async/await，如何正确使用

Question

I am trying to study c# async/await, all the tutorials I found are the same stuff and I can't really understand anything, for instance, why this code doesn't work? Could you please explain the how's and why's of async/await or give some useful links to truly understand given that I don't have experience with it.

class Program
    {
        static void Main(string[] args)
        {
            Waiter();
        }

        public static async void Waiter()
        {
            await HeavyStuff();
            Console.WriteLine("1");
        }

        public static async Task HeavyStuff()
        {
            await Task.Run(() => {
                Thread.Sleep(1000);
                Console.WriteLine("2");
            });
        }
    }

Answer 1

await the Waiter() in the Main(). Otherwise it will call Waiter(), but Main() will go on without waiting for Waiter() to finish. That is called "fire and forget". Main() will finish and the program ends before Waiter() is done.

class Program
{
    static async Task Main(string[] args)
    {
        await Waiter();
    }

    public static async Task Waiter()
    {
        Console.WriteLine("1");
        await HeavyStuff();
        Console.WriteLine("4"); // this will be instantly shown after "3"
    }

    public static async Task HeavyStuff()
    {
        Console.WriteLine("2");  // this will be instantly shown after "1"
        await Task.Delay(1000);  // short version of Task.Run(() => Thread.Sleep(1000))
        Console.WriteLine("3");  // this will be shown after 1 second
    }
}

A fire forget version without async Main. In this version we prevent the Main from finishing (= ending the Program) before Waiter can finish.

class Program
{
    static void Main(string[] args)
    {
        Waiter();
        Thread.Sleep(2000);  // this gives Waiter() time to finish
    }

    public static async Task Waiter()
    {
        Console.WriteLine("1");
        await HeavyStuff();
        Console.WriteLine("4");
    }

    public static async Task HeavyStuff()
    {
        Console.WriteLine("2");
        await Task.Delay(1000);
        Console.WriteLine("3");
    }
}

Answer 2

I would urge you to read a blog post by Stephen Cleary for a rather comprehensive explanation of what the point of async/await is.

To your question specifically though, the use of async in the method signature prompts the compiler to create a state machine. You must put the async keyword in the method in order to enable the use of await within the method body (as you likely found when writing Waiter() and HeavyStuff() ).

What ultimately happens here is that when you call Waiter from Main, it does actually call the Waiter method, but because you're not awaiting the result of the call, it treats it as a "fire-and-forget". As such, you experience the program closing.

Similarly, while Waiter() awaits HeavyStuff() (if Waiter() were awaited from Main() ), the program will wait for that method to return before showing the "1" on the console.

The blog post above covers this, but you'll likely not want to return void from Waiter() because that's typically used for event handling where there's nothing actually waiting on the result of the method. While you're not returning a value with your method, you are awaiting it, so it's better practice to return Task instead.

Since C# 7.1, you've been allowed to use the async keyword in your Main() entry point, thus your code should be as follows:

class Program
{
    static async Task Main(string[] args)
    {
        await Waiter();
    }

    public static async Task Waiter()
    {
        await HeavyStuff();
        Console.WriteLine("1");
    }

    public static async Task HeavyStuff()
    {
        await Task.Delay(1000);
        Console.WriteLine("2");
    }
}