当void为返回值时，从async-await捕获异常

Question

I find myself using async fire-and-forget methods using void as the return value, but DO care about exceptions.

It seems to be the consensus that exceptions cannot be handled properly with async-await if no reference is hold to the executing Task and void should be.. well.. avoided..

What am I missing in the following code that apparently seems to do the job:

class Program
{
    static void Main()
    {
        var p = new Processor();
        p.ExceptionThrown += p_ExceptionThrown;
        for (var i = 0; i < 10; i++)
            p.ProcessAsync(i);
        Console.ReadKey();
    }

    static void p_ExceptionThrown(object sender, Exception e)
    {
        Console.WriteLine("Exception caught in Main : " + e);
    }
}

class Processor
{
    public async void ProcessAsync(int iteration)
    {
        try
        {
            await Task.Run(() => Process(iteration));
        }
        catch (Exception e)
        {
            OnException(e);
        }
    }

    public void Process(int iteration)
    {
        Thread.Sleep(500);
        if(iteration == 5)
            throw new Exception("AUUCH");
    }

    public event EventHandler<Exception> ExceptionThrown;

    void OnException(Exception e)
    {
        var handler = ExceptionThrown;
        if (handler != null)
            handler(this, e);
    }
}

Answer 1

Under the covers the async / await keywords actually generate a state-machine when they are compiled down to IL, please read about it here . The only time that you should ever use async void is on an event handler, as explained here . The issue is that when the state machine is built-out it uses the Task or Task<T> classes as the return type in order to manage the next state of the next asynchronous operation in the chain. However, when you define the method as void, it basically returns null to the state machine and then everything gets out of whack.

Exceptions from an async void can't be caught with catch

The quote from above is from the best practices article I pointed you to before. The below alteration does work, as I have tested it to verify that it does.

class Program
{
    static void Main()
    {
        var p = new Processor();
        p.ExceptionThrown += p_ExceptionThrown;
        for (var i = 0; i < 10; i++)
            p.ProcessAsync(i);
        Console.ReadKey();
    }

    static void p_ExceptionThrown(object sender, Exception e)
    {
        Console.WriteLine("Exception caught in Main : " + e);
    }
}

class Processor
{
    public async Task ProcessAsync(int iteration)
    {
        try
        {
            await Task.Run(() => Process(iteration));
        }
        catch (Exception e)
        {
            OnException(e);
        }
    }

    public void Process(int iteration)
    {
        Thread.Sleep(500);
        if(iteration == 5)
            throw new Exception("AUUCH");
    }

    public event EventHandler<Exception> ExceptionThrown;

    void OnException(Exception e)
    {
        var handler = ExceptionThrown;
        if (handler != null)
            handler(this, e);
    }
}