danielovich danielovich - 1 month ago 10
C# Question

Can't specify the 'async' modifier on the 'Main' method of a console app

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?

Answer

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 "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();
    }
}
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