be written without Task.WaitAll(getListTask)
.
GetList
throws you will have to catch an AggregateException
and interrogate its exceptions to determine the actual exception thrown. You can, however, call GetAwaiter()
to get the TaskAwaiter
for the Task
, and call GetResult()
on that, ie var list = getListTask.GetAwaiter().GetResult();
. When getting the result from the TaskAwaiter
(also a blocking call) any exceptions thrown will not be wrapped in an AggregateException
.
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<int> Main();
public static Task Main(string[] args);
public static Task<int> 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:
https://i.stack.imgur.com/bxXL4.png
From the language version drop-down menu, select "7.1" (or any higher value):
https://i.stack.imgur.com/7R7zk.png
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.
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) =>
{
e.Cancel = true;
cts.Cancel();
};
MainAsync(args, cts.Token).GetAwaiter.GetResult();
}
static async Task MainAsync(string[] args, CancellationToken token)
{
...
}
Wait()
as well?
Wait()
, it won't wait for the async code to finish -- it'll stop waiting and end the process immediately.
Wait()
method is passed the same token. What I'm trying to say is, that it doesn't seem to make any difference.
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:
https://i.stack.imgur.com/dKR3U.png
Method2: Modify PropertyGroup of .csproj manually
Add this property:
<LangVersion>7.1</LangVersion>
example:
<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
<PlatformTarget>AnyCPU</PlatformTarget>
<DebugSymbols>true</DebugSymbols>
<DebugType>full</DebugType>
<Optimize>false</Optimize>
<OutputPath>bin\Debug\</OutputPath>
<DefineConstants>DEBUG;TRACE</DefineConstants>
<ErrorReport>prompt</ErrorReport>
<WarningLevel>4</WarningLevel>
<Prefer32Bit>false</Prefer32Bit>
<LangVersion>7.1</LangVersion>
</PropertyGroup>
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<int>
. 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) =>
{
e.Cancel = !cts.IsCancellationRequested;
cts.Cancel();
};
try
{
return MainAsync(args, cts.Token).GetAwaiter().GetResult();
}
catch (OperationCanceledException)
{
return 1223; // Cancelled.
}
}
private static async Task<int> MainAsync(string[] args, CancellationToken cancellationToken)
{
// Your code...
return await Task.FromResult(0); // Success.
}
e.Cancel = true
is unconditional.
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
}
}
SynchronizationContext
associated with the main thread. So it won't deadlock because even without ConfigureAwait(false)
, all continuations will execute on the threadpool.
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
In Main try changing the call to GetList to:
Task.Run(() => bs.GetList());
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<List<TvChannel>> GetList()
{
return new GetPrograms().DownloadTvChannels();
}
DownloadTvChannels()
return? Presumably it returns a Task<List<TvChannel>>
doesn't it? If not, it's unlikely that you'd be able to await it. (Possible, given the awaiter pattern, but unlikely.) As for the Main
method - it still needs to be static... did you replace the static
modifier with the async
modifier perhaps?
public static async void Main() {}
? But if DownloadTvChannels()
already returns a Task<List<TvChannel>>
, presumably it's already asynchronous - so you don't need to add another layer. It's worth understanding this carefully.
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)
{
(...)
}
As of C# 7.1 the following signatures are valid for the Main
method.
public static void Main() { }
public static int Main() { }
public static void Main(string[] args) { }
public static int Main(string[] args) { }
public static async Task Main() { }
public static async Task<int> Main() { }
public static async Task Main(string[] args) { }
public static async Task<int> Main(string[] args) { }
So, now you can do async/await
static async Task Main(string[] args)
{
Console.WriteLine("Hello Asyn Main method!");
await Task.Delay(200);
}
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(() => 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 () =>
{
await Task.Delay(TimeSpan.FromSeconds(1.5));
return 42;
});
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 () => { await Task.WhenAll(jobslist.Select(nl => RunMulti(nl))); }).GetAwaiter().GetResult();
}
private static async Task RunMulti(List<string> joblist)
{
await ...
}
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<TvChannel> list = Task.Run((Func<Task<List<TvChannel>>>)bs.GetList).Result;
}
}
(the cast is only required to resolve ambiguity)
class Program
{
public static EventHandler AsyncHandler;
static void Main(string[] args)
{
AsyncHandler+= async (sender, eventArgs) => { await AsyncMain(); };
AsyncHandler?.Invoke(null, null);
}
private async Task AsyncMain()
{
//Your Async Code
}
}
This is hypothetical but I am thinking:
static void Main(string[] args)
{
var context = new Thread(() => /*do stuff*/);
context.Start();
context.Join();
}
Success story sharing
Wait
orResult
, and there's nothing wrong with that. But be aware that there are two important differences: 1) allasync
continuations run on the thread pool rather than the main thread, and 2) any exceptions are wrapped in anAggregateException
.<LangVersion>latest</LangVersion>
into the csproj file, as shown here.