It's recommended that one use ConfigureAwait(false) whenever when you can, especially in libraries because it can help avoid deadlocks and improve performance.
I have written a library that makes heavy use of async (accesses web services for a DB). The users of the library were getting a deadlock and after much painful debugging and tinkering I tracked it down to the single use of await Task.Yield(). Everywhere else that I have an await, I use .ConfigureAwait(false), however that is not supported on Task.Yield().
What is the recommended solution for situations where one needs the equivalent of Task.Yield().ConfigureAwait(false)?
I've read about how there was a SwitchTo method that was removed. I can see why that could be dangerous, but why is there no equivalent of Task.Yield().ConfigureAwait(false)?
Edit:
To provide further context for my question, here is some code. I am implementing an open source library for accessing DynamoDB (a distributed database as a service from AWS) that supports async. A number of operations return IAsyncEnumerable<T> as provided by the IX-Async library. That library doesn't provide a good way of generating async enumerables from data sources that provide rows in "chunks" i.e. each async request returns many items. So I have my own generic type for this. The library supports a read ahead option allowing the user to specify how much data should be requested ahead of when it is actually needed by a call to MoveNext().
Basically, how this works is that I make requests for chunks by calling GetMore() and passing along state between these. I put those tasks in a chunks queue and dequeue them and turn them into actual results that I put in a separate queue. The NextChunk() method is the issue here. Depending on the value of ReadAhead I will keeping getting the next chunk as soon as the last one is done (All) or not until a value is needed but not available (None) or only get the next chunk beyond the values that are currently being used (Some). Because of that, getting the next chunk should run in parallel/not block getting the next value. The enumerator code for this is:
private class ChunkedAsyncEnumerator<TState, TResult> : IAsyncEnumerator<TResult>
{
private readonly ChunkedAsyncEnumerable<TState, TResult> enumerable;
private readonly ConcurrentQueue<Task<TState>> chunks = new ConcurrentQueue<Task<TState>>();
private readonly Queue<TResult> results = new Queue<TResult>();
private CancellationTokenSource cts = new CancellationTokenSource();
private TState lastState;
private TResult current;
private bool complete; // whether we have reached the end
public ChunkedAsyncEnumerator(ChunkedAsyncEnumerable<TState, TResult> enumerable, TState initialState)
{
this.enumerable = enumerable;
lastState = initialState;
if(enumerable.ReadAhead != ReadAhead.None)
chunks.Enqueue(NextChunk(initialState));
}
private async Task<TState> NextChunk(TState state, CancellationToken? cancellationToken = null)
{
await Task.Yield(); // ** causes deadlock
var nextState = await enumerable.GetMore(state, cancellationToken ?? cts.Token).ConfigureAwait(false);
if(enumerable.ReadAhead == ReadAhead.All && !enumerable.IsComplete(nextState))
chunks.Enqueue(NextChunk(nextState)); // This is a read ahead, so it shouldn't be tied to our token
return nextState;
}
public Task<bool> MoveNext(CancellationToken cancellationToken)
{
cancellationToken.ThrowIfCancellationRequested();
if(results.Count > 0)
{
current = results.Dequeue();
return TaskConstants.True;
}
return complete ? TaskConstants.False : MoveNextAsync(cancellationToken);
}
private async Task<bool> MoveNextAsync(CancellationToken cancellationToken)
{
Task<TState> nextStateTask;
if(chunks.TryDequeue(out nextStateTask))
lastState = await nextStateTask.WithCancellation(cancellationToken).ConfigureAwait(false);
else
lastState = await NextChunk(lastState, cancellationToken).ConfigureAwait(false);
complete = enumerable.IsComplete(lastState);
foreach(var result in enumerable.GetResults(lastState))
results.Enqueue(result);
if(!complete && enumerable.ReadAhead == ReadAhead.Some)
chunks.Enqueue(NextChunk(lastState)); // This is a read ahead, so it shouldn't be tied to our token
return await MoveNext(cancellationToken).ConfigureAwait(false);
}
public TResult Current { get { return current; } }
// Dispose() implementation omitted
}
I make no claim this code is perfect. Sorry it is so long, wasn't sure how to simplify. The important part is the NextChunk method and the call to Task.Yield(). This functionality is used through a static construction method:
internal static class AsyncEnumerableEx
{
public static IAsyncEnumerable<TResult> GenerateChunked<TState, TResult>(
TState initialState,
Func<TState, CancellationToken, Task<TState>> getMore,
Func<TState, IEnumerable<TResult>> getResults,
Func<TState, bool> isComplete,
ReadAhead readAhead = ReadAhead.None)
{ ... }
}
The exact equivalent of Task.Yield().ConfigureAwait(false) (which doesn't exist since ConfigureAwait is a method on Task and Task.Yield returns a custom awaitable) is simply using Task.Factory.StartNew with CancellationToken.None, TaskCreationOptions.PreferFairness and TaskScheduler.Current. In most cases however, Task.Run (which uses the default TaskScheduler) is close enough.
You can verify that by looking at the source for YieldAwaiter and see that it uses ThreadPool.QueueUserWorkItem/ThreadPool.UnsafeQueueUserWorkItem when TaskScheduler.Current is the default one (i.e. thread pool) and Task.Factory.StartNew when it isn't.
You can however create your own awaitable (as I did) that mimics YieldAwaitable but disregards the SynchronizationContext:
async Task Run(int input)
{
await new NoContextYieldAwaitable();
// executed on a ThreadPool thread
}
public struct NoContextYieldAwaitable
{
public NoContextYieldAwaiter GetAwaiter() { return new NoContextYieldAwaiter(); }
public struct NoContextYieldAwaiter : INotifyCompletion
{
public bool IsCompleted { get { return false; } }
public void OnCompleted(Action continuation)
{
var scheduler = TaskScheduler.Current;
if (scheduler == TaskScheduler.Default)
{
ThreadPool.QueueUserWorkItem(RunAction, continuation);
}
else
{
Task.Factory.StartNew(continuation, CancellationToken.None, TaskCreationOptions.PreferFairness, scheduler);
}
}
public void GetResult() { }
private static void RunAction(object state) { ((Action)state)(); }
}
}
Note: I don't recommend actually using NoContextYieldAwaitable, it's just an answer to your question. You should be using Task.Run (or Task.Factory.StartNew with a specific TaskScheduler)
I noticed you edited your question after you accepted the existing answer, so perhaps you're interested in more rants on the subject. Here you go :)
It's recommended that one use ConfigureAwait(false) whenever when you
can, especially in libraries because it can help avoid deadlocks and
improve performance.
It's recommended so, only if you're absolutely sure that any API you're calling in your implementation (including Framework APIs) doesn't depend on any properties of synchronization context. That's especially important for a library code, and even more so if the library is suitable for both client-side and server-side use. E.g, CurrentCulture is a common overlook: it would never be an issue for a desktop app, but it well may be for an ASP.NET app.
Back to your code:
private async Task<TState> NextChunk(...)
{
await Task.Yield(); // ** causes deadlock
var nextState = await enumerable.GetMore(...);
// ...
return nextState;
}
Most likely, the deadlock is caused by the client of your library, because they use Task.Result (or Task.Wait, Task.WaitAll, Task.IAsyncResult.AsyncWaitHandle etc, let them search) somewhere in the outer frame of the call chain. Albeit Task.Yield() is redundant here, this is not your problem in the first place, but rather theirs: they shouldn't be blocking on the asynchronous APIs and should be using "Async All the Way", as also explained in the Stephen Cleary's article you linked.
Removing Task.Yield() may or may not solve this problem, because enumerable.GetMore() can also use some await SomeApiAsync() without ConfigureAwait(false), thus posting the continuation back to the caller's synchronization context. Moreover, "SomeApiAsync" can happen to be a well established Framework API which is still vulnerable to a deadlock, like SendMailAsync, we'll get back to it later.
Overall, you should only be using Task.Yield() if for some reason you want to return to the caller immediately ("yield" the execution control back to the caller), and then continue asynchronously, at the mercy of the SynchronizationContext installed on the calling thread (or ThreadPool, if SynchronizationContext.Current == null). The continuation well may be executed on the same thread upon the next iteration of the app's core message loop. Some more details can be found here:
Task.Yield - real usages?
So, the right thing would be to avoid blocking code all the way. However, say, you still want to make your code deadlock-proof, you don't care about synchronization context and you're sure the same is true about any system or 3rd party API you use in your implementation.
Then, instead of reinventing ThreadPoolEx.SwitchTo (which was removed for a good reason), you could just use Task.Run, as suggested in the comments:
private Task<TState> NextChunk(...)
{
// jump to a pool thread without SC to avoid deadlocks
return Task.Run(async() =>
{
var nextState = await enumerable.GetMore(...);
// ...
return nextState;
});
}
IMO, this is still a hack, with the same net effect, although a much more readable one than using a variation of ThreadPoolEx.SwitchTo(). Same as SwitchTo, it still has an associated cost: a redundant thread switch which may hurt ASP.NET performance.
There is another (IMO better) hack, which I proposed here to address the deadlock with aforementioned SendMailAsync. It doesn't incur an extra thread switch:
private Task<TState> NextChunk(...)
{
return TaskExt.WithNoContext(async() =>
{
var nextState = await enumerable.GetMore(...);
// ...
return nextState;
});
}
public static class TaskExt
{
public static Task<TResult> WithNoContext<TResult>(Func<Task<TResult>> func)
{
Task<TResult> task;
var sc = SynchronizationContext.Current;
try
{
SynchronizationContext.SetSynchronizationContext(null);
task = func(); // do not await here
}
finally
{
SynchronizationContext.SetSynchronizationContext(sc);
}
return task;
}
}
This hack works in the way it temporarily removes the synchronization context for the synchronous scope of the original NextChunk method, so it won't be captured for the 1st await continuation inside the async lambda, effectively solving the deadlock problem.
Stephen has provided a slightly different implementation while answering the same question. His IgnoreSynchronizationContext restores the original synchronization context on whatever happens to be the continuation's thread after await (which could be a completely different, random pool thread). I'd rather not restore it after await at all, as long as I don't care about it.
Inasmuch as the useful and legit API you're looking for is missing, I filed this request proposing its addition to .NET.
I also added it to vs-threading so that the next release of the Microsoft.VisualStudio.Threading NuGet package will include this API. Note that this library is not VS-specific, so you can use it in your app.
Related
I'm trying to design a public method that returns a quick result and, if needed, kicks off a long-running background task.
The result is retrieved within the method pretty quickly, and once it is retrieved it needs to be immediately available to the caller, without waiting for the potential background task to complete.
The background task is either not needed at all, or it must run for a significant amount of time - longer than the caller and all other logic in the program would take to complete without it.
Looking through the MS docs, the best design option I can come up with is to return two things from this method: the result, as well as a Task for the background task.
I don't see a better way to do this, but there are some downsides: first, the responsibility for making sure the background task completes falls on the caller of the method, even though the caller really just wants to consume the immediate result and not be concerned with the behind-the-scene stuff. Second, it is awkward to return null if the background task isn't needed to begin with: now the caller must ensure the task isn't null, in addition to making sure it completes if it is null.
What other options are available for this sort of situation?
Here's an example of what my current design looks like:
public async Task<Tuple<string, Task>> GetAndSave() {
var networkResult = await GetFromNetworkAsync();
if (NeedsToSave(networkResult)) {
var saveTask = SaveToDiskAsync(networkResult);
return new Tuple<string, Task>(networkResult, saveTask);
}
else {
return new Tuple<string, Task>(networkResult, null);
}
}
first, the responsibility for making sure the background task completes falls on the caller of the method, even though the caller really just wants to consume the immediate result and not be concerned with the behind-the-scene stuff.
If it's important to make sure the background task completes then instead of returning the Task you could hand it off to another object (that has been injected into the class that has your GetAndSave method). For example:
public class Foo
{
readonly Action<Task> _ensureCompletion;
public Foo(Action<Task> ensureCompletion)
{
_ensureCompletion = ensureCompletion;
}
public async Task<string> GetAndSaveAsync() // Your method
{
string networkResult = await GetFromNetworkAsync();
if (NeedsToSave(networkResult))
{
Task saveTask = SaveToDiskAsync(networkResult);
_ensureCompletion(saveTask); // This is a synchronous call... no await keyword here. And it returns `void`
}
return networkResult;
}
Task<string> GetFromNetworkAsync() {...}
bool NeedsToSave(string x) {...}
Task SaveToDiskAsync(string x) {...}
}
Now you can inject whatever follow-up behavior you desire for the saveTask. For example, you could write stuff out to the console depending on how it goes:
async Task DoStuff()
{
var foo = new Foo(async task =>
// ^^^^^ More on this in a moment
{
try
{
await task;
Console.Writeline("It worked!");
}
catch (Exception e)
{
Console.Writeline(e.ToString());
}
});
var immediateResult = await foo.GetAndSaveAsync();
// do stuff with `immediateResult`
}
Now, the thing that might be confusing about this (and the power behind this type of solution) is how you can have a synchronous call on the one hand:
_ensureCompletion(saveTask); // This is a synchronous call... no await keyword here
...that does asynchronous things:
var foo = new Foo(async task => ...);
// ^^^^^ This statement lambda is asynchronous
(The injected delegate might even write out to the console on a different thread than whatever thread called GetAndSaveAsync()!)
There's no magic here. It all comes down to SynchronizationContext and the inner workings of async/await.
When the compiler encounters the await keyword (in an async context) then it will do a few things:
Turn everything after the await into a continuation (basically a delegate with some state)
Replace the await keyword with something like SynchronizationContext.Current.Post(continuation)
In other words, this:
async void EnsureCompletion(Task task)
{
try
{
await task;
Console.Writeline("It worked!");
}
catch (Exception e)
{
Console.Writeline(e.ToString());
}
}
...gets turned into something like this:
void EnsureCompletion(Task task)
{
try
{
task.ContinueWith(t => SynchronizationContext.Current.Post(_ =>
{
if (t.IsCompletedSuccessfully)
{
Console.Writeline("It worked!");
}
else
{
Console.Writeline(task.Exception.ToString());
}
});
}
catch (Exception e)
{
Console.Writeline(e.ToString());
}
}
As you can see, EnsureCompletion is an entirely synchronous function that does (almost) the exact same thing as its asynchronous form. Notice how it returns void and everything. This is how you can jam an asynchronous statement lambda into a delegate parameter that has a synchronous signature.
I hinted that the console writing might happen on a totally different thread. That's because async/await is orthogonal to threading. It depends on whatever the currently assigned implementation of SynchronizationContext is programmed to do. Unless you're using WPF or WinForms then by default there will be no SynchronizationContext and continuations (the things that get passed to SynchronizationContext.Post) will just be tossed over to whatever thread happens to be free in the default thread pool.
Second, it is awkward to return null if the background task isn't needed to begin with: now the caller must ensure the task isn't null, in addition to making sure it completes if it is null.
I'm of the opinion that null was a design mistake in C#. (Ask me what some other ones are :) ). If you return null from this method:
public Task DoSomethingAsync()
{
return null; // Compiles just fine
}
...then anyone who uses it will encounter a NullReferenceException when they await it.
async Task Blah()
{
await DoSomethingAsync(); // Wham! NullReferenceException :(
}
(the reason why comes back to how the compiler desugurs the await keyword)
...and it's awkward to have to check for nulls everywhere.
Much better would be to just return Task.CompletedTask as #juharr said.
But if you just hand off the background task like I showed above then you don't have to worry about this.
I see microsoft enforces this pattern for async methods:
async Task<object> DoMyOperationAsync(int par1, string par2,..., CancellationToken token=default(CancellationToken))
{
...
CancellationToken.ThrowIfRequested();
...
}
Every single method should have that ugly CancellationToken token=default(CancellationToken) parameter even though most of the time it's not even being used, but just passed through at best.
Why instead of this we cannot just use some sort of CancellationTokenContext, and use it in methods that actually need it?
public class CancellationTokenContext
{
static AsyncLocal<CancellationToken> asyncContext = new AsyncLocal<CancellationToken>();
public static CancellationToken Current {
get {
return asyncContext.Value;
}
set {
asyncContext.Value = value;
}
}
public static void ThrowIfRequested() {
Current.ThrowIfCancellationRequested();
}
}
public class MyClassWithAsyncMethod{
public async Task<object> DoMyOperationAsync(int par1, string par2,...)
{
...
CancellationTokenContext.ThrowIfRequested();
...
}
}
The main reason for not using AsyncLocal is that it's much slower. In fact, it's a dictionary lookup, so it should be approximate 20x slower (~ 20ns vs 1ns).
One other factor is: explicit is usually better than implicit. Though I agree it's a very annoying part of async implementation.
Maybe the main reason is that CancellationToken is a structure. So if you pass it as a parameter, it will be kept in a call stack, without need to allocate memory in heap. But if you use AsyncLocal instead, it will use boxing/unboxing to deal with a cancellation token. In case when async operations are heavily used, i.e. some ASP.NET Core app, it may even affect overall performance because of additional workload for garbage collector.
Other possible problem which I see here, is need to setup up and then restore AsyncLocal for every sub-call which require different cancellation token. For example:
private async Task DoSomeWork()
{
var currentCancallationToken = _ambientCancellationToken.Value;
_ambientCancellationToken.Value = CancellationToken.None;
// i.e. we should finish logging no matter of cancellation signal for calling method, for whatever reason
await _myCustomLogger.AsyncInfo("Do 1st step");
// restore ambient cancellation token behavior to make next step of async flow
_ambientCancellationToken.Value = currentCancallationToken;
await Do1StepOfWork();
_ambientCancellationToken.Value = CancellationToken.None;
// ...and here we go again
await _myCustomLogger.AsyncInfo("Do 2nd step");
// ...it became annoying...
_ambientCancellationToken.Value = currentCancallationToken;
await Do2StepOfWork();
}
But from other prospective, if you are sure, that your app (not a public library/framework) will not make several thousandths async flows per second, and you sure, that you will not use mixed cancellation contexts, it seems ok to use such approach.
Which of these is the best option when working with caching in C#?
I am interested at the compiler level which of these is the most elegant / performant solution.
E.g does the .net compiler use any tricks to know that when code will run synchronously and avoid creating/running unnecessary async await code?
Option 1, Use async/await and use Task.FromResult for cached values;
public async Task<T> GetValue<T>(string key)
{
if (_cache.containsKey(key))
{
// 99% of the time will hit this
return Task.FromResult(_cache.GetItem(key));
}
return await _api.GetValue(key);
}
Option 2, Avoid async/await and use something like GetAwaiter().GetResult() for the few times the API Endpoint will be hit.
public T GetValue<T>(string key)
{
if (_cache.containsKey(key))
{
// 99% of the time will hit this
return _cache.GetItem(key);
}
return _api.GetValue(key).GetAwaiter().GetResult();
}
Any insights would be very much appreciated.
The official approach is to cache the Task<T>, and not the T.
This also has the advantage that if someone requests the value, you can kick off the request to fetch the value and then cache the resulting, in-progress Task<T>. If someone else requests the cached value before the request has completed, they're also given the same in-progress Task<T>, and you don't end up making two requests.
For example:
public Task<T> GetValue<T>(string key)
{
// Prefer a TryGet pattern if you can, to halve the number of lookups
if (_cache.containsKey(key))
{
return _cache.GetItem(key);
}
var task = _api.GetValue(key);
_cache.Add(key, task);
return task;
}
Note that you need to think about failure in this case: if the request to the API fails, then you'll be caching a Task which contains an exception. This might be what you want, but it might not be.
If for some reason you can't do this, then the official advice is to use ValueTask<T> for high-performance scenarios. This type has some gotchas (such as you can't await it twice), so I recommend reading this. If you don't have high performance requirements, Task.FromResult is fine.
Your first won't work. The simplest, and the one to go for most of the time is:
public async Task<T> GetValueAsync<T>(string key)
{
if (_cache.ContainsKey(key))
{
return _cache.GetItem(key);
}
T result = await _api.GetValueAysnc(key);
_cache.Add(key, result);
return result;
}
Or better still if possible:
public async Task<T> GetValueAsync<T>(string key)
{
if (_cache.TryGet(key, out T result))
{
return result;
}
result = await _api.GetValueAysnc(key);
_cache.Add(key, result);
return result;
}
This works fine and will return an already-completed task when the value was in the cache, so awaiting it will continue immediately.
However if the value is in the cache much of the time and the method is called often enough for the extra apparatus around async to make a difference then you can avoid it entirely in such a case:
public Task<T> GetValueAsync<T>(string key)
{
if (_cache.TryGet(key, out Task<T> result))
{
return result;
}
return GetAndCacheValueAsync(string key);
}
private async Task<T> GetAndCacheValueAsync<T>(string key)
{
var task = _api.GetValueAysnc(key);
result = await task;
_cache.Add(key, task);
return result;
}
Here if the value is cached we avoid both the state-machine around async and also the creation of a new Task<T> since we have stored an actual Task. Each of these are only done in the first case.
What you are looking for probably is memoization.
A implementation might be something like this:
public static Func<T, TResult> Memoize<T, TResult>(this Func<T, TResult> f)
{
var cache = new ConcurrentDictionary<T, TResult>();
return a => cache.GetOrAdd(a, f);
}
Measure(() => slowSquare(2)); // 00:00:00.1009680
Measure(() => slowSquare(2)); // 00:00:00.1006473
Measure(() => slowSquare(2)); // 00:00:00.1006373
var memoizedSlow = slowSquare.Memoize();
Measure(() => memoizedSlow(2)); // 00:00:00.1070149
Measure(() => memoizedSlow(2)); // 00:00:00.0005227
Measure(() => memoizedSlow(2)); // 00:00:00.0004159
Source
First of all, this calls for linking the speed rant:
https://ericlippert.com/2012/12/17/performance-rant/
Microoptimisations like these are usually left to the JiT. My rule of thumb is that if you really need that difference, then you are propably dealing with realtime programming. And for Realtime Proramming a Garbage Collected runtime like .NET was propably the wrong environment to begin with. Something with direct memory management like unsafe code - even native C++ or Assembler - would have been better.
Secondly, a task might just be the wrong tool here. Maybe what you actually want is something like Lazy[T]? Or any of the 5 different Chache classes? (as with timer, there is about one for specific User Interface technology).
It is possible to use any tool for many purposes. But tasks are for Multitasking and there are better tools for caching and lazy initialisation. And Lazy[T] is even inherently Thread save.
I've searched for the answer to this but according to many guides and SO questions this code still appears correct to me, yet it runs synchronously.
private void CheckConditions()
{
foreach (var obj in myObjects)
{
if (obj.ConditionMet)
{
HandleConditionAsync(obj);
}
}
DoOtherWork();
}
private async void HandleConditionAsync(MyObject obj)
{
// shouldn't control transfer back to CheckConditions() here while we wait for user input?
string userInput = await obj.MessagePromptAsync("hello user");
DoSomeBookkeeping(obj);
}
// (MyObject.cs)
private MessagePrompt messagePrompt; // inherits from UserControl
public async Task<string> MessagePromptAsync(string prompt)
{
return await Task.FromResult<string>(messagePrompt.Prompt(prompt));
}
// (MessagePrompt.cs)
public string Prompt(string prompt)
{
this.UIThread(() => this.SetMessagePrompt(prompt));
userInputAutoResetEvent.WaitOne();
return myResult; // set in a button handler that also sets the AutoResetEvent
}
I'm intending for CheckConditions() to continue along merrily but instead it is stuck on MessagePrompt's AutoResetEvent despite my async/awaits. The only thing I can figure might be wrong is that perhaps MessagePrompt's methods aren't able to run asynchronously due to some restriction from UserControl, its use of a UI thread reference, or maybe non-async methods at the top of the stack.
There's nothing in your code that's asynchronous. The only task you have, you created from a result value, meaning the Prompt() method has to complete and return its result before you'll even get the Task object back to wait on. That object will already be completed, so any await on it will complete immediately, once it has the Task to wait on.
Maybe you meant this instead:
public async Task<string> MessagePromptAsync(string prompt)
{
return await Task.Run(() => messagePrompt.Prompt(prompt));
}
Or alternatively (if you really do have nothing else in the MessagePromptAsync() method):
public Task<string> MessagePromptAsync(string prompt)
{
return Task.Run(() => messagePrompt.Prompt(prompt));
}
Note that this may lead to a different problem, depending on what DoOtherWork() and UIThread() actually do. If your UI thread gets tied up in DoOtherWork() and the UIThread() method is wrapping Dispatcher.Invoke() or similar, then you'll have a deadlock.
If that does not address your problem, please provide a good Minimal, Complete, and Verifiable code example that reliably reproduces the problem.
You need to make CheckConditions() async as well, and then await the call to HandleConditionAsync(MyObject obj). CheckConditions() runs synchronously in your sample.
private async Task CheckConditionsAsync()
{
foreach (var obj in myObjects)
{
if (obj.ConditionMet)
{
await HandleConditionAsync(obj);
}
}
DoOtherWork();
}
Also, and this is just a best practices thing, an async method should always return a Task when possible. The only time I've ever had to use async void is for compatibility with an event handler. You can see I've changed CheckConditions() this way, and HandleConditionAsync(MyObject obj) should be modified similarly. I also changed the method name to represent it's asynchronous behaviour.
If you need to run a method that returns a Task synchronously (and you shouldn't do this, this is an indication of something incorrect about your design), you can run it with Task.FromResult(MyMethodAsync()). Again, avoid doing this wherever you can, it defeats the purpose of making a method asynchronous to in the first place.
I have an interface that reads/writes an object to storage. In one case the storage is a database with async methods. In the other case it's just a cookie.
I gather that its recommended to use async back along the path ending at an async call, so it seems to make sense for the interface to be async as well. But in the cookie case, I'm just setting a couple fields and sticking it in the response so there isn't any async there yet. I can wrap that bit in await Task.Run() to match the new interface but I don't know if this is advisable or if it has some negative impact on performance.
What to do?
public interface IProfileStore
{
async Task SetProfile(UserProfile profile);
}
public async Task SetProfile(UserProfile profile)
{
// Look mom, I'm needlessly async
await Task.Run(() =>
{
var cookie = new HttpCookie(AnonymousCookieName);
cookie["name"] = profile.FullName;
HttpContext.Current.Response.Cookies.Add(cookie);
});
}
You should not do that; you're just creating needless threadpool churn.
Instead, remove the async keyword from the method and simply return Task.FromResult(0) to return a synchronously-completed task
If you're performing a very short quickly completed operation then you're quite right that there is likely no need to use Task.Run to push the work to another thread. The act of scheduling the code in the thread pool is likely going to take longer than just doing it.
As for how to do that, just remove the await Task.Run that you have no need for and voila, you're all set. You have a synchronous operation that is still wrapped in a Task and so still matches the required interface.
Almost as SLaks suggests if you were doing something async but return the Task, so:
public Task SetProfile(UserProfile profile)
{
return Task.Run(() =>
{
var cookie = new HttpCookie(AnonymousCookieName);
cookie["name"] = profile.FullName;
HttpContext.Current.Response.Cookies.Add(cookie);
});
}
However as he suggests in this case:
public Task SetProfile(UserProfile profile)
{
var cookie = new HttpCookie(AnonymousCookieName);
cookie["name"] = profile.FullName;
HttpContext.Current.Response.Cookies.Add(cookie);
return Task.FromResult(null);
}
Return null as its a system cached completed Task.