avoid using async lambda when delegate type returns void

In the above example, the QueueOrder should have been declared with async Task instead of async void. Were passing in an async lambda that will give back a Task, which means the TResult in Func is actually Task, such that the delegate provided to StartNew is a Func>. If your codebase is heavily async and you have no legitimate or limited legitimate uses for async void, your best bet is to add an analyzer to your project. When you invoke an async method, it starts running synchronously. Func<Task> myIOBoundTask = async () => { MyType other = MyType (a, b); await other.ProcessIOBoundOperationAsync (); }; Please remember to click "Mark as Answer" the responses that resolved your issue, and to click "Unmark as Answer" if not. Mutually exclusive execution using std::atomic? You can easily create lambda expressions and statements that incorporate asynchronous processing by using the async and await keywords. When the await completes, it attempts to execute the remainder of the async method within the captured context. Figure 2 Exceptions from an Async Void Method Cant Be Caught with Catch. This allows you to easily get a delegate to represent an asynchronous operation, e.g. The delegate's Invoke method doesn't check attributes on the lambda expression. The C# language provides built-in support for tuples. Did any DOS compatibility layers exist for any UNIX-like systems before DOS started to become outmoded? However, the language can figure out that if you have an async lambda, you likely want it to return a Task. However, await operator is applicable to any async method with return type which differs from supported task types without limitations. This means that were really only timing the invocation of the async method up until the await, but not including the time to await the task or what comes after it. Sign up for a free GitHub account to open an issue and contact its maintainers and the community. Give feedback. So far, Ive shown two problems with blocking on async code: possible deadlocks and more-complicated error handling. Code Inspection: Avoid using 'async' lambda when delegate type returns 'void' Last modified: 28 December 2022 You can suppress this inspection to ignore specific issues, change its severity level to make the issues less or more noticeable, or disable it altogether. The first problem is task creation. It will immediately yield, returning an incomplete task, but when it resumes it will synchronously block whatever thread is running. Why is there a voltage on my HDMI and coaxial cables? Just because your code is asynchronous doesnt mean that its safe. Its possible to install a SynchronizationContext that detects when all async void methods have completed and collects any exceptions, but its much easier to just make the async void methods return Task instead. But if you have a method that is just a wrapper, then there's no need to await. As a general rule, async lambdas should only be used if they're converted to a delegate type that returns Task (for example, Func<Task>). The operand of the await operator is usually of one of the following .NET types: Task, Task<TResult . If you do that, you'll create an async void lambda. It looks like Resharper lost track here. StartNew accepts a Func and returns a Task. The problem is that, when passing async lambdas to methods that don't expect them, the compiler generates no warnings. It's essentially generating an async void method, IE: That makes sense, but I'm getting no warning. It will still run async so don't worry about having async in the razor calling code. async/await - when to return a Task vs void? Beginning with C# 9.0, you can use discards to specify two or more input parameters of a lambda expression that aren't used in the expression: Lambda discard parameters may be useful when you use a lambda expression to provide an event handler. This time, when the await completes, it attempts to execute the remainder of the async method within the thread pool context. (Yes, I'm aware that Foo can be refactored to accept a Func but this isn't always possible!). If you want to create a task wrapper for an existing asynchronous operation or event, use TaskCompletionSource. The guidelines are summarized in Figure 1; Ill discuss each in the following sections. It's not unexpected behaviour, because regular non-awaited calls behave much in the same way. I like the extension method, as you say, makes it clearer. Even though it's confusing in this context, what you're experiencing is by design: Specifically, an anonymous function F is compatible with a delegate type D provided: 3. Adds a bit of noise to the code, but fixes the warning (and presumably the underlying issue that comes with it). await DoSomething() .Match(x => OnSuccess(x), async ex => OnFailure(ex)); .where DoSomething returns a TryAsync and OnSuccess . Returning void from a calling method can, therefore, be a way of isolating the contagion, as it were. Both TPL Dataflow and Rx have async-ready methods and work well with asynchronous code. If the method doesnt have any awaits in it, or if all of the awaits in the method are on awaitables that are already completed by the time theyre awaited, then the method will run entirely synchronously. This can be beneficial to other community members reading this thread. From what I can tell from what you're sharing here, there's no reason for C# to have given you a warning before or after your refactoring because your code was valid C#. If that is the case, @Mister Magoo's answer is wrong, and I shouldn't have upvoted his answer. I'll open a bug report on the jetbrains tracker to get rid of the original warning which seems displayed by error. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. Because the function is asynchronous, you get this response as soon as the process has been started, instead of having to wait until the process has completed. The body of an expression lambda can consist of a method call. Short story taking place on a toroidal planet or moon involving flying, How to handle a hobby that makes income in US. To summarize this first guideline, you should prefer async Task to async void. A quick google search will tell you to avoid using async void myMethod () methods when possible. It seems counter-intuitive at first, but given that there are valid motivations behind it, and given that I was able to fix my issue, I'll rest my case. // or Since your actual code has an await in the lambda, there's warning. Shared resources still need to be protected, and this is complicated by the fact that you cant await from inside a lock. This code will work just fine in a console application but will deadlock when called from a GUI or ASP.NET context. The method is able to complete, which completes its returned task, and theres no deadlock. This inspection reports usages of void delegate types in the asynchronous context. CS4010 How to convert async lambda expression to delegate type 'TaskAction'. Async void methods have different composing semantics. First, avoid using async lambdas as arguments to methods that expect Action and don't provide an overload that expects a Func<Task>. You signed in with another tab or window. but this seems odd. ASP.Net Core - debbuger starts Chrome, but doesn't go to application URL, input text value: revert to previous value, Swagger UI on '.net Core hosted' Blazor WASM solution Web API project, What does IIS do when \\?\c:\filename instead of pulling an actual path, 'IApplicationBuilder' does not contain a definition for 'UseWebAssemblyDebugging', Dynamically set the culture by user preference does not work, Get Data From external API with Blazor WASM, DataAnnotationsValidator not working for Composite model in Blazor, Getting error in RenderFragment in a template grid component in ASP.NET BLAZOR Server, How to call child component method from parent component with foreach. As asynchronous GUI applications grow larger, you might find many small parts of async methods all using the GUI thread as their context. The warning had to do with the original example you gave. Figure 2 illustrates that exceptions thrown from async void methods cant be caught naturally. You can use the await operator only in a method, lambda expression, or anonymous method that is modified by the async keyword. Alternatively, AsyncEx provides AsyncCollection, which is an async version of BlockingCollection. Figure 8 shows a minor modification of Figure 7. Ill explain the reasoning behind each guideline so that its clear when it does and does not apply. That means that this call to StartNew is actually returning a Task>. A place where magic is studied and practiced? Stephen Toub works on the Visual Studio team at Microsoft. Second implementation of async task without await. However, when you synchronously block on a Task using Task.Wait or Task.Result, all of the exceptions are wrapped in an AggregateException and thrown. If the method doesn't have any awaits in it, or if all of the awaits in the method are on awaitables that are already completed by the time they're awaited, then the method will run entirely synchronously. Is there an easier way to determine that a Blazor App (PWA) has an update available? In these cases, the delegate for the lambda method should always have the return type Task or Task<T>. It's safe to use this method in a synchronous context, for example. Its easy to start several async void methods, but its not easy to determine when theyve finished. @G3Kappa The warning associated with your original example had to do with the fact that you had an async method with no await -- method referring to the lambda rather than Foo. Figure 4 demonstrates this exception to the guideline: The Main method for a console application is one of the few situations where code may block on an asynchronous method. When the man enquired what the turtle was standing on, the lady replied, Youre very clever, young man, but its turtles all the way down! As you convert synchronous code to asynchronous code, youll find that it works best if asynchronous code calls and is called by other asynchronous codeall the way down (or up, if you prefer). The exception to this guideline is the Main method for console applications, orif youre an advanced usermanaging a partially asynchronous codebase. Over in the property page for that control, click on the lightning-bolt icon to list all of the events that are sourced by that control. The following example produces a sequence that contains all elements in the numbers array that precede the 9, because that's the first number in the sequence that doesn't meet the condition: The following example specifies multiple input parameters by enclosing them in parentheses. In addition, there is msdn example, but it is a little bit more verbose: And now shortened code looks like your code. From the C# reference on Async Return Types, Async methods can have the following return types: Task<TResult>, for an async method that returns a value. And in many cases there are ways to make it possible. Unfortunately, they run into problems with deadlocks. Any lambda expression can be converted to a delegate type. The root cause of this deadlock is due to the way await handles contexts. Why is my Blazor Server App waiting to render until data has been retrieved, even when using async? Our Time method accepts an Action, so the compiler is going to map our async () => { } to being a void-returning async method, and the Action passed into the Time method will be for that void method. It's a blazor WASM project with .net 6. When calling functions from razor don't call Task functions. The question is about Resharper, not all arguments can be auto-filled. You can provide a tuple as an argument to a lambda expression, and your lambda expression can also return a tuple. Within AWS Lambda, functions invoked synchronously and asynchronously are . Disconnect between goals and daily tasksIs it me, or the industry? This inspection reports usages of void delegate types in the asynchronous context. . That is different than methods and local functions. When converting from synchronous to asynchronous code, any method returning a type T becomes an async method returning Task, and any method returning void becomes an async method returning Task. [], The design is a little wordy (as to be expected), but basically any lambda (async or not) will implicitly convert to a delegate with a void return type. In fact, I discovered this due to the DbContext concurrency issues that arose while debugging an ASP.NET application. await Task.Delay(1000); Because there are valid reasons for async void methods, Code analysis won't flag them. Anyone able to advise what is the best way to do this? RunThisAction(() => Console.WriteLine("Test")); RunThisAction(async () => await Task.Delay(1000)); These days theres a wealth of information about the new async and await support in the Microsoft .NET Framework 4.5. This is in part due to the fact that async methods that return Task are "contagious", such that their calling methods' often must also become async. So it will prefer that. { Figure 10 SemaphoreSlim Permits Asynchronous Synchronization. protected virtual async Task Foo(int id, Func beforeCommit), and I've made sure to await beforeCommit, but either way, there were no warnings whatsoever that prompted me to do this and happening upon the fix was rather serendipitous. By clicking Sign up for GitHub, you agree to our terms of service and I would still always use the short form though. Even if youre writing an ASP.NET application, if you have a core library thats potentially shared with desktop applications, consider using ConfigureAwait in the library code. The second Warnings comes from the fact that non-Action overloads of Match are marked as Pure, so you should do something with its return value. Staging Ground Beta 1 Recap, and Reviewers needed for Beta 2, Why must a lambda expression be cast when supplied as a plain Delegate parameter, convert a list of objects from one type to another using lambda expression, HttpClient.GetAsync() never returns when using await/async. Async Task methods enable easier error-handling, composability and testability. When you don't need any argument or when Blazor can auto add it then you can follow @MisterMagoo's answer. However, some semantics of an async void method are subtly different than the semantics of an async Task or async Task method. but using it in an asynchronous context, for example. We have 7 rules for async programming (so no, it does not cover all the uses cases you described): - S3168 - "async" methods should not return "void". . Is there a way to update a binding variable attached to an Input text Item in Blazor when using Ctrl +V combination keys? Here is an example: suppose we decided to expand the lambda to throw an exception: Because our doSomething delegate is void, the exception will never affect the caller thread and will not be caught with catch. Avoid using 'async' lambda when delegate type returns 'void', https://www.jetbrains.com/help/resharper/AsyncVoidLambda.html. This discussion was converted from issue #965 on December 15, 2021 10:43. Most methods today that accept as a parameter a delegate that returns void (e.g. A statement lambda resembles an expression lambda except that its statements are enclosed in braces: The body of a statement lambda can consist of any number of statements; however, in practice there are typically no more than two or three. Func delegates are useful for encapsulating user-defined expressions that are applied to each element in a set of source data. TPL Dataflow provides a BufferBlock that acts like an async-ready producer/consumer queue. We can fix this by modifying our Time function to accept a Func instead of an Action: public static double Time(Func func, int iters=10) { var sw = Stopwatch.StartNew(); for (int i = 0; i < iters; i++) func().Wait(); return sw.Elapsed.TotalSeconds / iters; }. Async void methods are difficult to test. Asking for help, clarification, or responding to other answers. Asynchronous code is often used to initialize a resource thats then cached and shared. Figure 9 is a quick reference of solutions to common problems. Making statements based on opinion; back them up with references or personal experience. Note that console applications dont cause this deadlock. When calling functions from razor don't call Task functions. Unbound breakpoints when debugging in Blazor Webassembly when using certain attributes/classes, Blazor InputText call async Method when TextChanged, Blazor Client side get CORS error when accessing Azure Function using Azure Active directory, Object reference not set when using keypress to trigger a button in Blazor. However, it's sometimes convenient to speak informally of the "type" of a lambda expression. UI Doesn't Hold Checkbox Value Of Selected Item In Blazor, Differences between Program.cs and App.razor, I can not use a C# class in a .razor page, in a blazor server application, Get value of input field in table row on button click in Blazor. Every Task will store a list of exceptions. In the following example, the lambda expression x => x * x, which specifies a parameter that's named x and returns the value of x squared, is assigned to a variable of a delegate type: Expression lambdas can also be converted to the expression tree types, as the following example shows: You can use lambda expressions in any code that requires instances of delegate types or expression trees, for example as an argument to the Task.Run(Action) method to pass the code that should be executed in the background. If a lambda expression doesn't return a value, it can be converted to one of the Action delegate types; otherwise, it can be converted to one of the Func delegate types. There are exceptions to each of these guidelines. That is true. How do I perform CRUD operations on the current authenticated users account information, in Blazor WASM? Within an async method, you can't use the await operator in the body of a synchronous function, inside the block of a lock statement, and in an unsafe context.. Continue with Recommended Cookies. In the case of a void method, though, no handle is handed back. A lambda expression with an expression on the right side of the => operator is called an expression lambda. Would you be able to take a look and see what I did wrong? The lambda must contain the same number of parameters as the delegate type. You can add the same event handler by using an async lambda. This inspection reports usages of void delegate types in the asynchronous context. When you specify an Expression argument, the lambda is compiled to an expression tree. The only reason it is considered async Task here is because Task.Run has an overload for Func. @StanJav Ooh, I didn't realise it was part of the library (obvious really, it's too useful to have been missed!). The MSTest asynchronous testing support only works for async methods returning Task or Task. This context is the current SynchronizationContext unless its null, in which case its the current TaskScheduler. To solve this problem, the SemaphoreSlim class was augmented with the async-ready WaitAsync overloads. That makes the two Select calls to look similar although in fact the type of objects created from the lambdas is different. Any lambda expression can be converted to a delegate type. Should all work - it is just a matter of your preference for style. For backwards compatibility, if only a single input parameter is named _, then, within a lambda expression, _ is treated as the name of that parameter. Huh? To learn more, see our tips on writing great answers. What is the point of Thrower's Bandolier? Obviously, an async method can create a task, and thats the easiest option. C# allows you to define async delegates or lambdas and use them in contexts that accept void-returning delegates, thus creating an async void method such as is forbidden by VSTHRD100, but is much harder to catch when simply looking at the code because for the same syntax, the C# compiler will create an async Func<Task> delegate or an async void . When you await a Task, the first exception is re-thrown, so you can catch the specific exception type (such as InvalidOperationException). For more information about C# tuples, see Tuple types. StartNew will then complete the Task> that it handed back, since the delegate associated with that task has completed its synchronous execution. This context behavior can also cause another problemone of performance. Reload the page to restore functionality header. Otherwise, it synthesizes a delegate type. For ASP.NET apps, this includes any code that uses HttpContext.Current or builds an ASP.NET response, including return statements in controller actions. I can summarize it like this: It generates compiler warnings; If an exception is uncaught there, your application is dead; You won't probably have a proper call stack to debug with To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Refer again to Figure 4. async/await - when to return a Task vs void? And in many cases there are ways to make it possible. However, when the method encounters the first await that yields, the async method returns. : Task LogicMethodAsync (int id) { return _dataAcess.DoActionAsync (id) } Task.Run ( async ()=> await Task.Delay (1000)); Is there a proper earth ground point in this switch box? Is there a compelling reason for this or was it just an oversight? You should not use ConfigureAwait when you have code after the await in the method that needs the context. If that method never uses await (or you do but whatever you await is already completed) then the method will execute synchronously. asp.net web api6.2 asp.net web apijsonxml!"" For example, consider the following declaration: The compiler can infer parse to be a Func.

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