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I have this simple code:

public static async Task<int> SumTwoOperationsAsync() { var firstTask = GetOperationOneAsync(); var secondTask = GetOperationTwoAsync(); return await firstTask + await secondTask; } private async Task<int> GetOperationOneAsync() { await Task.Delay(500); // Just to simulate an operation taking time return 10; } private async Task<int> GetOperationTwoAsync() { await Task.Delay(100); // Just to simulate an operation taking time return 5; }

Great. This compiles.

But let’s say I have a console application and I want to run the code above (calling SumTwoOperationsAsync()).

static void Main(string[] args) { SumTwoOperationsAsync(); }

But I've read that (when using sync) I have to sync all the way up and down:

Does this mean that my Main function should be marked as async?

Well, it can't be because there is a compilation error:

an entry point cannot be marked with the 'async' modifier

If I understand the async stuff , the thread will enter the Main function → SumTwoOperationsAsync → will call both functions and will be out. But until the SumTwoOperationsAsync

What am I missing?

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    Here's a proper solution in C# 7.1 Commented May 30, 2017 at 5:47
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    @nawful seemed to miss out the crucial link, so here it is ... learn.microsoft.com/en-us/dotnet/csharp/whats-new/… - In your visual studio 2017 project, go to project properties -> build -> advanced, then change your language version to 7.1 (or higher) Commented Sep 16, 2017 at 8:02

3 Answers 3

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367

In most project types, your async "up" and "down" will end at an async void event handler or returning a Task to your framework.

However, Console apps do not support this.

You can either just do a Wait on the returned task:

static void Main() { MainAsync().Wait(); // or, if you want to avoid exceptions being wrapped into AggregateException: // MainAsync().GetAwaiter().GetResult(); } static async Task MainAsync() { ... }

or you can use your own context like the one I wrote:

static void Main() { AsyncContext.Run(() => MainAsync()); } static async Task MainAsync() { ... }

More information for async Console apps is on my blog.

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12 Comments

If it were not console app , does my observation ( last line of question) is correct ? The thread is stopeed at SumTwoOperationsAsync ...right ? (assuming the Main calling method is not marked as async)
I have an intro on my blog that explains how threads work with async via the "context".
Re-reading your answer - are you saying that it is pointless to use async in a console app ?
@RoyiNamir: No. It's not as common, though, unless you're consuming an async-only API. Console (and other desktop) apps are generally less caring about wasting threads.
@user2173353: No, it's guaranteed to be caught in a catch. If that's not what you're seeing, I recommend asking your own question with a minimal code example.
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103

Here is the simplest way to do this

static void Main(string[] args) { Task t = MainAsync(args); t.Wait(); } static async Task MainAsync(string[] args) { await ... }
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4

As a quick and very scoped solution:

Task.Result

Both Task.Result and Task.Wait won't allow to improving scalability when used with I/O, as they will cause the calling thread to stay blocked waiting for the I/O to end.

When you call .Result on an incomplete Task, the thread executing the method has to sit and wait for the task to complete, which blocks the thread from doing any other useful work in the meantime. This negates the benefit of the asynchronous nature of the task.

notasync

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