异步执行代码

Question

Sometimes when I run code made asynchronous using async and await , I find that some parts of the code don't even get executed. For example in the following code, "Sleeping2" is not shown on the console screen:

public static void Sleeping(int millis)
{
    System.Console.WriteLine("Sleeping1");
    System.Threading.Thread.Sleep(millis);
    System.Console.WriteLine("Sleeping2");
}

public static async void SleepingAsync(int millis)
{
    await System.Threading.Tasks.Task.Run(() => Sleeping(millis));
}

public static async void DoSleepingMain()
{
    System.Console.WriteLine("DoSleepingMain1");
    SleepingAsync(12000);
    System.Console.WriteLine("DoSleepingMain2");
}

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

Another example is the following in which neither Sleeping1 or Sleeping2 are displayed. I don't understand this because I await the Task in the DoSleepingMain method.

public static void Sleeping(int millis)
{
    System.Console.WriteLine("Sleeping1");
    System.Threading.Thread.Sleep(millis);
    System.Console.WriteLine("Sleeping2");
}

public static async Task SleepingAsync(int millis)
{
    System.Threading.Tasks.Task.Run(() => Sleeping(millis)); //warning: call not awaited
}

public static async void DoSleepingMain()
{
    System.Console.WriteLine("DoSleepingMain1");
    await SleepingAsync(12000);
    System.Console.WriteLine("DoSleepingMain2");
}

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

Any explanation (or pointer to explanations) would be appreciated! Thanks

Answer 1

You've just discovered why you should never write async void .

If you write an async void method, you have no way of knowing when the asynchronous part finishes.

This includes Main() ; your program is exiting as soon as it hits the first await .

async methods are mostly useless in console programs; if you really want to try them, you'll need to call .Wait() on the Task s returned to Main() .