Introduced in C# 5 async/await keywords significantly simplify asynchronous code and improve code readability and maintainability. However, when it comes to exception handling, it becomes a bit trickier.

Let’s consider the following example:

static void Main(string[] args)
{
    try
    {
        AsyncMethod();
    }
    catch
    {
        Console.WriteLine("Exception in Main");
    }

    Console.ReadLine();
}

private async static void AsyncMethod()
{
    throw new NotImplementedException();
}

When executed, instead of displaying “Exception in Main” message, this program crashes. Similar to this console application, all exceptions raised from async void methods crash Windows 8 apps too.

In console and desktop programs unhandled exceptions can be intercepted using AppDomain.UnhandledException event. Only intercepted, for example for logging - it doesn’t prevent crash of the program. In Windows 8 apps even interception is not possible because AppDomain is not part of .NET profile for Windows Store apps.

Windows 8.1 changes the way the async-related exceptions are handled. Now it is possible to catch exception raised from async void method by handling Application.UnhandledException event:

sealed partial class MyApp : Application
{
    public MyApp()
    {
        this.InitializeComponent();

        UnhandledException += Application_UnhandledException;

    }

    private void Application_UnhandledException(object sender, UnhandledExceptionEventArgs e)
    {
        e.Handled = true;
        // ...
    }
}

With UnhandledException event it is possible to not only log exception but also prevent process termination (e.Handled = true).

The following example demonstrate code that will behave differently in Windows 8 and Windows 8.1:

private async void Button_Click(object sender, Windows.UI.Xaml.RoutedEventArgs e)
{
    throw new NotImplementedException();
}

In Windows 8 it will raise exception and terminate process, without possibility to catch it using Application.UnhandledException event. In Windows 8.1 Application.UnhandledException event will be fired in case of such exception.