Question

Using the async CTP from Microsoft for .NET,
is it possible to catch an exception thrown by an async method in the calling method?

public async void Foo()
{
    var x = await DoSomethingAsync();

    /* Handle the result, but sometimes an exception might be thrown.
       For example, DoSomethingAsync gets data from the network
       and the data is invalid... a ProtocolException might be thrown. */
}

public void DoFoo()
{
    try
    {
        Foo();
    }
    catch (ProtocolException ex)
    {
          /* The exception will never be caught.
             Instead when in debug mode, VS2010 will warn and continue.
             The deployed the app will simply crash. */
    }
}

So basically I want the exception from the async code to bubble up into my calling code
if that is even possible at all.

Answer 1

It’s somewhat weird to read but yes, the exception will bubble up to the calling code – but only if you await or Wait() the call to Foo.

public async Task Foo()
{
    var x = await DoSomethingAsync();
}

public async void DoFoo()
{
    try
    {
        await Foo();
    }
    catch (ProtocolException ex)
    {
          // The exception will be caught because you've awaited
          // the call in an async method.
    }
}

//or//

public void DoFoo()
{
    try
    {
        Foo().Wait();
    }
    catch (ProtocolException ex)
    {
          /* The exception will be caught because you've
             waited for the completion of the call. */
    }
} 

As Stephen Cleary wrote in Async/Await – Best Practices in Asynchronous Programming:

Async void methods have different error-handling semantics. When an exception is thrown out of an async Task or async Task method, that exception is captured and placed on the Task object. With async void methods, there is no Task object, so any exceptions thrown out of an async void method will be raised directly on the SynchronizationContext that was active when the async void method started.

Note that using Wait() may cause your application to block, if .NET decides to execute your method synchronously.

This explanation http://www.interact-sw.co.uk/iangblog/2010/11/01/csharp5-async-exceptions is pretty good – it discusses the steps the compiler takes to achieve this magic.