Asynchronous operation and thread in C#

Question

Asynchronous programming is a technique that calls a long running method in the background so that the UI thread remains responsive. It should be used while calling a web service or database query or any I/O bound operation. when the asynchronous method completes, it returns the result to the main thread. In this way, the program's main thread does not have to wait for the result of an I/O bound operation and continues to execute further without blocking/freezing the UI. This is ok.

As far as I know the asynchronous method executes on a background worker thread. The runtime makes availabe the thread either from the threadpool or it may create a brand new thread for its execution.

But I have read in many posts that an asynchronous operation may execute on a separate thread or without using any thread. Now I am very confused.

1) Could you please help clarifying in what situation an asynchronous operation will not use a thread?

2) What is the role of processor core in asynchronous operation?

3) How it is different from multithreading? I know one thing that multithreading is usefult with compute-bound operation.

Please help.

Answer 1

IO (let's say a database-operation over the network) is a good example for all three:

1. you basically just register a callback the OS will finally call (maybe on a then newly created thread) when the IO-Operation finished. There is no thread sitting around and waiting - the resurrection will be triggered by hardware-events (or at least by a OS process usually outside user-space)

2. it might have none (see 1)

3. in Multithreading you use more than one thread (your background-thread) and there one might idle sit there doing nothing (but using up system-resources) - this is of course different if you have something to compute (so the thread is not idle waiting for external results) - there it makes sense to use a background-worker-thread

Answer 2

Asynchronous operations don't actually imply much of anything about how they are processed, only that they would like the option to get back to you later with your results. By way of example:

• They may (as you've mentioned) split off a compute-bound task onto an independent thread, but this is not the only use case.
• They may sometimes complete synchronously within the call that launches them, in which case no additional thread is used. This may happen with an I/O request if there is already enough buffer content (input) or free buffer space (output) to service the request.
• They may simply drop off a long-running I/O request to the system; in this case the callback is likely to occur on a background thread after receiving notification from an I/O completion port.
• On completion, a callback may be delivered later on the same thread; this is especially common with events within a UI framework, such as navigation in a WebBrowser .

Answer 3

Asynchronity doesn't say anything about thread. Its about having some kind of callbacks which will be handled inside a "statemachine" (not really correct but you can think of it like events ). Asynchronity does not raise threads nor significantly allocate system ressources. You can run as many asynchronous methods as you want to.

Threads do have a real imply on your system and you have a hughe but limited number you can have at once.

Io operations are mostly related to others controllers (HDD, NIC,...) What now happens if you create a thread is that a thread of your application which has nothing to do waits for the controllers to finish. In async as Carsten and Jeffrey already mentioned you just get some kind of callback mechanism so your thread continues to do other work, methods and so on.

Also keep in mind that each thread costs ressources (RAM, Performance,handles Garbage Collection got worse,...) and may even and up in exceptions (OutOfMemoryException...)

So when to use Threads? Absolutly only if you really need it. If there is a async api use it unless you have really important reasons to not use it. In past days the async api was really painfull, thats why many people used threads when ever they need just asynchronity.

For example node.js refuses the use of mulptile thread at all!

This is specially important if you handle multiple requests for example in services / websites where there is always work to do. There is also a this short webcast with Jeffrey Richter about this which helped me to understand

Also have a look at this MSDN article PS: As a side effect sourcecode with async and await tend to be more readable