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Why is this code running synchronously?

I am trying to understand concurrency by doing it in code. I have a code snippet which I thought was running asynchronously. But when I put the debug writeline statements in, I found that it is running synchronously. Can someone explain what I need to do differently to push ComputeBB() onto another thread using Task.Something?
Clarification I want this code to run ComputeBB in some other thread so that the main thread will keep on running without blocking.
Here is the code:
{
// part of the calling method
Debug.WriteLine("About to call ComputeBB");
returnDTM.myBoundingBox = await Task.Run(() => returnDTM.ComputeBB());
Debug.WriteLine("Just called await ComputBB.");
return returnDTM;
}
private ptsBoundingBox2d ComputeBB()
{
Debug.WriteLine("Starting ComputeBB.");
Stopwatch sw = new Stopwatch(); sw.Start();
var point1 = this.allPoints.FirstOrDefault().Value;
var returnBB = new ptsBoundingBox2d(
point1.x, point1.y, point1.z, point1.x, point1.y, point1.z);
Parallel.ForEach(this.allPoints,
p => returnBB.expandByPoint(p.Value.x, p.Value.y, p.Value.z)
);
sw.Stop();
Debug.WriteLine(String.Format("Compute BB took {0}", sw.Elapsed));
return returnBB;
}
Here is the output in the immediate window:
About to call ComputeBB
Starting ComputeBB.
Compute BB took 00:00:00.1790574
Just called await ComputBB.
Clarification If it were really running asynchronously it would be in this order:
About to call ComputeBB
Just called await ComputBB.
Starting ComputeBB.
Compute BB took 00:00:00.1790574
But it is not.
Elaboration
The calling code has signature like so: private static async Task loadAsBinaryAsync(string fileName) At the next level up, though, I attempt to stop using async. So here is the call stack from top to bottom:
static void Main(string[] args)
{
aTinFile = ptsDTM.CreateFromExistingFile("TestSave.ptsTin");
// more stuff
}
public static ptsDTM CreateFromExistingFile(string fileName)
{
ptsDTM returnTin = new ptsDTM();
Task<ptsDTM> tsk = Task.Run(() => loadAsBinaryAsync(fileName));
returnTin = tsk.Result; // I suspect the problem is here.
return retunTin;
}
private static async Task<ptsDTM> loadAsBinaryAsync(string fileName)
{
// do a lot of processing
Debug.WriteLine("About to call ComputeBB");
returnDTM.myBoundingBox = await Task.Run(() => returnDTM.ComputeBB());
Debug.WriteLine("Just called await ComputBB.");
return returnDTM;
}

I have a code snippet which I thought was running asynchronously. But when I put the debug writeline statements in, I found that it is running synchronously.
await is used to asynchronously wait an operations completion. While doing so, it yields control back to the calling method until it's completion.
what I need to do differently to push ComputeBB() onto another thread
It is already ran on a thread pool thread. If you don't want to asynchronously wait on it in a "fire and forget" fashion, don't await the expression. Note this will have an effect on exception handling. Any exception which occurs inside the provided delegate would be captured inside the given Task, if you don't await, there is a chance they will go about unhandled.
Edit:
Lets look at this piece of code:
public static ptsDTM CreateFromExistingFile(string fileName)
{
ptsDTM returnTin = new ptsDTM();
Task<ptsDTM> tsk = Task.Run(() => loadAsBinaryAsync(fileName));
returnTin = tsk.Result; // I suspect the problem is here.
return retunTin;
}
What you're currently doing is synchronously blocking when you use tsk.Result. Also, for some reason you're calling Task.Run twice, once in each method. That is unnecessary. If you want to return your ptsDTM instance from CreateFromExistingFile, you will have to await it, there is no getting around that. "Fire and Forget" execution doesn't care about the result, at all. It simply wants to start whichever operation it needs, if it fails or succeeds is usually a non-concern. That is clearly not the case here.
You'll need to do something like this:
private PtsDtm LoadAsBinary(string fileName)
{
Debug.WriteLine("About to call ComputeBB");
returnDTM.myBoundingBox = returnDTM.ComputeBB();
Debug.WriteLine("Just called ComputeBB.");
return returnDTM;
}
And then somewhere up higher up the call stack, you don't actually need CreateFromExistingFiles, simply call:
Task.Run(() => LoadAsBinary(fileName));
When needed.
Also, please, read the C# naming conventions, which you're currently not following.

await's whole purpose is in adding the synchronicity back in asynchronous code. This allows you to easily partition the parts that are happenning synchronously and asynchronously. Your example is absurd in that it never takes any advantage whatsoever of this - if you just called the method directly instead of wrapping it in Task.Run and awaiting that, you would have had the exact same result (with less overhead).
Consider this, though:
await
Task.WhenAll
(
loadAsBinaryAsync(fileName1),
loadAsBinaryAsync(fileName2),
loadAsBinaryAsync(fileName3)
);
Again, you have the synchronicity back (await functions as the synchronization barrier), but you've actually performed three independent operations asynchronously with respect to each other.
Now, there's no reason to do something like this in your code, since you're using Parallel.ForEach at the bottom level - you're already using the CPU to the max (with unnecessary overhead, but let's ignore that for now).
So the basic usage of await is actually to handle asynchronous I/O rather than CPU work - apart from simplifying code that relies on some parts of CPU work being synchronised and some not (e.g. you have four threads of execution that simultaneously process different parts of the problem, but at some point have to be reunited to make sense of the individual parts - look at the Barrier class, for example). This includes stuff like "making sure the UI doesn't block while some CPU intensive operation happens in the background" - this makes the CPU work asynchronous with respect to the UI. But at some point, you still want to reintroduce the synchronicity, to make sure you can display the results of the work on the UI.
Consider this winforms code snippet:
async void btnDoStuff_Click(object sender, EventArgs e)
{
lblProgress.Text = "Calculating...";
var result = await DoTheUltraHardStuff();
lblProgress.Text = "Done! The result is " + result;
}
(note that the method is async void, not async Task nor async Task<T>)
What happens is that (on the GUI thread) the label is first assigned the text Calculating..., then the asynchronous DoTheUltraHardStuff method is scheduled, and then, the method returns. Immediately. This allows the GUI thread to do whatever it needs to do. However - as soon as the asynchronous task is complete and the GUI is free to handle the callback, the execution of btnDoStuff_Click will continue with the result already given (or an exception thrown, of course), back on the GUI thread, allowing you to set the label to the new text including the result of the asynchronous operation.
Asynchronicity is not an absolute property - stuff is asynchronous to some other stuff, and synchronous to some other stuff. It only makes sense with respect to some other stuff.
Hopefully, now you can go back to your original code and understand the part you've misunderstood before. The solutions are multiple, of course, but they depend a lot on how and why you're trying to do what you're trying to do. I suspect you don't actually need to use Task.Run or await at all - the Parallel.ForEach already tries to distribute the CPU work over multiple CPU cores, and the only thing you could do is to make sure other code doesn't have to wait for that work to finish - which would make a lot of sense in a GUI application, but I don't see how it would be useful in a console application with the singular purpose of calculating that single thing.
So yes, you can actually use await for fire-and-forget code - but only as part of code that doesn't prevent the code you want to continue from executing. For example, you could have code like this:
Task<string> result = SomeHardWorkAsync();
Debug.WriteLine("After calling SomeHardWorkAsync");
DoSomeOtherWorkInTheMeantime();
Debug.WriteLine("Done other work.");
Debug.WriteLine("Got result: " + (await result));
This allows SomeHardWorkAsync to execute asynchronously with respect to DoSomeOtherWorkInTheMeantime but not with respect to await result. And of course, you can use awaits in SomeHardWorkAsync without trashing the asynchronicity between SomeHardWorkAsync and DoSomeOtherWorkInTheMeantime.
The GUI example I've shown way above just takes advantage of handling the continuation as something that happens after the task completes, while ignoring the Task created in the async method (there really isn't much of a difference between using async void and async Task when you ignore the result). So for example, to fire-and-forget your method, you could use code like this:
async void Fire(string filename)
{
var result = await ProcessFileAsync(filename);
DoStuffWithResult(result);
}
Fire("MyFile");
This will cause DoStuffWithResult to execute as soon as result is ready, while the method Fire itself will return immediately after executing ProcessFileAsync (up to the first await or any explicit return someTask).
This pattern is usually frowned upon - there really isn't any reason to return void out of an async method (apart from event handlers); you could just as easily return Task (or even Task<T> depending on the scenario), and let the caller decide whether he wants his code to execute synchronously in respect to yours or not.
Again,
async Task FireAsync(string filename)
{
var result = await ProcessFileAsync(filename);
DoStuffWithResult(result);
}
Fire("MyFile");
does the same thing as using async void, except that the caller can decide what to do with the asynchronous task. Perhaps he wants to launch two of those in parallel and continue after all are done? He can just await Task.WhenAll(Fire("1"), Fire("2")). Or he just wants that stuff to happen completely asynchronously with respect to his code, so he'll just call Fire("1") and ignore the resulting Task (of course, ideally, you at the very least want to handle possible exceptions).

SignalR invoke not returning when used in an async method

I'm not sure if this is a SignalR issue or an async/await issue. When my client app (WPF) starts up, it does some initialisation:-
public async void Initialise()
{
// Get data from the server - async, as it may be long-running.
var data = await _hubProxy.Invoke<FooData>("Method1");
_dataProcessor.ProcessData(data);
}
The _dataProcessor is a helper class that does some stuff with the data passed to it, then at some point calls a different server method using a line similar to:-
var moreData = _hubProxy.Invoke<BarData>("Method2").Result;
None of the code in the helper class is async.
The first server invoke (in Initialise()) works fine - it gets back the data and passes it to the helper class. This proceeds without issue until its invoke - it gets called but never returns, and the thread never proceeds past this line. I've put a breakpoint in the server method and can confirm that it is being called and returning a value, but for some reason this is not getting back to the client.
The strange thing is, if I take out the async/await keywords in the Initialise() method, everything works fine. What am I doing wrong? Why would these keywords affect the synchronous code in the helper class?
(I realise you shouldn't normally use async void, but as the Initialise() method is "fire and forget", I thought it was okay in this scenario).

This line causes a deadlock on the UI thread:
var moreData = _hubProxy.Invoke<BarData>("Method2").Result;
Make ProcessData an async method and await _hubProxy.Invoke inside it:
var moreData = await _hubProxy.Invoke<BarData>("Method2");
Then, await _dataProcessor.ProcessData in your Initialise:
await _dataProcessor.ProcessData(data);
Make sure you don't use .Result or .Wait anywhere else.
Another way of solving it is just this:
public async void Initialise()
{
// Get data from the server - async, as it may be long-running.
var data = await _hubProxy.Invoke<FooData>("Method1").ConfigureAwait(false);
_dataProcessor.ProcessData(data);
}
Note ConfigureAwait(false). In this case the continuation after await will happen on a non-UI thread. This will eliminate the deadlock, but this is not an ideal solution, rather a workaround. Moreover, your logic may require the continuation on the UI thread, for UI access or some thread-safety concerns.
The best solution would be to use both ConfigureAwait(false) (if possible) and avoid blocking with .Result or .Wait, at the same time.

Why does async method-callers behave strangely?

I'm working on a Windows 8.1 store app in which the user can save text to a file.
I've been trying to understand how to best use async and await.
This is what I've come up with:
private async void userText_KeyDown(object sender, KeyRoutedEventArgs e)
{
if (e.Key == Windows.System.VirtualKey.Enter)
{
if (addUserImput)
{
userStringlist.Add(userBox.Text);
userBox.Text = "";
addUserImput = false;
}
await WriteToFileAsync();
addUserImput = true;
}
}
And the async-method looks like this:
private async Task WriteToFileAsync()
{
string name = "userStrings.txt";
var option = CreationCollisionOption.ReplaceExisting;
var folder = Windows.Storage.ApplicationData.Current.LocalFolder;
var file = await folder.CreateFileAsync(name, option);
await Windows.Storage.FileIO.WriteLinesAsync(file, userStringlist);
}
As soon as WriteToFileAsync reaches the await-keyword the execution will start over. In order to prevent duplicates in my list I had to add the if-statement.
It just strikes me as odd. I'm still new to this, so I might've missed something. Why does the keydown event resume from the top, doing work that has already been done?
My "workaround" works, I just don't get the logic behind the event's behaviour.

Yes, that's how asynchronous solutions work. When you hit your first actually asynchronous operation (in this case, CreateFileAsync) the method returns to its caller, which returns to its caller, and it eventually works it's way out of the entire method and back up to your application's message loop. It then continues on processing other UI messages. Some of those messages may be key down events (and they could end up being run before your asynchronous operation is completed). Other events could be things like paint events or mouse click events that lets your form do whatever it needs to interact with the user. This is what prevents it from freezing.
What you want to do is to prevent the given section of code that you have from being run concurrently. If this weren't asynchronous this is something that you would generally solve using the lock keyword, but that isn't an option for an asynchronous method. What you need is some method of preventing access to the code until any other executions of that code block finish. Fortunately there are tools available to do this. You could use a boolean, as you are, but this is somewhat fragile and fairly easy to make a mistake with as the complexity of the application grows. A Semaphore is specifically designed for this task:
private SemaphoreSlim semaphore = new SemaphoreSlim(1);
private async void Bar()
{
try
{
await semaphore.WaitAsync();
//do stuff
}
finally
{
semaphore.Release();
}
}
The SemaphoreSlim class has a WaitAsync method specifically designed for use in asynchronous methods, such as yours. You can wait until the semaphore is free, do your code, and then ensure that you release the semaphore when done so that other code can then move into the code block.

You may need to use handled = true in this case . check if http://msdn.microsoft.com/en-us/library/system.windows.forms.keyeventargs.handled(v=vs.110).aspx works

Block async method manually

My knowledge of how async/await methods are run and on what thread is a little hazy. I would like to block a method until an event is fired. To do this I tried using the ManualResetEvent however this blocks every call to the async method until the last waitHandle.Set( ) is called at which point all async methods complete (or so it seems).
I need a way to block async calls individually as I'm trying to convert the event based asynchronous approach to a Task based async/await one.
How can/should I go about it?
Update:
Based on the information provided by Stephen and Noseratio, I have now changed my asynchronous methods to something a long the lines of this:
public async Task<IEnumerable<CustomerNoteInfo>> LoadClientNotesAsync(int id)
{
return await _session.InvokeOperationAsync((client) =>
{
var tcs = new TaskCompletionSource<IEnumerable<CustomerNoteInfo>>( );
client.GetCustomerNotesCompleted += (sender, e) =>
{
if (e.Error != null) tcs.TrySetException(e.Error);
else tcs.TrySetResult(e.Result);
};
client.GetCustomerNotesAsync(id);
return tcs.Task;
}).Unwrap( );
}
I'm sure this is going to look ugly to everyone who has ever done any async/await Task based programming, but am I on the right track for this? It seems to have fixed the problem I was having with the manual reset event and I can now run this method multiple times asynchronously. :)

My knowledge of how async/await methods are run and on what thread is a little hazy.
I recommend you start with my async intro, which covers the keywords and how they determine which thread to use.
I'm trying to convert the event based asynchronous approach to a Task based async/await one.
The MSDN documentation on this is quite good.
You do not need to block an async method; just create a TAP (Task-based) wrapper for the EAP (method/event pair). Then you can just call your TAP method and await the returned task.

What's a good non-networked example of the new C# Async feature?

Microsoft just announced the new C# Async feature. Every example I've seen so far is about asynchronously downloading something from HTTP. Surely there are other important async things?
Suppose I'm not writing a new RSS client or Twitter app. What's interesting about C# Async for me?
Edit I had an Aha! moment while watching Anders' PDC session. In the past I have worked on programs that used "watcher" threads. These threads sit waiting for something to happen, like watching for a file to change. They aren't doing work, they're just idle, and notify the main thread when something happens. These threads could be replaced with await/async code in the new model.

Ooh, this sounds interesting. I'm not playing with the CTP just yet, just reviewing the whitepaper. After seeing Anders Hejlsberg's talk about it, I think I can see how it could prove useful.
As I understand, async makes writing asynchronous calls easier to read and implement. Very much in the same way writing iterators is easier right now (as opposed to writing out the functionality by hand). This is essential blocking processes since no useful work can be done, until it is unblocked. If you were downloading a file, you cannot do anything useful until you get that file letting the thread go to waste. Consider how one would call a function which you know will block for an undetermined length and returns some result, then process it (e.g., store the results in a file). How would you write that? Here's a simple example:
static object DoSomeBlockingOperation(object args)
{
// block for 5 minutes
Thread.Sleep(5 * 60 * 1000);
return args;
}
static void ProcessTheResult(object result)
{
Console.WriteLine(result);
}
static void CalculateAndProcess(object args)
{
// let's calculate! (synchronously)
object result = DoSomeBlockingOperation(args);
// let's process!
ProcessTheResult(result);
}
Ok good, we have it implemented. But wait, the calculation takes minutes to complete. What if we wanted to have an interactive application and do other things while the calculation took place (such as rendering the UI)? This is no good, since we called the function synchronously and we have to wait for it to finish effectively freezing the application since the thread is waiting to be unblocked.
Answer, call the function expensive function asynchronously. That way we're not bound to waiting for the blocking operation to complete. But how do we do that? We'd call the function asynchronously and register a callback function to be called when unblocked so we may process the result.
static void CalculateAndProcessAsyncOld(object args)
{
// obtain a delegate to call asynchronously
Func<object, object> calculate = DoSomeBlockingOperation;
// define the callback when the call completes so we can process afterwards
AsyncCallback cb = ar =>
{
Func<object, object> calc = (Func<object, object>)ar.AsyncState;
object result = calc.EndInvoke(ar);
// let's process!
ProcessTheResult(result);
};
// let's calculate! (asynchronously)
calculate.BeginInvoke(args, cb, calculate);
}
Note: Sure we could start another thread to do this but that would mean we're spawning a thread that just sits there waiting to be unblocked, then do some useful work. That would be a waste.
Now the call is asynchronous and we don't have to worry about waiting for the calculation to finish and process, it's done asynchronously. It will finish when it can. An alternative to calling code asynchronously directly, you could use a Task:
static void CalculateAndProcessAsyncTask(object args)
{
// create a task
Task<object> task = new Task<object>(DoSomeBlockingOperation, args);
// define the callback when the call completes so we can process afterwards
task.ContinueWith(t =>
{
// let's process!
ProcessTheResult(t.Result);
});
// let's calculate! (asynchronously)
task.Start();
}
Now we called our function asynchronously. But what did it take to get it that way? First of all, we needed the delegate/task to be able to call it asynchronously, we needed a callback function to be able to process the results, then call the function. We've turned a two line function call to much more just to call something asynchronously. Not only that, the logic in the code has gotten more complex then it was or could be. Although using a task helped simplify the process, we still needed to do stuff to make it happen. We just want to run asynchronously then process the result. Why can't we just do that? Well now we can:
// need to have an asynchronous version
static async Task<object> DoSomeBlockingOperationAsync(object args)
{
//it is my understanding that async will take this method and convert it to a task automatically
return DoSomeBlockingOperation(args);
}
static async void CalculateAndProcessAsyncNew(object args)
{
// let's calculate! (asynchronously)
object result = await DoSomeBlockingOperationAsync(args);
// let's process!
ProcessTheResult(result);
}
Now this was a very simplified example with simple operations (calculate, process). Imagine if each operation couldn't conveniently be put into a separate function but instead have hundreds of lines of code. That's a lot of added complexity just to gain the benefit of asynchronous calling.
Another practical example used in the whitepaper is using it on UI apps. Modified to use the above example:
private async void doCalculation_Click(object sender, RoutedEventArgs e) {
    doCalculation.IsEnabled = false;
    await DoSomeBlockingOperationAsync(GetArgs());
    doCalculation.IsEnabled = true;
}
If you've done any UI programming (be it WinForms or WPF) and attempted to call an expensive function within a handler, you'll know this is handy. Using a background worker for this wouldn't be that much helpful since the background thread will be sitting there waiting until it can work.
Suppose you had a way to control some external device, let's say a printer. And you wanted to restart the device after a failure. Naturally it will take some time for the printer to start up and be ready for operation. You might have to account for the restart not helping and attempt to restart again. You have no choice but to wait for it. Not if you did it asynchronously.
static async void RestartPrinter()
{
Printer printer = GetPrinter();
do
{
printer.Restart();
printer = await printer.WaitUntilReadyAsync();
} while (printer.HasFailed);
}
Imagine writing the loop without async.
One last example I have. Imagine if you had to do multiple blocking operations in a function and wanted to call asynchronously. What would you prefer?
static void DoOperationsAsyncOld()
{
Task op1 = new Task(DoOperation1Async);
op1.ContinueWith(t1 =>
{
Task op2 = new Task(DoOperation2Async);
op2.ContinueWith(t2 =>
{
Task op3 = new Task(DoOperation3Async);
op3.ContinueWith(t3 =>
{
DoQuickOperation();
}
op3.Start();
}
op2.Start();
}
op1.Start();
}
static async void DoOperationsAsyncNew()
{
await DoOperation1Async();
await DoOperation2Async();
await DoOperation3Async();
DoQuickOperation();
}
Read the whitepaper, it actually has a lot of practical examples like writing parallel tasks and others.
I can't wait to start playing with this either in the CTP or when .NET 5.0 finally makes it out.

The main scenarios are any scenario that involves high latency. That is, lots of time between "ask for a result" and "obtain a result". Network requests are the most obvious example of high latency scenarios, followed closely by I/O in general, and then by lengthy computations that are CPU bound on another core.
However, there are potentially other scenarios that this technology will mesh nicely with. For example, consider scripting the logic of a FPS game. Suppose you have a button click event handler. When the player clicks the button you want to play a siren for two seconds to alert the enemies, and then open the door for ten seconds. Wouldn't it be nice to say something like:
button.Disable();
await siren.Activate();
await Delay(2000);
await siren.Deactivate();
await door.Open();
await Delay(10000);
await door.Close();
await Delay(1000);
button.Enable();
Each task gets queued up on the UI thread, so nothing blocks, and each one resumes the click handler at the right point after its job is finished.

I've found another nice use-case for this today: you can await user interaction.
For example, if one form has a button that opens another form:
Form toolWindow;
async void button_Click(object sender, EventArgs e) {
if (toolWindow != null) {
toolWindow.Focus();
} else {
toolWindow = new Form();
toolWindow.Show();
await toolWindow.OnClosed();
toolWindow = null;
}
}
Granted, this isn't really any simpler than
toolWindow.Closed += delegate { toolWindow = null; }
But I think it nicely demonstrates what await can do. And once the code in the event handler is non-trivial, await make programming much easier. Think about the user having to click a sequence of buttons:
async void ButtonSeries()
{
for (int i = 0; i < 10; i++) {
Button b = new Button();
b.Text = i.ToString();
this.Controls.Add(b);
await b.OnClick();
this.Controls.Remove(b);
}
}
Sure, you could do this with normal event handlers, but it would require you to take apart the loop and convert it into something much harder to understand.
Remember that await can be used with anything that gets completed at some point in the future. Here's the extension method Button.OnClick() to make the above work:
public static AwaitableEvent OnClick(this Button button)
{
return new AwaitableEvent(h => button.Click += h, h => button.Click -= h);
}
sealed class AwaitableEvent
{
Action<EventHandler> register, deregister;
public AwaitableEvent(Action<EventHandler> register, Action<EventHandler> deregister)
{
this.register = register;
this.deregister = deregister;
}
public EventAwaiter GetAwaiter()
{
return new EventAwaiter(this);
}
}
sealed class EventAwaiter
{
AwaitableEvent e;
public EventAwaiter(AwaitableEvent e) { this.e = e; }
Action callback;
public bool BeginAwait(Action callback)
{
this.callback = callback;
e.register(Handler);
return true;
}
public void Handler(object sender, EventArgs e)
{
callback();
}
public void EndAwait()
{
e.deregister(Handler);
}
}
Unfortunately it doesn't seem possible to add the GetAwaiter() method directly to EventHandler (allowing await button.Click;) because then the method wouldn't know how to register/deregister that event.
It's a bit of boilerplate, but the AwaitableEvent class can be re-used for all events (not just UI). And with a minor modification and adding some generics, you could allow retrieving the EventArgs:
MouseEventArgs e = await button.OnMouseDown();
I could see this being useful with some more complex UI gestures (drag'n'drop, mouse gestures, ...) - though you'd have to add support for cancelling the current gesture.

There are some samples and demos in the CTP that don't use the Net, and even some that don't do any I/O.
And it does apply to all multithreaded / parallel problem areas (that already exist).
Async and Await are a new (easier) way of structuring all parallel code, be it CPU-bound or I/O bound. The biggest improvement is in areas where before C#5 you had to use the APM (IAsyncResult) model, or the event model (BackgroundWorker, WebClient). I think that is why those examples lead the parade now.

A GUI clock is a good example; say you want to draw a clock, that updates the time shown every second. Conceptually, you want to write
while true do
sleep for 1 second
display the new time on the clock
and with await (or with F# async) to asynchronously sleep, you can write this code to run on the UI thread in a non-blocking fashion.
http://lorgonblog.wordpress.com/2010/03/27/f-async-on-the-client-side/

The async extensions are useful in some cases when you have an asynchronous operation. An asynchronous operation has a definite start and completion. When asynchronous operations complete, they may have a result or an error. (Cancellation is treated as a special kind of error).
Asynchronous operations are useful in three situations (broadly speaking):
Keeping your UI responsive. Any time you have a long-running operation (whether CPU-bound or I/O-bound), make it asynchronous.
Scaling your servers. Using asynchronous operations judiciously on the server side may help your severs to scale. e.g., asynchronous ASP.NET pages may make use of async operations. However, this is not always a win; you need to evaluate your scalability bottlenecks first.
Providing a clean asynchronous API in a library or shared code. async is excellent for reusability.
As you begin to adopt the async way of doing things, you'll find the third situation becoming more common. async code works best with other async code, so asynchronous code kind of "grows" through the codebase.
There are a couple of types of concurrency where async is not the best tool:
Parallelization. A parallel algorithm may use many cores (CPUs, GPUs, computers) to solve a problem more quickly.
Asynchronous events. Asynchronous events happen all the time, independent of your program. They often do not have a "completion." Normally, your program will subscribe to an asynchronous event stream, receive some number of updates, and then unsubscribe. Your program can treat the subscribe and unsubscribe as a "start" and "completion", but the actual event stream never really stops.
Parallel operations are best expressed using PLINQ or Parallel, since they have a lot of built-in support for partitioning, limited concurrency, etc. A parallel operation may easily be wrapped in an awaitable by running it from a ThreadPool thread (Task.Factory.StartNew).
Asynchronous events do not map well to asynchronous operations. One problem is that an asynchronous operation has a single result at its point of completion. Asynchronous events may have any number of updates. Rx is the natural language for dealing with asynchronous events.
There are some mappings from an Rx event stream to an asynchronous operation, but none of them are ideal for all situations. It's more natural to consume asynchronous operations by Rx, rather than the other way around. IMO, the best way of approaching this is to use asynchronous operations in your libraries and lower-level code as much as possible, and if you need Rx at some point, then use Rx from there on up.

Here is probably a good example of how not to use the new async feature (that's not writing a new RSS client or Twitter app), mid-method overload points in a virtual method call. To be honest, i am not sure there is any way to create more than a single overload point per method.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Threading;
namespace AsyncText
{
class Program
{
static void Main(string[] args)
{
Derived d = new Derived();
TaskEx.Run(() => d.DoStuff()).Wait();
System.Console.Read();
}
public class Base
{
protected string SomeData { get; set; }
protected async Task DeferProcessing()
{
await TaskEx.Run(() => Thread.Sleep(1) );
return;
}
public async virtual Task DoStuff() {
Console.WriteLine("Begin Base");
Console.WriteLine(SomeData);
await DeferProcessing();
Console.WriteLine("End Base");
Console.WriteLine(SomeData);
}
}
public class Derived : Base
{
public async override Task DoStuff()
{
Console.WriteLine("Begin Derived");
SomeData = "Hello";
var x = base.DoStuff();
SomeData = "World";
Console.WriteLine("Mid 1 Derived");
await x;
Console.WriteLine("EndDerived");
}
}
}
}
Output Is:
Begin Derived
Begin Base
Hello
Mid 1 Derived
End Base
World
EndDerived
With certain inheritance hierarchies (namely using command pattern) i find myself wanting to do stuff like this occasionally.

here is an article about showing how to use the 'async' syntax in a non-networked scenario that involves UI and multiple actions.