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Is it bad practice to have async call within Task.Run?

I have a C# console app processing around 100,000 JSON messages from RabbitMQ every 1 min
After getting each/a bunch of messages from RabbitMQ I then call
await Task.Run(async () =>
{
//do lots of CPU stuff here, including 2 external API calls using await async call
}
Everything I've read says use await Task.Run for CPU bound operations. And use await async for the HTTP external calls.
If I change it to:
await Task.Run(() =>
Then it complains as I have an async API call in the lines below, so it needs the async keyword in the Task.Run statement.
There are about 2000+ (complex if then business rules) lines of code in this section, and the sometimes the API call is not needed.
So I'm faced with either a massive restructure of the application, with lots of testing needed, or if its ok to do API calls alongside the CPU bound operations then I'll leave it as is.
To summarise, is this bad practice, or is it ok to have CPU bound work and API calls inside the same task? The task is processing one JSON message.

Everything I've read says use await task.run for cpu bound operations . And use await async for the http external calls
The general guideline is to use async/await for I/O. Task.Run is useful for CPU-bound operations if you need to offload them from a UI thread. For example, in server scenarios such as ASP.NET, you wouldn't want to use Task.Run for CPU-bound code. This is because ASP.NET already schedules your code on a separate thread pool thread.
In your case, you have a Console application, which doesn't have a UI thread. But it also doesn't have that automatic scheduling onto a thread pool thread that ASP.NET gives you.
if its ok to do api calls alongside the cpu bound operations then i'll leave it as is.
This is fine either way. Since the code is awaiting the Task.Run, it won't continue (presumably processing the next message) until the operation completes on another thread pool thread. So the Task.Run isn't helping much, but it isn't hurting much, either.
If you need more performance - specifically, processing messages concurrently - then you should look into something like TPL Dataflow or System.Threading.Channels that would allow you to replace the Task.Run with a queue of work that can run in parallel. That would give you something more like what ASP.NET provides out of the box.

General
(...) use await Task.Run for CPU bound operations. And use await async for the HTTP external calls.
This advice comes from the fact that if you run code that doesn't 'let go' enough then you may not get a lot of the benefit Tasks give because the current thread / thread pool just cannot handle the work. In the extreme case when you run 100% synchronous code you won't get any parallelism because the current thread cannot let go and cannot do any other work - your tasks would be executed sequentially. It's important to remember that this is not a style issue; running busy synchronous code with Tasks just work well in some scenarios. In this sense the problem polices itself: if you structure the solution incorrectly it doesn't do what you need.
If you run a mixture of busy and waiting then Task.Run may or may not be great and it will depend on the specific workload. If it works for you, it's fine - you're not doing anything incorrect.
Generally the picture is nuanced and tasks can be and are used to do all kinds of jobs. In certain circumstances the situation is clear cut - e.g. if you run long running work in the UI thread you will lock the UI which is bad or if you have (long-running) busy synchronous code. It's worth keeping in mind that this has been a problem before C# had Tasks.
BTW. If you look at the reference documentation for Task.WhenAll Method it contains examples with both I/O (ping) and CPU (dummy for loop) style work. Yes, these are toy examples but it shows it isn't incorrect to run both types of work with tasks.
Parallel.ForEachAsync?
If you can use .NET 6, Parallel.ForEachAsync could improve performance of your solution and/or make the code look cleaner. Example on Twitter (as picture!).

Proper way to start and fire-and-forget asynchronous calls?

I have an async call (DoAsyncWork()), that I would like to start in a fire-and-forget way, i.e. I'm not interesting in its result and would like the calling thread to continue even before the async method is finished.
What is the proper way to do this? I need this in both, .NET Framework 4.6 as well as .NET Core 2, in case there are differences.
public async Task<MyResult> DoWorkAsync(){...}
public void StarterA(){
Task.Run(() => DoWorkAsync());
}
public void StarterB(){
Task.Run(async () => await DoWorkAsync());
}
Is it one of those two or something different/better?
//edit: Ideally without any extra libraries.

What is the proper way to do this?
First, you need to decide whether you really want fire-and-forget. In my experience, about 90% of people who ask for this actually don't want fire-and-forget; they want a background processing service.
Specifically, fire-and-forget means:
You don't care when the action completes.
You don't care if there are any exceptions when executing the action.
You don't care if the action completes at all.
So the real-world use cases for fire-and-forget are astoundingly small. An action like updating a server-side cache would be OK. Sending emails, generating documents, or anything business related is not OK, because you would (1) want the action to be completed, and (2) get notified if the action had an error.
The vast majority of the time, people don't want fire-and-forget at all; they want a background processing service. The proper way to build one of those is to add a reliable queue (e.g., Azure Queue / Amazon SQS, or even a database), and have an independent background process (e.g., Azure Function / Amazon Lambda / .NET Core BackgroundService / Win32 service) processing that queue. This is essentially what Hangfire provides (using a database for a queue, and running the background process in-proc in the ASP.NET process).
Is it one of those two or something different/better?
In the general case, there's a number of small behavior differences when eliding async and await. It's not something you would want to do "by default".
However, in this specific case - where the async lambda is only calling a single method - eliding async and await is fine.

It depends on what you mean by proper :)
For instance: are you interested in the exceptions being thrown in your "fire and forget" calls? If not, than this is sort of fine. Though what you might need to think about is in what environment the task lives.
For instance, if this is a asp.net application and you do this inside the lifetime of a thread instantiated due to a call to a .aspx or .svc. The Task becomes a background thread of that (foreground)thread. The foreground thread might get cleaned up by the application pool before your "fire and forget" task is completed.
So also think about in which thread your tasks live.
I think this article gives you some useful information on that:
https://www.hanselman.com/blog/HowToRunBackgroundTasksInASPNET.aspx
Also note that if you do not return a value in your Tasks, a task will not return exception info. Source for that is the ref book for microsoft exam 70-483
There is probably a free version of that online somewhere ;P https://www.amazon.com/Exam-Ref-70-483-Programming-C/dp/0735676828
Maybe useful to know is that if your have an async method being called by a non-async and you wish to know its result. You can use .GetAwaiter().GetResult().
Also I think it is important to note the difference between async and multi-threading.
Async is only useful if there are operations that use other parts of a computer that is not the CPU. So things like networking or I/O operations. Using async then tells the system to go ahead and use CPU power somewhere else instead of "blocking" that thread in the CPU for just waiting for a response.
multi-threading is the allocation of operations on different threads in a CPU (for instance, creating a task which creates a background thread of the foreground thread... foreground threads being the threads that make up your application, they are primary, background threads exist linked to foreground threads. If you close the linked foreground thread, the background thread closes as well)
This allows the CPU to work on different tasks at the same time.
Combining these two makes sure the CPU does not get blocked up on just 4 threads if it is a 4 thread CPU. But can open more while it waits for async tasks that are waiting for I/O operations.
I hope this gives your the information needed to do, what ever it is you are doing :)

How do a library provide async methods if it is a bad practice to do Task.Run?

In this article: https://blog.stephencleary.com/2013/11/taskrun-etiquette-examples-dont-use.html , it is advised against using Task.Run. however there are lot of libraries that provide methods that ends with Async and hence I expect those methods to return a running task that I can await (which however is not necessary, since those libraries could decide to return a synchronous task).
The context is a ASP.NET application. How am I supposed to make a method running in parallel?
What I understand is that async calls are executed in parallel if they contain at least one "await" operator inside, the problem is that the innermost call, should be parallel to achieve that, and to do that I have somewhat to resort to Task.Run
I have also seen some examples using TaskCompletionSource, is this necessary to implement the "inner most async method" to run a method in parallel in a ASP.NET application?

In an ASP.Net application we tend to value requests/s over individual response times1 - certainly if we're directly trading off one versus the other. So we don't try to focus more CPU power at satisfying one request.
And really, focussing more CPU power at a task is what Task.Run is for - it's for when you have a distinct chunk of work to be done, you can't do it on the current thread (because its got its own work to do) and when you're free to use as much CPU as possible.
In ASP.Net, where async shines is when we're dealing with I/O. Nasty slow things like accessing the file system or talking to a database across the network. And wonderfully, at the lowest level, the windows I/O system is async already and we don't have to devote a thread just to waiting for things to finish.
So, you won't be using Task.Run. Instead you'll be looking for I/O related objects that expose Async methods. And those methods themselves will not, as above, be using Task.Run. What this does allow us to do is to stop using any threads for servicing our particular request whilst there's no work to be done, and so improve out requests/s metric.
1This is a generalization but single user/request ASP.Net sites are rare in my experience.

Web API Sync Calls Best Practice

Probably this question has already been made, but I never found a definitive answer. Let's say that I have a Web API 2.0 Application hosted on IIS. I think I understand that best practice (to prevent deadlocks on client) is always use async methods from the GUI event to the HttpClient calls. And this is good and it works. But what is the best practice in case I had client application that does not have a GUI (e.g. Window Service, Console Application) but only synchronous methods from which to make the call? In this case, I use the following logic:
void MySyncMethodOnMyWindowServiceApp()
{
list = GetDataAsync().Result().ToObject<List<MyClass>>();
}
async Task<Jarray> GetDataAsync()
{
list = await Client.GetAsync(<...>).ConfigureAwait(false);
return await response.Content.ReadAsAsync<JArray>().ConfigureAwait(false);
}
But unfortunately this can still cause deadlocks on client that occur at random times on random machines.
The client app stops at this point and never returns:
list = await Client.GetAsync(<...>).ConfigureAwait(false);

If it's something that can be run in the background and isn't forced to be synchronous, try wrapping the code (that calls the async method) in a Task.Run(). I'm not sure that'll solve a "deadlock" problem (if it's something out of sync, that's another issue), but if you want to benefit from async/await, if you don't have async all the way down, I'm not sure there's a benefit unless you run it in a background thread. I had a case where adding Task.Run() in a few places (in my case, from an MVC controller which I changed to be async) and calling async methods not only improved performance slightly, but it improved reliability (not sure that it was a "deadlock" but seemed like something similar) under heavier load.
You will find that using Task.Run() is regarded by some as a bad way to do it, but I really couldn't see a better way to do it in my situation, and it really did seem to be an improvement. Perhaps this is one of those things where there's the ideal way to do it vs. the way to make it work in the imperfect situation that you're in. :-)
[Updated due to requests for code]
So, as someone else posted, you should do "async all the way down". In my case, my data wasn't async, but my UI was. So, I went async down as far as I could, then I wrapped my data calls with Task.Run in such as way that it made sense. That's the trick, I think, to figure out if it makes sense that things can run in parallel, otherwise, you're just being synchronous (if you use async and immediately resolve it, forcing it to wait for the answer). I had a number of reads that I could perform in parallel.
In the above example, I think you have to async up as far as makes sense, and then at some point, determine where you can spin off a t hread and perform the operation independent of the other code. Let's say you have an operation that saves data, but you don't really need to wait for a response -- you're saving it and you're done. The only thing you might have to watch out for is not to close the program without waiting for that thread/task to finish. Where it makes sense in your code is up to you.
Syntax is pretty easy. I took existing code, changed the controller to an async returning a Task of my class that was formerly being returned.
var myTask = Task.Run(() =>
{
//...some code that can run independently.... In my case, loading data
});
// ...other code that can run at the same time as the above....
await Task.WhenAll(myTask, otherTask);
//..or...
await myTask;
//At this point, the result is available from the task
myDataValue = myTask.Result;
See MSDN for probably better examples:
https://msdn.microsoft.com/en-us/library/hh195051(v=vs.110).aspx
[Update 2, more relevant for the original question]
Let's say that your data read is an async method.
private async Task<MyClass> Read()
You can call it, save the task, and await on it when ready:
var runTask = Read();
//... do other code that can run in parallel
await runTask;
So, for this purpose, calling async code, which is what the original poster is requesting, I don't think you need Task.Run(), although I don't think you can use "await" unless you're an async method -- you'll need an alternate syntax for Wait.
The trick is that without having some code to run in parallel, there's little point in it, so thinking about multi-threading is still the point.

Using Task<T>.Result is the equivalent of Wait which will perform a synchronous block on the thread. Having async methods on the WebApi and then having all the callers synchronously blocking them effectively makes the WebApi method synchronous. Under load you will deadlock if the number of simultaneous Waits exceeds the server/app thread pool.
So remember the rule of thumb "async all the way down". You want the long running task (getting a collection of List) to be async. If the calling method must be sync you want to make that conversion from async to sync (using either Result or Wait) as close to the "ground" as possible. Keep they long running process async and have the sync portion as short as possible. That will greatly reduce the length of time that threads are blocked.
So for example you can do something like this.
void MySyncMethodOnMyWindowServiceApp()
{
List<MyClass> myClasses = GetMyClassCollectionAsync().Result;
}
Task<List<MyClass>> GetMyListCollectionAsync()
{
var data = await GetDataAsync(); // <- long running call to remote WebApi?
return data.ToObject<List<MyClass>>();
}
The key part is the long running task remains async and not blocked because await is used.
Also don't confuse the responsiveness with scalability. Both are valid reasons for async. Yes responsiveness is a reason for using async (to avoid blocking on the UI thread). You are correct this wouldn't apply to a back end service however this isn't why async is used on a WebApi. The WebApi is also a non GUI back end process. If the only advantage of async code was responsiveness of the UI layer then WebApi would be sync code from start to finish. The other reason for using async is scalability (avoiding deadlocks) and this is the reason why WebApi calls are plumbed async. Keeping the long running processes async helps IIS make more efficient use of a limited number of threads. By default there are only 12 worker threads per core. This can be raised but that isn't a magic bullet either as threads are relatively expensive (about 1MB overhead per thread). await allows you to do more with less. More concurrent long running processes on less threads before a deadlock occurs.

The problem you are having with deadlocks must stem from something else. Your use of ConfigureAwait(false) prevents deadlocks here. Solve the bug and you are fine.
See Should we switch to use async I/O by default? to which the answer is "no". You should decide on a case by case basis and choose async when the benefits outweigh the costs. It is important to understand that async IO has a productivity cost associated with it. In non-GUI scenarios only a few targeted scenarios derive any benefit at all from async IO. The benefits can be enormous, though, but only in those cases.
Here's another helpful post: https://stackoverflow.com/a/25087273/122718

When should I use Async Controllers in ASP.NET MVC?

I have some concerns using async actions in ASP.NET MVC. When does it improve performance of my apps, and when does it not?
Is it good to use async action everywhere in ASP.NET MVC?
Regarding awaitable methods: shall I use async/await keywords when I want to query a database (via EF/NHibernate/other ORM)?
How many times can I use await keywords to query the database asynchronously in one single action method?

You may find my MSDN article on the subject helpful; I took a lot of space in that article describing when you should use async on ASP.NET, not just how to use async on ASP.NET.
I have some concerns using async actions in ASP.NET MVC. When it improves performance of my apps, and when - not.
First, understand that async/await is all about freeing up threads. On GUI applications, it's mainly about freeing up the GUI thread so the user experience is better. On server applications (including ASP.NET MVC), it's mainly about freeing up the request thread so the server can scale.
In particular, it won't:
Make your individual requests complete faster. In fact, they will complete (just a teensy bit) slower.
Return to the caller/browser when you hit an await. await only "yields" to the ASP.NET thread pool, not to the browser.
First question is - is it good to use async action everywhere in ASP.NET MVC?
I'd say it's good to use it everywhere you're doing I/O. It may not necessarily be beneficial, though (see below).
However, it's bad to use it for CPU-bound methods. Sometimes devs think they can get the benefits of async by just calling Task.Run in their controllers, and this is a horrible idea. Because that code ends up freeing up the request thread by taking up another thread, so there's no benefit at all (and in fact, they're taking the penalty of extra thread switches)!
Shall I use async/await keywords when I want to query database (via EF/NHibernate/other ORM)?
You could use whatever awaitable methods you have available. Right now most of the major players support async, but there are a few that don't. If your ORM doesn't support async, then don't try to wrap it in Task.Run or anything like that (see above).
Note that I said "you could use". If you're talking about ASP.NET MVC with a single database backend, then you're (almost certainly) not going to get any scalability benefit from async. This is because IIS can handle far more concurrent requests than a single instance of SQL server (or other classic RDBMS). However, if your backend is more modern - a SQL server cluster, Azure SQL, NoSQL, etc - and your backend can scale, and your scalability bottleneck is IIS, then you can get a scalability benefit from async.
Third question - How many times I can use await keywords to query database asynchronously in ONE single action method?
As many as you like. However, note that many ORMs have a one-operation-per-connection rule. In particular, EF only allows a single operation per DbContext; this is true whether the operation is synchronous or asynchronous.
Also, keep in mind the scalability of your backend again. If you're hitting a single instance of SQL Server, and your IIS is already capable of keeping SQLServer at full capacity, then doubling or tripling the pressure on SQLServer is not going to help you at all.

Asynchronous action methods are useful when an action must perform several independent long running operations.
A typical use for the AsyncController class is long-running Web
service calls.
Should my database calls be asynchronous ?
The IIS thread pool can often handle many more simultaneous blocking requests than a database server. If the database is the bottleneck, asynchronous calls will not speed up the database response. Without a throttling mechanism, efficiently dispatching more work to an overwhelmed database server by using asynchronous calls merely shifts more of the burden to the database. If your DB is the bottleneck, asynchronous calls won’t be the magic bullet.
You should have a look at 1 and 2 references
Derived from #PanagiotisKanavos comments:
Moreover, async doesn't mean parallel. Asynchronous execution frees a
valuable threadpool thread from blocking for an external resource, for
no complexity or performance cost. This means the same IIS machine can
handle more concurrent requests, not that it will run faster.
You should also consider that blocking calls start with a
CPU-intensive spinwait. During stress times, blocking calls will
result in escalating delays and app pool recycling. Asynchronous calls
simply avoid this

is it good to use async action everywhere in ASP.NET MVC?
As usual in programming, it depends. There is always a trade-off when going down a certain path.
async-await shines in places where you know you'll receiving concurrent requests to your service and you want to be able to scale out well. How does async-await help with scaling out? In the fact that when you invoke a async IO call synchronously, such as a network call or hitting your database, the current thread which is responsible for the execution is blocked waiting for the request to finish. When you use async-await, you enable the framework to create a state machine for you which makes sure that after the IO call is complete, your method continues executing from where it left off.
A thing to note is that this state machine has a subtle overhead. Making a method asynchronous does not make it execute faster, and that is an important factor to understand and a misconception many people have.
Another thing to take under consideration when using async-await is the fact that it is async all the way, meaning that you'll see async penetrate your entire call stack, top to buttom. This means that if you want to expose synchronous API's, you'll often find yourself duplicating a certain amount of code, as async and sync don't mix very well.
Shall I use async/await keywords when I want to query database (via
EF/NHibernate/other ORM)?
If you choose to go down the path of using async IO calls, then yes, async-await will be a good choice, as more and more modern database providers expose async method implementing the TAP (Task Asynchronous Pattern).
How many times I can use await keywords to query database
asynchronously in ONE single action method?
As many as you want, as long as you follow the rules stated by your database provider. There is no limit to the amount of async calls you can make. If you have queries which are independent of each other and can be made concurrently, you can spin a new task for each and use await Task.WhenAll to wait for both to complete.

async actions help best when the actions does some I\O operations to DB or some network bound calls where the thread that processes the request will be stalled before it gets answer from the DB or network bound call which you just invoked. It's best you use await with them and it will really improve the responsiveness of your application (because less ASP input\output threads will be stalled while waiting for the DB or any other operation like that). In all my applications whenever many calls to DB very necessary I've always wrapped them in awaiatable method and called that with await keyword.

My 5 cents:
Use async/await if and only if you do an IO operation, like DB or external service webservice.
Always prefer async calls to DB.
Each time you query the DB.
P.S. There are exceptional cases for point 1, but you need to have a good understanding of async internals for this.
As an additional advantage, you can do few IO calls in parallel if needed:
Task task1 = FooAsync(); // launch it, but don't wait for result
Task task2 = BarAsync(); // launch bar; now both foo and bar are running
await Task.WhenAll(task1, task2); // this is better in regard to exception handling
// use task1.Result, task2.Result

As you know, MVC supports asynchronous controllers and you should take advantage of it. In case your Controller, performs a lengthy operation, (it might be a disk based I/o or a network call to another remote service), if the request is handled in synchronous manner, the IIS thread is busy the whole time. As a result, the thread is just waiting for the lengthy operation to complete. It can be better utilized by serving other requests while the operation requested in first is under progress. This will help in serving more concurrent requests.
Your webservice will be highly scalable and will not easily run into C10k problem.
It is a good idea to use async/await for db queries. and yes you can use them as many number of times as you deem fit.
Take a look here for excellent advise.

My experience is that today a lot of developers use async/await as a default for controllers.
My suggestion would be, use it only when you know it will help you.
The reason is, as Stephen Cleary and others already mentioned, it can introduce performance issues, rather than resolving them, and it will help you only in a specific scenario:
High-traffic controllers
Scalable backend

Is it good to use async action everywhere in ASP.NET MVC?
It's good to do so wherever you can use an async method especially when you have performance issues at the worker process level which happens for massive data and calculation operations. Otherwise, no need because unit testing will need casting.
Regarding awaitable methods: shall I use async/await keywords when I
want to query a database (via EF/NHibernate/other ORM)?
Yes, it's better to use async for any DB operation as could as possible to avoid performance issues at the level of worker processes.
Note that EF has created many async alternatives for most operations, such as:
.ToListAsync()
.FirstOrDefaultAsync()
.SaveChangesAsync()
.FindAsync()
How many times can I use await keywords to query the database
asynchronously in one single action method?
The sky is the limit