avoid using async lambda when delegate type returns void avoid using async lambda when delegate type returns void

As long as ValidateFieldAsync () still returns async Task this is still async and awaitable, just with a little less overhead. EDIT: The example I provided is wrong, as my problematic Foo implementation actually returns a Task. Apparently it can't 'predict' the code generated by Razor. He specializes in areas related to parallelism and asynchrony. Ill explain the error-handling problem now and show how to avoid the deadlock problem later in this article. Both TPL Dataflow and Rx have async-ready methods and work well with asynchronous code. The method is able to complete, which completes its returned task, and theres no deadlock. Every Task will store a list of exceptions. Removing async void | John Thiriet You can't use statement lambdas to create expression trees. Func> getContentsLowerCaseAsync = async url => { string contents = await DownloadString(url); return contents.ToLower(); }; Async methods in C# and Visual Basic can return void, Task, or Task, which means they can be mapped to delegates that return void, Task, or Task. My code is GPL licensed, can I issue a license to have my code be distributed in a specific MIT licensed project? Now when I compile and run our async lambda, I get the following output thats what Id expect: Seconds: 1.0078671 Press any key to continue . c# blazor avoid using 'async' lambda when delegate type returns 'void', Blazor Reusable RenderFragments in code with event : Cannot convert lambda expression to intended delegate type, Using the Blazor InputFile tag- how can I control the file type shown when I browse. Figure 8 shows a minor modification of Figure 7. c# blazor avoid using 'async' lambda when delegate type returns 'void' 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. As far as I know, that warning means that if anything throws an exception in the async OnFailure method, the exception won't be caught, as it will be in the returned Task that isn't handled, as the compiler is assuming the failure lambda is void. LINQ to Objects, among other implementations, has an input parameter whose type is one of the Func family of generic delegates. In fact, I discovered this due to the DbContext concurrency issues that arose while debugging an ASP.NET application. Because there are valid reasons for async void methods, Code analysis won't flag them. If you can use ConfigureAwait at some point within a method, then I recommend you use it for every await in that method after that point. Returning void from a calling method can, therefore, be a way of isolating the contagion, as it were. The following code snippet illustrates a synchronous void-returning method and its asynchronous equivalent: Void-returning async methods have a specific purpose: to make asynchronous event handlers possible. Ordinarily, the fields of a tuple are named Item1, Item2, and so on. These outer variables are the variables that are in scope in the method that defines the lambda expression, or in scope in the type that contains the lambda expression. My question is basically an offshoot of this best practice: What does the lambda expression below evaluate to? but using it in an asynchronous context, for example. Any lambda expression can be converted to a delegate type. It's a blazor WASM project with .net 6. 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. From the POV of the library maintainer, there's no reason to believe that callback wouldn't block. There are a few techniques for incrementally converting a large codebase to async code, but theyre outside the scope of this article. How to use Slater Type Orbitals as a basis functions in matrix method correctly? Thanks also for the explanation about the pure warning. EditContext OnFieldChanged reporting wrong return type. Since your actual code has an await in the lambda, there's warning. // or throw new NotImplementedException(); If the body of F is an expression, and either D has a void return type or F is async and D has the return type Task, then when each parameter of F is given the type of the corresponding parameter in D, the body of F is a valid expression (wrt Expressions) that would be permitted as a statement_expression ( Expression statements ). You can use them to keep code concise, and to capture closures, in exactly the same way you would in non-async code. Code Inspection: Avoid using 'async' lambda when delegate type returns How do I avoid "Avoid using 'async' lambdas when delegate return type is void" when the success delegate is sync? By default, when an incomplete Task is awaited, the current context is captured and used to resume the method when the Task completes. If this method is called from a GUI context, it will block the GUI thread; if its called from an ASP.NET request context, it will block the current ASP.NET request thread. }. For example, Func defines a delegate with two input parameters, int and string, and a return type of bool. How to inject Blazor-WebAssembly-app extension-UI in webpage. Get only the string of the error from ValidationMessage in blazor? This is bad advice - you should only use async void for an EventHandler - all Blazor EventCallbacks should return a Task when they are asynchronous. I would still always use the short form though. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. Async Task methods enable easier error-handling, composability and testability. 3. 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. One consequence of this decision is that the System.Diagnostics.ConditionalAttribute cannot be applied to a lambda expression. Otherwise, it synthesizes a delegate type. Anyway to avoid making a whole chain of methods to async methods? This discussion was converted from issue #965 on December 15, 2021 10:43. Asynchronous code works best if it doesnt synchronously block. The return type of the delegate representing lambda function should have one of the following return types: Task; Task<T> . What is the point of Thrower's Bandolier? @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. So far, Ive shown two problems with blocking on async code: possible deadlocks and more-complicated error handling. In my last post, I discussed building an asynchronous version of a manual-reset event. Match ( Succ: _ => Foo (), Fail: _ => Bar ()); Also, avoid using async without await. To summarize this third guideline, you should use ConfigureAwait when possible. Task, for an async method that performs an operation but returns no value. Have a question about this project? You can specify the types explicitly as shown in the following example: Input parameter types must be all explicit or all implicit; otherwise, a CS0748 compiler error occurs. References. Its usually wrong to provide an async implementation (or override) of a void-returning method on an interface (or base class). The following example shows how to add attributes to a lambda expression: You can also add attributes to the input parameters or return value, as the following example shows: As the preceding examples show, you must parenthesize the input parameters when you add attributes to a lambda expression or its parameters. When I run this, I see the following written out to the console: Seconds: 0.0000341 Press any key to continue . To summarize this first guideline, you should prefer async Task to async void. I was looking for it as an extension method, not a standalone method (I know, I should read people's replies more carefully!). Allowing async to grow through the codebase is the best solution, but this means theres a lot of initial work for an application to see real benefit from async code. 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. Second implementation of async task without await. Action, Action, etc.) This is behavior is typically due to one of two things, or variations off of these: The exception to this guideline is asynchronous event handlers, which must return void. Anyone able to advise what is the best way to do this? However, if you're creating expression trees that are evaluated outside the context of the .NET Common Language Runtime (CLR), such as in SQL Server, you shouldn't use method calls in lambda expressions. Rx is more powerful and efficient but has a more difficult learning curve. Also if you like reading on dead trees, there's a woefully out-of-date annotated version of the C# 4 spec you might be able to find used. Whether turtles or zombies, its definitely true that asynchronous code tends to drive surrounding code to also be asynchronous. One thing you could do, if your return value is Unit and you're using your Match call for impure code, is to write _ = await /* */ to tell the analyzer explicitly that you don't care about the return value. Most methods today that accept as a parameter a delegate that returns void (e.g. MSB4018 The "GenerateServiceWorkerAssetsManifest" task failed unexpectedly, Unable to determine the desired template from the input template name: blazorserverside, Blazor error: The hash algorithm must be one of 'sha256', 'sha384', or 'sha512', followed by a '-' character. This doesn't match the current behaviour for non-awaited async method calls, which correctly generate a CS4014 warning. To solve this problem, the SemaphoreSlim class was augmented with the async-ready WaitAsync overloads. An example of data being processed may be a unique identifier stored in a cookie. The warning had to do with the original example you gave. The guidelines are summarized in Figure 1; Ill discuss each in the following sections. Lambda expressions - Lambda expressions and anonymous functions 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. Consider applying the 'await' operator to the result of the call." You should not use ConfigureAwait when you have code after the await in the method that needs the context. beforeCommit was being called like a normal action in-between two other asynchronous functions. All rights reserved. Should all work - it is just a matter of your preference for style. Figure 5 is a cheat sheet of async replacements for synchronous operations. A more complicated but still problematic example is a generic method that accepts an Action as a parameter and returns a Task, or that accepts a Func<,TResult> as a parameter and returns a Task, such as Task.Factory.StartNew. This inspection reports usages of void delegate types in the asynchronous context. Manage Settings It looks like Resharper lost track here. For example, this produces no error and the lambda is treated as async void: That is different than if you passed it a named async Task method, which would cause a compiler error: So be careful where you use it. 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. Adds a bit of noise to the code, but fixes the warning (and presumably the underlying issue that comes with it). StartNew accepts a Func and returns a Task. The following example uses the Count standard query operator: The compiler can infer the type of the input parameter, or you can also specify it explicitly. 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#. To learn more, see our tips on writing great answers. Some of our partners may process your data as a part of their legitimate business interest without asking for consent. Just in case you haven't seen it, there is Unit ignore(A anything) => unit; also in this library. As for why this is possible (or async void exists at all) was to enable using async method with existing event handlers and calling back interfaces. How would I run an async Task method synchronously? This article is intended as a second step in learning asynchronous programming; I assume that youve read at least one introductory article about it. Not the answer you're looking for? For asynchronous streams, you can use either TPL Dataflow or Reactive Extensions (Rx). 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. 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 . An outer variable must be definitely assigned before it can be consumed in a lambda expression. 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. In C#6, it can also be an extension method. Stephen Clearyis a husband, father and programmer living in northern Michigan. However, some semantics of an async void method are subtly different than the semantics of an async Task or async Task method. This technique is particularly useful if you need to gradually convert an application from synchronous to asynchronous. Consider this simple example: This method isnt fully asynchronous. Error handling is much easier to deal with when you dont have an AggregateException, so I put the global try/catch in MainAsync. The task created by StartNew will invoke the Func>, which will run synchronously until the first await that yields, at which point the Func> will return, handing back the result Task that represents the async lambdas execution. This inspection reports usages of void delegate types in the asynchronous context. Was this translation helpful? Heres an example of async code that can corrupt shared state if it executes twice, even if it always runs on the same thread: The problem is that the method reads the value and suspends itself at the await, and when the method resumes it assumes the value hasnt changed. - S4462 - Calls to "async" methods should not be blocking. Attributes on lambda expressions are useful for code analysis, and can be discovered via reflection. This behavior can be confusing, especially considering that stepping through the debugger implies that its the await that never completes. The following example demonstrates these rules: The following rules apply to variable scope in lambda expressions: Beginning with C# 9.0, you can apply the static modifier to a lambda expression to prevent unintentional capture of local variables or instance state by the lambda: A static lambda can't capture local variables or instance state from enclosing scopes, but may reference static members and constant definitions. Imagine you have an existing synchronous method that is called . Async Task methods enable easier error-handling, composability and testability. As a general rule, async lambdas should only be used if theyre converted to a delegate type that returns Task (for example, Func). AWS Lambda will send a response that the video encoding function has been invoked and started successfully. await Task.Delay(1000); Blazor the type or namespace name 'App' could not be found (are you missing a using directive or an assembly reference? The following Func delegate, when it's invoked, returns Boolean value that indicates whether the input parameter is equal to five: You can also supply a lambda expression when the argument type is an Expression, for example in the standard query operators that are defined in the Queryable type. A variable that is captured won't be garbage-collected until the delegate that references it becomes eligible for garbage collection. The compiler chooses an available Func or Action delegate, if a suitable one exists. This allows you to easily get a delegate to represent an asynchronous operation, e.g. However, when the method encounters the first await that yields, the async method returns. They raise their exceptions directly on the SynchronizationContext, which is similar to how synchronous event handlers behave. Huh? { The MSTest asynchronous testing support only works for async methods returning Task or Task. The core functionality of the MongoDB support can be used directly, with no need to invoke the IoC services of the Spring Container. : Task LogicMethodAsync (int id) { return _dataAcess.DoActionAsync (id) } @StanJav Hmm, just tried it, and it can't resolve the symbol ignore even though I have using static LanguageExt.Prelude, I'm trying this on the end of a call to TryAsync.Match(). Why are Suriname, Belize, and Guinea-Bissau classified as "Small Island Developing States"? The problem here is the same as with async void methods but it is much harder to spot. I realise now that in such a case I need to wrap the OnSuccess in Task.Run() to convince the compiler to call the overload I want. For this, you can use, for example, a type Func<Task, T> lambda. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Figure 8 Each Async Method Has Its Own Context. Thanks for contributing an answer to Stack Overflow! VSTHRD101 Avoid unsupported async delegates. Each async method has its own context, so if one async method calls another async method, their contexts are independent. To understand this effect, we need to remember how async methods operate. Asking for help, clarification, or responding to other answers. 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. Is equivalent to this, if you were to express it with a named method: But it is important to note that async lambdas can be inferred to be async void. Any lambda expression can be converted to a delegate type. Thank you! The only reason it is considered async Task here is because Task.Run has an overload for Func. rev2023.3.3.43278. Aside from performance, ConfigureAwait has another important aspect: It can avoid deadlocks. This time, when the await completes, it attempts to execute the remainder of the async method within the thread pool context. 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. My code is GPL licensed, can I issue a license to have my code be distributed in a specific MIT licensed project? AWS Lambda: Sync or Async? - Stackery It's safe to use this method in a synchronous context, for example. Figure 6 Handling a Returned Task that Completes Before Its Awaited. Figure 1 Summary of Asynchronous Programming Guidelines. EDIT: The example I provided is wrong, as my problematic Foo implementation actually returns a Task. return "OK"; }); suppress this inspection to ignore specific issues, change its severity level to make the issues less or more noticeable, Code Inspection: Heuristically unreachable switch arm due to integer analysis, Code Inspection: Use preferred namespace body style. Making statements based on opinion; back them up with references or personal experience. @PathogenDavid I'm saying that I'm getting no warning at all, not now nor before the refactoring, I think you misunderstood me. Suppose I have code like this. The operand of the await operator is usually of one of the following .NET types: Task, Task<TResult . A place where magic is studied and practiced? When an exception is thrown out of an async Task or async Task method, that exception is captured and placed on the Task object. For GUI apps, this includes any code that manipulates GUI elements, writes data-bound properties or depends on a GUI-specific type such as Dispatcher/CoreDispatcher. but this seems odd. The nature of simulating nature: A Q&A with IBM Quantum researcher Dr. Jamie We've added a "Necessary cookies only" option to the cookie consent popup. await Task.Delay(1000); this is still async and awaitable, just with a little less overhead. Is a PhD visitor considered as a visiting scholar? No CS4014 when passing an async lambda to a function that expects a synchronous function, the example given in the C# language reference, the newer language features are in separate documents, woefully out-of-date annotated version of the C# 4 spec. Lambda expressions are invoked through the underlying delegate type. await DoSomething() .Match(x => OnSuccess(x), async ex => OnFailure(ex)); .where DoSomething returns a TryAsync and OnSuccess . Wait()) or asynchronously (e.g. public class CollectionWithAdd: IEnumerable {public void Add < T >(T item) {Console. Yeah, sometimes stuff in the language can seem a bit strange, but there's usually a reason for it (that reason usually being legacy nonsense or it isn't strange when you consider other contexts.). This article just highlights a few best practices that can get lost in the avalanche of available documentation. In Dungeon World, is the Bard's Arcane Art subject to the same failure outcomes as other spells? Instead of void return type use Task or ValueTask. Theyre each waiting for the other, causing a deadlock. And in many cases there are ways to make it possible. The expression await Task.Delay(1000) doesn't really return anything in itself. 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. The root cause of this deadlock is due to the way await handles contexts. This is an especially common problem for programmers who are dipping their toes into asynchronous programming, converting just a small part of their application and wrapping it in a synchronous API so the rest of the application is isolated from the changes. These exceptions can be observed using AppDomain.UnhandledException or a similar catch-all event for GUI/ASP.NET applications, but using those events for regular exception handling is a recipe for unmaintainability. If you follow this solution, youll see async code expand to its entry point, usually an event handler or controller action. That is different than methods and local functions. The problem here is the same as with async void methods but it is much harder to spot. What is the difference between asynchronous programming and multithreading? Mixed async and blocking code can cause deadlocks, more-complex error handling and unexpected blocking of context threads. This can be beneficial to other community members reading this thread. When you don't need any argument or when Blazor can auto add it then you can follow @MisterMagoo's answer. I hope the guidelines and pointers in this article have been helpful. How do I avoid "Avoid using 'async' lambdas when delegate return type First, avoid using async lambdas as arguments to methods that expect Action and don't provide an overload that expects a Func<Task>. In these cases, the delegate for the lambda method should always have the return type Task or Task<T>. In particular, its usually a bad idea to block on async code by calling Task.Wait or Task.Result. Code Inspection: Avoid using 'async' lambda when delegate type returns This context behavior can also cause another problemone of performance. You use a lambda expression to create an anonymous function. How can this new ban on drag possibly be considered constitutional? avoid using 'async' lambda when delegate type returns 'void' With async void methods, there is no Task object, so any exceptions thrown out of an async void method will be raised directly on the SynchronizationContext that was active when the async void method started. For backwards compatibility, if only a single input parameter is named _, then, within a lambda expression, _ is treated as the name of that parameter. Thanks to the following technical expert for reviewing this article: Stephen Toub Call void functions because that is what is expected. These delegates use type parameters to define the number and type of input parameters, and the return type of the delegate. A place where magic is studied and practiced? Avoid async void methods | You've Been Haacked This context is the current SynchronizationContext unless its null, in which case its the current TaskScheduler. but using it in an asynchronous context, for example. For more information, see the Anonymous function expressions section of the C# language specification. And in many cases there are ways to make it possible. RunThisAction(() => Console.WriteLine("Test")); RunThisAction(async () => await Task.Delay(1000)); 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>. One subtle trap is passing an async lambda to a method taking an Action parameter; in this case, the async lambda returns void and inherits all the problems of async void methods. 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. An expression lambda returns the result of the expression and takes the following basic form: C#. 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. Because of the differences in error handling and composing, its difficult to write unit tests that call async void methods. Synchronous event handlers are usually private, so they cant be composed or directly tested. The problem here is the same as with async void Performance considerations for When this annotation is applied to the parameter of delegate type, IDE checks the input argument of this parameter: * When lambda expression or anonymous method is passed as an argument, IDE verifies that the passed We rely on the default exchange in the broker .