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How a program is executed asynchronously: [duplicate]

I thought that they were basically the same thing — writing programs that split tasks between processors (on machines that have 2+ processors). Then I'm reading this, which says:
Async methods are intended to be non-blocking operations. An await
expression in an async method doesn’t block the current thread while
the awaited task is running. Instead, the expression signs up the rest
of the method as a continuation and returns control to the caller of
the async method.
The async and await keywords don't cause additional threads to be
created. Async methods don't require multithreading because an async
method doesn't run on its own thread. The method runs on the current
synchronization context and uses time on the thread only when the
method is active. You can use Task.Run to move CPU-bound work to a
background thread, but a background thread doesn't help with a process
that's just waiting for results to become available.
and I'm wondering whether someone can translate that to English for me. It seems to draw a distinction between asynchronicity (is that a word?) and threading and imply that you can have a program that has asynchronous tasks but no multithreading.
Now I understand the idea of asynchronous tasks such as the example on pg. 467 of Jon Skeet's C# In Depth, Third Edition
async void DisplayWebsiteLength ( object sender, EventArgs e )
{
label.Text = "Fetching ...";
using ( HttpClient client = new HttpClient() )
{
Task<string> task = client.GetStringAsync("http://csharpindepth.com");
string text = await task;
label.Text = text.Length.ToString();
}
}
The async keyword means "This function, whenever it is called, will not be called in a context in which its completion is required for everything after its call to be called."
In other words, writing it in the middle of some task
int x = 5;
DisplayWebsiteLength();
double y = Math.Pow((double)x,2000.0);
, since DisplayWebsiteLength() has nothing to do with x or y, will cause DisplayWebsiteLength() to be executed "in the background", like
processor 1 | processor 2
-------------------------------------------------------------------
int x = 5; | DisplayWebsiteLength()
double y = Math.Pow((double)x,2000.0); |
Obviously that's a stupid example, but am I correct or am I totally confused or what?
(Also, I'm confused about why sender and e aren't ever used in the body of the above function.)

Your misunderstanding is extremely common. Many people are taught that multithreading and asynchrony are the same thing, but they are not.
An analogy usually helps. You are cooking in a restaurant. An order comes in for eggs and toast.
Synchronous: you cook the eggs, then you cook the toast.
Asynchronous, single threaded: you start the eggs cooking and set a timer. You start the toast cooking, and set a timer. While they are both cooking, you clean the kitchen. When the timers go off you take the eggs off the heat and the toast out of the toaster and serve them.
Asynchronous, multithreaded: you hire two more cooks, one to cook eggs and one to cook toast. Now you have the problem of coordinating the cooks so that they do not conflict with each other in the kitchen when sharing resources. And you have to pay them.
Now does it make sense that multithreading is only one kind of asynchrony? Threading is about workers; asynchrony is about tasks. In multithreaded workflows you assign tasks to workers. In asynchronous single-threaded workflows you have a graph of tasks where some tasks depend on the results of others; as each task completes it invokes the code that schedules the next task that can run, given the results of the just-completed task. But you (hopefully) only need one worker to perform all the tasks, not one worker per task.
It will help to realize that many tasks are not processor-bound. For processor-bound tasks it makes sense to hire as many workers (threads) as there are processors, assign one task to each worker, assign one processor to each worker, and have each processor do the job of nothing else but computing the result as quickly as possible. But for tasks that are not waiting on a processor, you don't need to assign a worker at all. You just wait for the message to arrive that the result is available and do something else while you're waiting. When that message arrives then you can schedule the continuation of the completed task as the next thing on your to-do list to check off.
So let's look at Jon's example in more detail. What happens?
Someone invokes DisplayWebSiteLength. Who? We don't care.
It sets a label, creates a client, and asks the client to fetch something. The client returns an object representing the task of fetching something. That task is in progress.
Is it in progress on another thread? Probably not. Read Stephen's article on why there is no thread.
Now we await the task. What happens? We check to see if the task has completed between the time we created it and we awaited it. If yes, then we fetch the result and keep running. Let's suppose it has not completed. We sign up the remainder of this method as the continuation of that task and return.
Now control has returned to the caller. What does it do? Whatever it wants.
Now suppose the task completes. How did it do that? Maybe it was running on another thread, or maybe the caller that we just returned to allowed it to run to completion on the current thread. Regardless, we now have a completed task.
The completed task asks the correct thread -- again, likely the only thread -- to run the continuation of the task.
Control passes immediately back into the method we just left at the point of the await. Now there is a result available so we can assign text and run the rest of the method.
It's just like in my analogy. Someone asks you for a document. You send away in the mail for the document, and keep on doing other work. When it arrives in the mail you are signalled, and when you feel like it, you do the rest of the workflow -- open the envelope, pay the delivery fees, whatever. You don't need to hire another worker to do all that for you.

In-browser Javascript is a great example of an asynchronous program that has no multithreading.
You don't have to worry about multiple pieces of code touching the same objects at the same time: each function will finish running before any other javascript is allowed to run on the page. (Update: Since this was written, JavaScript has added async functions and generator functions. These functions do not always run to completion before any other javascript is executed: whenever they reach a yield or await keyword, they yield execution to other javascript, and can continue execution later, similar to C#'s async methods.)
However, when doing something like an AJAX request, no code is running at all, so other javascript can respond to things like click events until that request comes back and invokes the callback associated with it. If one of these other event handlers is still running when the AJAX request gets back, its handler won't be called until they're done. There's only one JavaScript "thread" running, even though it's possible for you to effectively pause the thing you were doing until you have the information you need.
In C# applications, the same thing happens any time you're dealing with UI elements--you're only allowed to interact with UI elements when you're on the UI thread. If the user clicked a button, and you wanted to respond by reading a large file from the disk, an inexperienced programmer might make the mistake of reading the file within the click event handler itself, which would cause the application to "freeze" until the file finished loading because it's not allowed to respond to any more clicking, hovering, or any other UI-related events until that thread is freed.
One option programmers might use to avoid this problem is to create a new thread to load the file, and then tell that thread's code that when the file is loaded it needs to run the remaining code on the UI thread again so it can update UI elements based on what it found in the file. Until recently, this approach was very popular because it was what the C# libraries and language made easy, but it's fundamentally more complicated than it has to be.
If you think about what the CPU is doing when it reads a file at the level of the hardware and Operating System, it's basically issuing an instruction to read pieces of data from the disk into memory, and to hit the operating system with an "interrupt" when the read is complete. In other words, reading from disk (or any I/O really) is an inherently asynchronous operation. The concept of a thread waiting for that I/O to complete is an abstraction that the library developers created to make it easier to program against. It's not necessary.
Now, most I/O operations in .NET have a corresponding ...Async() method you can invoke, which returns a Task almost immediately. You can add callbacks to this Task to specify code that you want to have run when the asynchronous operation completes. You can also specify which thread you want that code to run on, and you can provide a token which the asynchronous operation can check from time to time to see if you decided to cancel the asynchronous task, giving it the opportunity to stop its work quickly and gracefully.
Until the async/await keywords were added, C# was much more obvious about how callback code gets invoked, because those callbacks were in the form of delegates that you associated with the task. In order to still give you the benefit of using the ...Async() operation, while avoiding complexity in code, async/await abstracts away the creation of those delegates. But they're still there in the compiled code.
So you can have your UI event handler await an I/O operation, freeing up the UI thread to do other things, and more-or-less automatically returning to the UI thread once you've finished reading the file--without ever having to create a new thread.

Is it good practice to always wait on a task to complete?

Sorry if it is a dumb question. I'm confused about the wait() and its variants in regards to the task parallel library.
Every single example I've seen waits on tasks to complete - is this considered good practice?
My scenario is this, that I'm developing a windows service that will run continuously. I would like to engage a number of tasks, but I don't care if they will run to completion - I will set a cancellation-token with an expiration, that will throw an error if something goes awry. So I don't see the need for a wait-to-complete, but every darn example uses it...

It really depends on what your situations needs. If for instance, you want to launch a sub process to do a procedure, say for instance, fire off an email in parallel you can do without waiting.
However, if you will need to act upon what ever result or structure which is affected by some behavior you will need to wait.
If your tasks are self contained and do not interact and/or depend on each other, then I do not see why you would need to wait.

You only need to wait on a task if the code that is waiting requires the output of the task before it can proceed. If you don't need that output, don't wait.

How do I cancel a schedule for a task?

I need to create a scheduled task. For that I'm trying to learn how to do that, and am currently thinking of using TaskSchedulerClass from TaskScheduler Class Library. I got a code examples here (and there's another one I might try here). But - I don't want to actually run it without making sure I know how to cancel it when needed.
So my question is: How do I cancel such a task-registration in code?
(Just "Try it!" won't suffice in this case. Because the point is I don't want to get stuck with something I can't stop.)

What about TryDequeue Method?
It's attempts to dequeue a Task that was previously queued to this scheduler.
You need to use the same Task you sent to QueueTask function.
If you don't have the instance of this Task you can try and retrieve it using GetScheduledTasks, but it stated it in use For debugger support only

How do I make a .NET 4 thread wait for asynchronous requests to finish?

I am using HttpWebRequest.BeginGetRequest() to make 500 asynchronous HTTP requests from a single method. I would like that method to wait until I get a response from all the requests or they timeout.
What is the best way to do this?
I'm currently wrapping the asynchronous calls within a List of Task objects to use Tasks.WaitAll(), but I don't want to go too far down the rabbit hole before I know that this is a good solution.
Any ideas?
EDIT
I implemented counters, and they work, but I'm curious about using delegates like shown on this page.
Multi-threading and Async Examples
Has anybody done something like this before? Is it overkill?

I'm currently wrapping the asynchronous calls within a List of Task objects to use Tasks.WaitAll()
This is a fairly clean solution if you truly want to force these "tasks" to synchronize and block at this point. This is the main rationale behind Task.WaitAll(), and is nice since it (optionally) allows you to cancel the blocking operation after a timeout, if you so choose.

Personally I wouldn't block the thread, it defeats the purpose of the async model.
If I absolutely had to wait for these web requests to finish before continuing I would instead keep a counter that is incremented each time you get called back on a successful or failed request.
Check the counter on each callback and if it has hit your desired count then let the thread continue...
This way you can also keep your UI nice and responsive and perhaps update a counter/progress bar - Even if you're not kicking these off on the UI thread it's nice to provide some visual feed back tot he user about what is going on.

C# Sleep for 500 milliseconds

Could you please tell me how do I go about pausing my program for 500 milliseconds and then continue?
I read Thread.Sleep(500) is not good as it holds up the GUI thread.
Using a timer it fires a callback ...
I just want to wait 500ms and then continue to the next statement.
Please advise.
EDIT: I need to display a status bar message for 500ms and then update the message with a different one. Sorry, I meant 500 not 50.
EDIT: I do understand what all you have said. but: [I just want to wait 500ms and then continue to the next statement.] I think because it is such a short interval i am going do a Thread.Sleep(500) on the main GUI thread. Otherwise i would have to rewrite a lot of code to accomodate this brief interval of 500 milliseconds.
EDIT: i will try to reformat my status message so the pause is not needed.

Hmya, what you're trying to do is pretty fundamentally incompatible with the Windows programming model. A native Windows program is event driven. Your program is always idle, sitting inside a loop started by Application.Run(), waiting for Windows to tell it that something interesting happened that it should respond to. Paint requests, mouse clicks, timer expirations, stuff like that.
Your program should respond to this and filter what is interesting to you. When you drop a button on a form, you are always interested in the Click event, generated when Windows sends the MouseDown notification message. Your Click event handler runs some kind of custom code that you write. Like updating a status bar message in your case.
Updating the status bar message half a second later doesn't make a whole heckofalot of sense. What exactly happened during those 500 milliseconds that changed the way your program responds to events? You can call the Update() method of the StatusBar so the new message is visible, then call System.Threading.Thread.Sleep(500) to get what you want. You'll get away with it, the "Not Responding" ghost that Windows puts up takes your program going dead for several seconds.
But that doesn't make a lot of sense, nothing happened during that half second, the state of your program didn't change. It couldn't change, it was dead to Windows and not receiving any messages that would allow it to change state.
Well, that's about as far as I can take this. Please update your question and explain why you need to do this. Just in case: if you're contemplating this to fake doing something important for half a second, your user will not be impressed. She'll eventually notice your UI is dead for half a second without anything to show for it.

You have two choices:
Use a timer as you suggested. Split your method up into two methods, foo1 and foo2. Use the foo1 to start the timer and run foo2 in the callback.
Use a BackgroundWorker for running the entire function and use Thread.Sleep on the worker thread.
From your update it seems that the only thing you want to do is change a single field. I would definitely recommend the first method: using a timer. Starting a BackgroundWorker for this task is overkill and will just give you unnecessary extra work and complications.

Instead of pausing the UI directly for 500 ms, you can always use a BackgroundWorker. That will cause your callback to run in a separate thread, where you can use Thread.Sleep to pause it without blocking the UI. Then when you are done, just update the status bar with your new message.

More context to the question would be helpful.
Thread.Sleep(50) will pause the current thread for 50 milliseconds. If you're doing this in the UI thread, then yes, it will freeze the UI for 50 milliseconds. However, if you use a different thread to do this processing, then calling Sleep on that thread will pause it for 50 milliseconds without freezing your UI thread.
See Marc's answer to this question for an example on using a BackgroundWorker instance to do what you need.

In C# your best bet is to use the Timer and fire a callback.
In F# there is an awesome way to do what you want, see
F# async on the client side
which shows how to write straight-line code and have the language take care of the callbacks for you.

You need to allocate another thread. In that thread you Sleep(500) and change the needed data. Caution: you would need to use the original thread's dispatcher, because the data related to UI should be usually updated from the GUI thread.