Async and asynchronous methods clarification?

问题:

AFAIK - ( and I read a lot about it), asynchronous methods (not asynchronous delegates !) exists to solve the "thread is blocked" problem when dealing with I/O operations like : reading a file or downloading a file :

Richter shows it quite clearly here :

• Task<T> is not related to the i/o blocking issue. it is simply just like open a thread ( plus extra efficiency + functionality ) - but it still causes a thread to consume cpu quanta etc.

And here is my question :

I've read (msdn) that :

An async method provides a convenient way to do potentially long-running work without blocking the caller's thread. The caller of an async method can resume its work without waiting for the async method to finish.

• Is it just like creating a Task<t> with ContinueWith ?

• Doesn't the terminology is confusing ? asynchronous methods are for i/o operations (where there are zero threads waiting while i/o operation is made and no thread is dealing with it). But to call a code (which use async) as: asynchronous methods is a bit confusing. don't you think ? because I assume there is another thread which is executing...(which is my first question actually).

Where is the confusion from ?

Because Albahari tend to emphasize what Asynchronous methods are for :

p.s. I've read a few question here at SO regarding this topic , but found none which talks about the misclassification that asynchronous methods are here to deal with io operations

回答1:

Task<T> is not related to the I/O blocking issue. It is simply just like open a thread (plus extra efficiency and functionality) - but it still causes a thread to consume CPU quanta etc.

Not necessarily. Basically there are two kinds of Tasks: one executes a synchronous piece of code and completes when that code finishes executing. This kind of Task blocks a Thread the whole time since it starts executing until it's completed (successfully or not).

But there is another kind of Task: one that completes when something happens. This kind of Task is what .Net 4.5 and C# 5.0 use heavily and it doesn't block a Thread (at least not directly). You can create such Task yourself by using TaskCompletionSource<T>.

(Another point is that a blocked thread doesn't consume any CPU, but that's not really relevant here.)

Is it just like creating a Task<t> with ContinueWith?

Yes, await t is quite similar to t.ContinueWith(rest of the method).

Doesn't the terminology is confusing? Asynchronous methods are for I/O operations (where there are zero threads waiting while I/O operation is made and no thread is dealing with it). But to call a code (which use async) as: asynchronous methods is a bit confusing. Don't you think? Because I assume there is another thread which is executing.

I don't see the confusion. Classic asynchronous method (like BeginRead(); this is called “Asynchronous Programming Model” or APM) is a way of starting an operation and being notified when it completes (though a callback). Modern async method (like ReadAsync(); this is called “Task-based Asynchronous Pattern” or TAP) is also a way of starting an operation and being notified when it completes (using await).

In both cases, there can be some code that executes before the method returns (the code before the first await in the TAP case).

In both cases, the usual way of being notified about the result doesn't block any threads (callback for APM, await for TAP).

In both cases, you can use blocking wait if you want (immediately calling the EndXxx() method for APM, Wait() for TAP).

Both cases can be used to execute synchronous code on a background thread (BeginInvoke() on a delegate for APM, Task.Factory.StartNew() for TAP).

Again, I don't see the confusion, the two models seem very similar to me.

回答2:

Async methods are not just for IO - they can be for anything - and yes, it's just another way of saying that the work of the method is executed in a separate thread. Any method where the calling thread offloads the work to a separate thread can be correctly called an "async" method. It is the same as Task<T> and ContinueWith -- ContinueWith is just another way of talking about a callback, really.

The word "asynchronous" simply means "not at the same time" - it can refer to any actions that happen independently of each other.