I have a question regarding SignalR's official documentation - Hubs API Guide - .NET Client. In the section - How to establish a connection. It has been written the following thing:
The Start method executes asynchronously. To make sure that subsequent
lines of code don't execute until after the connection is established,
use await in an ASP.NET 4.5 asynchronous method or .Wait() in a
synchronous method. Don't use .Wait() in a WinRT client.
Does anyone know what is the specific reason not to call Wait()? Also, does also this apply when I have a WinRT client where I have a call with hubProxy.Invoke() to the server?
Thank you for your help!
From the comment:
There is nothing mentioned in relation to asynchronous or synchronous
code. Is the code WinRT asynchronous by default or there is something
else that I don't know or don't understand?
In a client-side UI app (including WinRT), there're at least two reason to not block:
avoid blocking the UI and keeping it fluent;
avoid the deadlocks as described in Stephen Cleary's blog, for cases when there's an await in the current chain of calls, on the upper stack frame;
The latter is especially important for WinRT, where asynchrony is pervasive. The whole WinRT framework has been designed to be "async all the way down", so it's very likely you'd create a deadlock if you use task.Wait or task.Result anywhere on the UI thread.
For a server-side ASP.NET app, there're also at least two reasons to not block:
blocking makes your web app less scalable, because the blocked thread could be serving other incoming HTTP requests.
the same deadlock scenario.
You may want to go through the list of links in async-await wiki for further reading materials.
Related
Here is my scenario:
We have a library which has a "MethodA" in a class which makes a http call using C# HttpClient and retrieves all notification which are in the queue. We are using .NET Core 3.1. I want to add another class in the same library which will call this "MethodA" on a scheduled interval. I have used Task.Run and Task.Delay to achieve this in my test app and it works. Only thing is this is an I/O bound call and not CPU bound operation and it is recommended by Stephen Cleary to avoid using Task.Run on I/O bound calls here.
How can I achieve this?
Thanks,
Sal
I/O bound calls are already handled by your operating system in an event driven way. Windows for example does it with overlapped file calls.
So the answer to your question is, don't use the thread pool, await on the OS call with Stream.ReadAsync and friends.
Edit: I just noticed you're talking about HttpClient, and the same answer applies there. Get the response stream and use async read calls on it instead of tying up your thread pool for no reason.
I've been reading about the new async and await operators in C# and tried to figure out in which circumstances they would possibly be useful to me. I studied several MSDN articles and here's what I read between the lines:
You can use async for Windows Forms and WPF event handlers, so they can perform lengthy tasks without blocking the UI thread while the bulk of the operation is being executed.
async void button1_Click(object sender, EventArgs e)
{
// even though this call takes a while, the UI thread will not block
// while it is executing, therefore allowing further event handlers to
// be invoked.
await SomeLengthyOperationAsync();
}
A method using await must be async, which means that the usage of any async function somewhere in your code ultimately forces all methods in the calling sequence from the UI event handlers up until the lowest-level async method to be async as well.
In other words, if you create a thread with an ordinary good old ThreadStart entry point (or a Console application with good old static int Main(string[] args)), then you cannot use async and await because at one point you would have to use await, and make the method that uses it async, and hence in the calling method you also have to use await and make that one async and so on. But once you reach the thread entry point (or Main()), there's no caller to which an await would yield control to.
So basically you cannot use async and await without having a GUI that uses the standard WinForms and WPF message loop. I guess all that makes indeed sense, since MSDN states that async programming does not mean multithreading, but using the UI thread's spare time instead; when using a console application or a thread with a user defined entry point, multithreading would be necessary to perform asynchronous operations (if not using a compatible message loop).
My question is, are these assumptions accurate?
So basically you cannot use async and await without having a GUI that uses the standard WinForms and WPF message loop.
That's absolutely not the case.
In Windows Forms and WPF, async/await has the handy property of coming back to the UI thread when the asynchronous operation you were awaiting has completed, but that doesn't mean that's the only purpose to it.
If an asynchronous method executes on a thread-pool thread - e.g. in a web service - then the continuation (the rest of the asynchronous method) will simply execute in any thread-pool thread, with the context (security etc) preserved appropriately. This is still really useful for keeping the number of threads down.
For example, suppose you have a high traffic web service which mostly proxies requests to other web services. It spends most of its time waiting for other things, whether that's due to network traffic or genuine time at another service (e.g. a datbase). You shouldn't need lots of threads for that - but with blocking calls, you naturally end up with a thread per request. With async/await, you'd end up with very few threads, because very few requests would actually need any work performed for them at any one point in time, even if there were a lot of requests "in flight".
The trouble is that async/await is most easily demonstrated with UI code, because everyone knows the pain of either using background threads properly or doing too much work in the UI thread. That doesn't mean it's the only place the feature is useful though - far from it.
Various server-side technologies (MVC and WCF for example) already have support for asynchronous methods, and I'd expect others to follow suit.
A method using await must be async, which means that the usage of any async function somewhere in your code ultimately forces all methods in the calling sequence from the UI event handlers up until the lowest-level async method to be async as well.
Not true - methods marked async just mean they can use await, but callers of those methods have no restrictions. If the method returns Task or Task<T> then they can use ContinueWith or anything else you could do with tasks in 4.0
A good non-UI example is MVC4 AsyncController.
Ultimately, async/await is mostly about getting the compiler rewriting so you can write what looks like synchronous code and avoid all the callbacks like you had to do before async/await was added. It also helps with the SynchronizationContext handling, useful for scenarios with thread affinity (UI frameworks, ASP.NET), but even without those, it's still useful. Main can always do DoStuffAsync().Wait(); for instance. :)
My question is, are these assumptions accurate?
No.
You can use async for Windows Forms and WPF event handlers, so they can perform lengthy tasks without blocking the UI thread while the bulk of the operation is being executed.
True. Also true for other UI applications including Silverlight and Windows Store.
And also true for ASP.NET. In this case, it's the HTTP request thread that is not blocked.
A method using await must be async, which means that the usage of any async function somewhere in your code ultimately forces all methods in the calling sequence from the UI event handlers up until the lowest-level async method to be async as well.
This is a best practice ("async all the way down"), but it's not strictly required. You can block on the result of an asynchronous operation; many people choose to do this in Console applications.
an ordinary good old ThreadStart entry point
Well... I do have to take issue with "ordinary good old". As I explain on my blog, Thread is pretty much the worst option you have for doing background operations.
I recommend you review my introduction to async and await, and follow up with the async / await FAQ.
async-await is only wrapper for Task class manipulations, which is part of so named Tasks Parallel Library - TPL(published before async-await auto code generation tech.)
So fact is you may not use any references to UI controls within async - await.
Typically async-await is powerfull tool for any web and server relations, loading resources, sql. It works with smart waiting data with alive UI.
Typically TPL application: from simple big size loop till multi stages parallel calculations in complex calculations based on shared data (ContinueWith and so on)
I'm confused about async IO operations. In this article Stephen Cleary explains that we should not use Task.Run(() => SomeIoMethod()) because truly async operations should use
standard P/Invoke asynchronous I/O system in .NET
http://blog.stephencleary.com/2013/11/there-is-no-thread.html
However, avoid “fake asynchrony” in libraries. Fake asynchrony is when
a component has an async-ready API, but it’s implemented by just
wrapping the synchronous API within a thread pool thread. That is
counterproductive to scalability on ASP.NET. One prominent example of
fake asynchrony is Newtonsoft JSON.NET, an otherwise excellent
library. It’s best to not call the (fake) asynchronous versions for
serializing JSON; just call the synchronous versions instead. A
trickier example of fake asynchrony is the BCL file streams. When a
file stream is opened, it must be explicitly opened for asynchronous
access; otherwise, it will use fake asynchrony, synchronously blocking
a thread pool thread on the file reads and writes.
And he advises to use HttpClient but internaly it use Task.Factory.StartNew()
Does this mean that HttpClient provides not truly async operations?
Does this mean that HttpClient provides not truly async operations?
Sort of. HttpClient is in an unusual position, since it's primary implementation uses HttpWebRequest, which is only partially asynchronous.
In particular, the DNS lookup is synchronous, and I think maybe the proxy resolution, too. After that, it's all asynchronous. So, for most scenarios, the DNS is fast (usually cached) and there isn't a proxy, so it acts asynchronously. Unfortunately, there are enough scenarios (particularly from within corporate networks) where the synchronous operations can cause significant lag.
So, when the team was writing HttpClient, they had three options:
Fix HttpWebRequest (and friends) allowing for fully-asynchronous operations. Unfortunately, this would have broken a fair amount of code. Due to the way inheritance is used as extension points in these objects, adding asynchronous methods would be backwards-incompatible.
Write their own HttpWebRequest equivalent. Unfortunately, this would take a lot of work and they'd lose all the interoperability with existing WebRequest-related code.
Queue requests to the thread pool to avoid the worst-case scenario (blocking synchronous code on the UI thread). Unfortunately, this has the side effects of degrading scalability on ASP.NET, being dependent on a free thread pool thread, and incurring the worst-case scenario cost even for best-case scenarios.
In an ideal world (i.e., when we have infinite developer and tester time), I would prefer (2), but I understand why they chose (3).
On a side note, the code you posted shows a dangerous use of StartNew, which has actually caused problems due to its use of TaskScheduler.Current. This has been fixed in .NET Core - not sure when the fix will roll back into .NET Framework proper.
No, your assumptions are wrong.
StartNew isn't equal to the Run method.
This code is from HttpClientHandler, not the HttpClient, and you didn't examine the this.startRequest code from this class. The code you're inspecting is a prepare method, which starts a task in new thread pool, and inside call actual code to start an http request.
HTTP-connection is created not on the .NET level of abstraction, and I'm sure that inside startRequest you'LL find some P/Invoke method, which will do actual work for:
DNS lookup
Socket connection
Sending the request
waiting for the answer
etc.
As you can see, all above are logic which really should be called in async manner, because it is outside the .NET framework, and some operation can be very time-consuming. This is exactly logic that should be called asynchroniously, and during the waiting for it .NET thread is being released in ThreadPool to process other tasks.
I've been reading about the new async and await operators in C# and tried to figure out in which circumstances they would possibly be useful to me. I studied several MSDN articles and here's what I read between the lines:
You can use async for Windows Forms and WPF event handlers, so they can perform lengthy tasks without blocking the UI thread while the bulk of the operation is being executed.
async void button1_Click(object sender, EventArgs e)
{
// even though this call takes a while, the UI thread will not block
// while it is executing, therefore allowing further event handlers to
// be invoked.
await SomeLengthyOperationAsync();
}
A method using await must be async, which means that the usage of any async function somewhere in your code ultimately forces all methods in the calling sequence from the UI event handlers up until the lowest-level async method to be async as well.
In other words, if you create a thread with an ordinary good old ThreadStart entry point (or a Console application with good old static int Main(string[] args)), then you cannot use async and await because at one point you would have to use await, and make the method that uses it async, and hence in the calling method you also have to use await and make that one async and so on. But once you reach the thread entry point (or Main()), there's no caller to which an await would yield control to.
So basically you cannot use async and await without having a GUI that uses the standard WinForms and WPF message loop. I guess all that makes indeed sense, since MSDN states that async programming does not mean multithreading, but using the UI thread's spare time instead; when using a console application or a thread with a user defined entry point, multithreading would be necessary to perform asynchronous operations (if not using a compatible message loop).
My question is, are these assumptions accurate?
So basically you cannot use async and await without having a GUI that uses the standard WinForms and WPF message loop.
That's absolutely not the case.
In Windows Forms and WPF, async/await has the handy property of coming back to the UI thread when the asynchronous operation you were awaiting has completed, but that doesn't mean that's the only purpose to it.
If an asynchronous method executes on a thread-pool thread - e.g. in a web service - then the continuation (the rest of the asynchronous method) will simply execute in any thread-pool thread, with the context (security etc) preserved appropriately. This is still really useful for keeping the number of threads down.
For example, suppose you have a high traffic web service which mostly proxies requests to other web services. It spends most of its time waiting for other things, whether that's due to network traffic or genuine time at another service (e.g. a datbase). You shouldn't need lots of threads for that - but with blocking calls, you naturally end up with a thread per request. With async/await, you'd end up with very few threads, because very few requests would actually need any work performed for them at any one point in time, even if there were a lot of requests "in flight".
The trouble is that async/await is most easily demonstrated with UI code, because everyone knows the pain of either using background threads properly or doing too much work in the UI thread. That doesn't mean it's the only place the feature is useful though - far from it.
Various server-side technologies (MVC and WCF for example) already have support for asynchronous methods, and I'd expect others to follow suit.
A method using await must be async, which means that the usage of any async function somewhere in your code ultimately forces all methods in the calling sequence from the UI event handlers up until the lowest-level async method to be async as well.
Not true - methods marked async just mean they can use await, but callers of those methods have no restrictions. If the method returns Task or Task<T> then they can use ContinueWith or anything else you could do with tasks in 4.0
A good non-UI example is MVC4 AsyncController.
Ultimately, async/await is mostly about getting the compiler rewriting so you can write what looks like synchronous code and avoid all the callbacks like you had to do before async/await was added. It also helps with the SynchronizationContext handling, useful for scenarios with thread affinity (UI frameworks, ASP.NET), but even without those, it's still useful. Main can always do DoStuffAsync().Wait(); for instance. :)
My question is, are these assumptions accurate?
No.
You can use async for Windows Forms and WPF event handlers, so they can perform lengthy tasks without blocking the UI thread while the bulk of the operation is being executed.
True. Also true for other UI applications including Silverlight and Windows Store.
And also true for ASP.NET. In this case, it's the HTTP request thread that is not blocked.
A method using await must be async, which means that the usage of any async function somewhere in your code ultimately forces all methods in the calling sequence from the UI event handlers up until the lowest-level async method to be async as well.
This is a best practice ("async all the way down"), but it's not strictly required. You can block on the result of an asynchronous operation; many people choose to do this in Console applications.
an ordinary good old ThreadStart entry point
Well... I do have to take issue with "ordinary good old". As I explain on my blog, Thread is pretty much the worst option you have for doing background operations.
I recommend you review my introduction to async and await, and follow up with the async / await FAQ.
async-await is only wrapper for Task class manipulations, which is part of so named Tasks Parallel Library - TPL(published before async-await auto code generation tech.)
So fact is you may not use any references to UI controls within async - await.
Typically async-await is powerfull tool for any web and server relations, loading resources, sql. It works with smart waiting data with alive UI.
Typically TPL application: from simple big size loop till multi stages parallel calculations in complex calculations based on shared data (ContinueWith and so on)
Ive a question about this example: Asynchronous Client Socket .
It says that the operations are asynchronous and they ARE (beginxxx/endxxx are presented), but they use ManualResetEvent and if i understand the sample code correctly - such calls:
Receive(client);
receiveDone.WaitOne();
Will block the thread, they were called from. So that if i have an application with UI and i call that async socket code from main thread - application will freeze...Or am i wrong? Sorry, i expected to send/receive some commands over tcp asynchronously without freezing main thread. Will i have to call all the socket operations from that sample to avoid freezing?
Yes, this code is set up so that it will run synchronously. However, you can easily use the Beginxxx and Endxxx calls yourself rather than wrapping them as this (poor) example of asynchronous action is written.
For further clarification, the article does run each method asynchronously, but then blocks using blocker.WaitOne(); A better approach to something like this might be to have your beginconnect callback trigger a send, which will have its callback trigger a receive, etc
You could also use the TPL and take advantage of their ContinueWith functionality to do this. Then your code might even look cleaner :)
It would block the UI. The MSDN example does demonstrate asynchronousy, but very poorly. Its also primitive. Please look at this topic which I also just answered regarding different asynchronous programming models. I discuss quite a few and provide links for all of them with full examples.
Link: Need help to implement multithreading in C#