Doing the Dishes

In my last post, I mentioned that you can still use try...catch blocks with .NET 4.5 async methods. There is some interesting magic that .NET injects for you under the covers in this case. For example, imagine you have the following (simplified and contrived) code which uses the newer .NET 4.5 SendAsync() method on UdpClient instead of the older, synchronous Send() version:

// Do something...

try
{
	var client = new UdpClient();
	client.SendAsync(datagram, datagram.Length, hostname, port);
}
catch (Exception e)
{
	// Handle exception here...
}

// Do something else...

Let’s assume that the host specified by hostname is down/invalid. The part about this flow that blew my mind was that the call to SendAsync() will return immediately (and not delay the code in // Do something else...), but the exception will still bubble up and be handled by the catch block. This post has an in depth explanation of how the compiler handles this. In the post, he mentions how you can even debug and step through code like this in Visual Studio, pass over an async method and see it return immediately, step through more code, and be jarringly brought back to the thrown exception from a method that has already returned!