I have an interface IAnimation which exposes a method BeginAsync(). That method should start the animation and return when it is completed.
What I would like to do is implement a null animation class NoAnimation that just returns when it executes BeginAsync().
Is this the right implementation?
public async Task BeginAsync()
{
await Task.Run(() => { });
}
I suspect that there is a more elegant approach than this. I also considered
creating an empty method. But that gives me a warning which I don't like either.
Just use Task.CompletedTask to return a completed task:
public Task BeginAsync()
{
return Task.CompletedTask;
}
If you have a Task<TResult> use Task.FromResult<TResult> to return a completed task with a result:
public Task<bool> BeginAsync()
{
return Task.FromResult(true);
}
Your current implementation is very inefficient, as it builds the state machine, and also uses a ThreadPool thread to run the empty task.
I appreciate this doesn't exactly answer the original question (or at least not in a way that isn't already in another answer). What it does it provide additional context for #ReedCopsey's answer, for those (including me! and others, based on the comments on that answer) who initially might find it hard to understand what is going on here.
It's very easy to fall into the trap of thinking that async must be part of the signature of any asynchronous method. It looks as if it must be; it (misleadingly!) reads as if it's whole purpose is to mark a method as asynchronous.
But it's not, Task or Task<T> are required, but not async. This is shown in #ReedCopsey's answer and in the following correct example (slightly adapted from this useful SO answer):
public Task<int> MethodTaskAsync(int arg0, int arg1)
{
Task<int> task = new Task<int>(() => Method(arg0, arg1));
task.Start(); // Hot task (started task) should always be returned.
return task;
}
That's why interface method signatures don't need, and can't have, the async keyword: an 'asynchronous' method per se, without regard to how it might be implemented, is just a method that returns a started (and, possibly, already finished) Task or Task<T>.
If async doesn't mark a method as asynchronous, what does it do? It's a way to write asynchronous methods more simply: once you have applied it to a method, you can use the await keyword to tell the compiler to correctly handle subsidiary asynchronous calls for you very simply (without it, you could try to handle subsidiary Tasks manually, using lots more code like that in the code block above, but to do so would be very complex and error prone).
So if you just want to return a do-nothing, already-completed task from an 'asynchronous' (i.e. Task-returning) method, you can and should do so synchronously(!), as in #ReedCopsey 's answers, using Task.CompletedTask or Task.FromResult(...).
Related
This is probably a more of a finesse question but I have the following method inside a ViewComponent class
public async Task<IViewComponentResult> InvokeAsync()
{
return View();
}
but the name InvokeAsync is underlined and gives the following warning
This async method lacks 'await' operators and will run synchronously.
Consider using the 'await' operator to await non-blocking API calls,
or 'await Task.Run(...)' to do CPU-bound work on a background thread
but if I try removing the async from the method then return View() is underlined with red and outputs the following error
'Microsoft.AspNetCore.Mvc.ViewComponents.ViewViewComponentResult' to
'System.Threading.Tasks.Task' MVCStateManagement
So my question is what approach should I take? Let the async there indiferently of the warning, or is there a workaround / fix for this warning? Does it have that much of an impact on my project?
Thanks!
It's unclear why the method was defined as an async method that returns a Task<IViewComponentResult> in the first place.
Since the method seems to be truly synchronous and simply returns a view, you should probably define it like this:
public IViewComponentResult Invoke()
{
return View();
}
A synchronous method doesn't magically become asynchronous just because you add the async keyword to it.
If you are implementing an interface and cannot change the signature of the method, you could use the Task.FromResult method to return an already completed task (you should still remove the async keyword):
public Task<IViewComponentResult> InvokeAsync()
{
return Task.FromResult<IViewComponentResult>(View());
}
The method runs on a Task of ViewcomponentResult.
You can call it without async by using this:
public Task<IViewComponentResult> InvokeAsync()
{
return Task.FromResult<IViewComponentResult>(View());
}
Your warning will no longer show and your code will run.
You could also leave it. Either will have no impact on the performance of your project.
You have to remove the async flag and the Task<>. Just return a IViewComponentResult.
You will tipically return a object wrapped into Task<> when you do async work. If you don´t it does not make any sense.
From MSDN:
The Task class represents a single operation that returns a
value and that usually executes asynchronously. Task objects
are one of the central components of the task-based asynchronous
pattern first introduced in the .NET Framework 4. Because the work
performed by a Task object typically executes asynchronously
on a thread pool thread rather than synchronously on the main
application thread, you can use the Status property, as well as the
IsCanceled, IsCompleted, and IsFaulted properties, to determine the
state of a task. Most commonly, a lambda expression is used to specify
the work that the task is to perform.
https://learn.microsoft.com/en-us/dotnet/api/system.threading.tasks.task-1?view=netframework-4.7.2
EDIT:
You can also try returning:
return Task.FromResult<IViewComponentResult>(View());
Consider a situation when the method returns a chain of tasks, like this one(I was working mainly with .net 3.5, so the example uses ContinueWith, but the question is the same for await):
static Task UnwrappedTask()
{
Task<Task> t = TestStr().ContinueWith( (
return GetTaskFromString(s).ContinueWith(
// returns a task
});
});
return t.Unwrap();
}
static async Task GetTaskFromString(string str)
{
await Task.Delay(3000);
}
static async Task<string> TestStr()
{
await Task.Delay(3000);
return "res";
}
Is there a new Task object created every time we return from ContinueWith(), or does the compiler reuse the instance somehow? I am interested in how the Value type Task.Result behaves in this scenario: is it copied all those times, or does the wrapper Task class provide an efficient way of transporting it to the outermost method?
ContinueWith will always create a new task. It can't possibly do anything else. The whole design of the method is that ContinueWith takes an existing Task, executes some arbitrary method after it has finished, and produces a new result. The result of ContinueWith isn't ever going to be identical to the task it is a continuation of, so it can't just re-use it. It is, by design, not mutating the task it's adding a continuation to (at least in any externally visible way, technically storing the continuations to fire is an internal mutation of the Task).
I have a async method returning a task. Do I need to return anything in the following situation?
public async Task UpdateProductSpecificationsAsync()
{
await _productRepository.UpdateProductSpecificationsAsync(); //Takes long time to execute
}
Is the following code examples equivalent to each other?
public Task UpdateProductSpecifications()
{
_productRepository.UpdateProductSpecifications(); //Takes long time to execute
return Task.FromResult(0);
}
public Task UpdateProductSpecifications()
{
_productRepository.UpdateProductSpecifications(); //Takes long time to execute
return Task.CompletedTask;
}
Is there any situation where I should await a Task.TaskCompleted?
I have a async method returning a task. Do I need to return anything in the following situation?
An async Task method is equivalent to a non-async void method. And like a non-async void method, an async Task method can't return anything.
That said, your example seems odd. If all you're going to do in the method is await some other async method, you should not bother with the async/await at all. Just return the Task directly:
public Task UpdateProductSpecificationsAsync()
{
return _productRepository.UpdateProductSpecificationsAsync();
}
Is the following code examples equivalent to each other?
Define "equivalent". To me, they are definitely not equivalent. The first creates a new Task<int> object, and returns that object as a Task. Calling code would have to cast it back to Task<int> to see the result value.
The second returns the static Task.CompletedTask property value, which is a singleton object, allocated only once per process. There is no result value for the caller to read.
The caller would have to go to extra work to directly observe those differences, but at the very least, returning a reference to a singleton object is more efficient than creating a new object every time. Whether this is significant or not depends, of course, on how often you call the method.
All that said, I don't understand why in that scenario, you wouldn't implement it like this (assuming there's no truly asynchronous version of the _productRepository method):
public Task UpdateProductSpecificationsAsync()
{
return Task.Run(() => _productRepository.UpdateProductSpecifications());
}
Then you'd get actual asynchronous behavior, which is what the caller would typically expect. Your versions force the caller to wait in spite of the method looking like it's asynchronous. IMHO, it's a very bad idea to write code that lies.
Is there any situation where I should await a Task.TaskCompleted [sic]?
You mean directly? Or by code that doesn't know it's been handed the Task.CompletedTask reference? I'm going to assume the former, because it's unreasonable to expect the latter to have any idea that's what it's doing.
That seems like an overly broad question to me. Any situation? That's open to interpretation.
That said, I would say "no" to that question. What would the point of awaiting something you know is already completed? The await won't yield back to the caller, because the awaitable is already completed. It seems like a waste to me.
I can imagine scenarios where one awaits a Task.CompletedTask in order to achieve some other desired side-effect. I've seen weirder things than that in production code. But I wouldn't recommend it. Whatever hypothetical side-effect we'd be talking about, I'm sure it's not a documented side-effect, so one would be relying on undocumented implementation details that could change in the future.
Don't write fragile code. Write code that relies only on documented behavior, and only does things that make obvious sense. (And the corollary: if you break that rule, at the very least write a very detailed comment explaining why you wrote weird, fragile code.)
This question already has answers here:
Why use async and return await, when you can return Task<T> directly?
(9 answers)
Closed 2 years ago.
Only the last line of the method below is using 'await', just before the method returns, so doesn't this mean that method is basically synchronous and should just be called "Get()" without the async modifier and the suffix Async?
public virtual async Task<TEntity> GetAsync(Guid id)
{
// some more code here
return await _dbSet.FindAsync(id);
}
doesn't this mean that method is basically synchronous
No. It's asynchronous. You're probably thinking of sequential (progressing from one thing to the next), not synchronous (blocking the current thread). An await will pause the method (sequentially) but not block the thread (asynchronously). For more information, see my async intro.
without the async modifier
While you could elide the async/await keywords, I would recommend that you do not. This is because // some more code here may throw an exception. I cover this and other considerations in my blog post on eliding async and await.
and the suffix Async?
No, that suffix is appropriate for any method that returns an awaitable (e.g., Task). So, even if you elide the async and await, it's still returning a task that should be awaited, so it should still have the Async suffix.
You can think of it this way: the Async suffix is part of the API interface. The async keyword is an implementation detail. They often go together, but not always.
Its a matter of opinion. Generally the naming convention is anything that returns Task or Task<T> has a suffix of Async.
Your example above though would be better written like this so you do not have the extra overhead of the async wrapper. This is because you do not need to await the result in that method. The consumer of the method would then await the result if it used it directly.
public virtual Task<TEntity> GetAsync(Guid id, params Expression<Func<TEntity, object>>[] includeProperties))
{
// some more code here
return _dbSet.FindAsync(id);
}
await doesn't block. It awaits asynchronously for an operation to complete. That means that the original thread is released while awaiting. In a desktop application, this means that the UI thread is released while waiting eg for an HTTP or database call to finish.
When that operation completes, execution is returned to the original thread (for desktop applications).
As it is, you can simplify your code by removing the async and await keywords and returning the Task immediatelly. The code doesn't do anything with the results of the operation, so it doesn't need to await for it to finish. This will be done by the caller of the method:
public virtual Task<TEntity> GetAsync(Guid id)
{
// some more code here
return _dbSet.FindAsync(id);
}
The real asynchronous operation in this code is FindAsync. The async keyword is just syntactic sugar that allows you to use the await keyword. If you don't need to await, you don't need the async keyword either
There are many cases where suffixing every async method with Async is simply redundant. If your method returns a Task or Task<T>, and its purpose is to get data from a database or another network resource, then of course it runs asynchronously. The compiler will also almost always give you an error if you forget to await something. With that in mind, there are some good reasons to suffix async methods with Async:
The method runs asynchronously but does not return a Task
There is an alternative other method by the same name that runs synchronously
Forgetting an await in calling code is unlikely to be caught by the compiler
Suffixing a method name with Async is meant to indicate that the method runs asynchronously, which this method does. Whether it does so using async/await or directly returns another Task is an implementation detail. Imagine you were reviewing some code calling the method that forgets to use await. Which of these makes the error more apparent?
var item1 = _example.Get(itemId1);
var item2 = _example.GetAsync(itemId2);
Based on nothing else, there's no reason to believe there's anything wrong with the first line. The second line will at least raise some eyebrows.
I was reading this article about Expression Bodied Members in C# 6.0, the author demonstrated this code:
public async Task<string> ReadFromWeb() => await RunWebRequest();
He says that it's not recommended to use async and await keywords in above code:
The compiler is performing some heavy lifting to implement the async
state machine for this method. Because of the structure of the method,
that extra work isn't really accomplishing much. It's creating a state
machine to wrap a task that simply unwraps a task returned from a
different method.
He proposed that we should write the code without async and await:
public Task<string> ReadFromWebSimple() => RunWebRequest();
I'd like to know more information about that.
Lets see what the author was talking about. When you mark a method as async, the compiler generates a state-machine on your behalf in order to allow the asynchronous style of execution to "feel like" it's executing synchronously.
When you write:
public async Task<string> ReadFromWeb() => await RunWebRequest();
The compiler generates:
[AsyncStateMachine(typeof(C.<RunWebRequest>d__1))]
public Task<string> RunWebRequest()
{
C.<RunWebRequest>d__1 <RunWebRequest>d__;
<RunWebRequest>d__.<>t__builder = AsyncTaskMethodBuilder<string>.Create();
<RunWebRequest>d__.<>1__state = -1;
AsyncTaskMethodBuilder<string> <>t__builder = <RunWebRequest>d__.<>t__builder;
<>t__builder.Start<C.<RunWebRequest>d__1>(ref <RunWebRequest>d__);
return <RunWebRequest>d__.<>t__builder.Task;
}
Since you're using an Expression Bodied Method, you actually have a single liner of a method. When do you generally await on something? when you want to manipulate the return value of the async method. With one-liners, that's never the case. This means that you can save the state-machine generation at the invocation level, and let only those who want to acutally await the result higher up the caller stack to await on your method. This will effectively transform your method to look like this:
public Task<string> ReadFromWeb()
{
return this.RunWebRequest();
}
Which saves you the rather already slim state-machine struct allocated by the compiler, which is actually quite redundant when you're creating an EBM.
Making a method async allows you to use await inside it. You don't really need await as you're not using the return value and you're not doing anything after that operation completes. In this case you can just return the task directly instead. That way your caller awaits the inner async method without the overhead of the method in between.
This avoids the work being done to make ReadFromWeb an async method. It's not a huge deal but in this case where all you have is a single invocation doing that is pretty harmless.