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

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. When the return type is Task, the caller knows its dealing with a future operation; when the return type is void, the caller might assume the method is complete by the time it returns. 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 ). Thanks also for the explanation about the pure warning. Now with that background, consider whats happening with our timing function. 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the example given in the C# language reference, the newer language features are in separate documents, woefully out-of-date annotated version of the C# 4 spec. Thanks to the following technical expert for reviewing this article: Stephen Toub . 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. Finally, some async-ready data structures are sometimes needed. Beginning with C# 10, a lambda expression may have a natural type. Short story taking place on a toroidal planet or moon involving flying, How to handle a hobby that makes income in US. This behavior is inherent in all types of asynchronous programming, not just the new async/await keywords. The root cause of this deadlock is due to the way await handles contexts. In the case of a void method, though, no handle is handed back. But if you use Reactive Extensions, there's an even better approach that I've written about before, Observable.FromEventPattern. @CK-LinoPro and @StanJav I have come across a similar issue, which I explained in a new discussion (as it's not quite the same as this one). The differences in semantics make sense for asynchronous event handlers. this is still async and awaitable, just with a little less overhead. 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. i.e. 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. This allows you to easily get a delegate to represent an asynchronous operation, e.g. You can also use lambda expressions when you write LINQ in C#, as the following example shows: When you use method-based syntax to call the Enumerable.Select method in the System.Linq.Enumerable class, for example in LINQ to Objects and LINQ to XML, the parameter is a delegate type System.Func. If you are using .NET asynchronous programming, the return type can be Task and Task<T> types and use async and await keywords. @StanJav Ooh, I didn't realise it was part of the library (obvious really, it's too useful to have been missed!). EDIT: The example I provided is wrong, as my problematic Foo implementation actually returns a Task. When you invoke an async method, it starts running synchronously. 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 code is GPL licensed, can I issue a license to have my code be distributed in a specific MIT licensed project? 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. When calling functions from razor don't call Task functions. They have a thread pool SynchronizationContext instead of a one-chunk-at-a-time SynchronizationContext, so when the await completes, it schedules the remainder of the async method on a thread pool thread. Recall that the context is captured only if an incomplete Task is awaited; if the Task is already complete, then the context isnt captured. The lambda must contain the same number of parameters as the delegate type. Making statements based on opinion; back them up with references or personal experience. For example, consider the following declaration: The compiler can infer parse to be a Func. This is very powerful, but it can also lead to subtle bugs if youre not careful. Void-returning methods arent the only potentially problematic area; theyre just the easiest example to highlight, because its very clear from the signature that they dont return anything and thus are only useful for their side-effects, which means that code invoking them typically needs them to run to completion before making forward progress (since it likely depends on those side-effects having taken place), and async void methods defy that. Oh, I see And now I understand the reasoning behind it. The await operator can be used for each call and the method returns Task, which allows you to wait for the calls of individual asynchronous lambda methods. Specify zero input parameters with empty parentheses: If a lambda expression has only one input parameter, parentheses are optional: Two or more input parameters are separated by commas: Sometimes the compiler can't infer the types of input parameters. What is the point of Thrower's Bandolier? Yup, the example given in the C# language reference is even using it for exactly that. 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. In such cases, the return type may be set to void. For backwards compatibility, if only a single input parameter is named _, then, within a lambda expression, _ is treated as the name of that parameter. return "OK"; As a simple example, consider a timing helper function, whose job it is to time how long a particular piece of code takes to execute: public static double Time(Action action, int iters=10) { var sw = Stopwatch.StartNew(); for(int i=0; i> that it handed back, since the delegate associated with that task has completed its synchronous execution. Makes sense. And it might just stop that false warning, I can't check now. So it will prefer that. When the await completes, it attempts to execute the remainder of the async method within the captured context. Tasks are great, but they can only return one object and only complete once. Variables that are captured in this manner are stored for use in the lambda expression even if the variables would otherwise go out of scope and be garbage collected. And in many cases there are ways to make it possible. The Task-based Async Pattern (TAP) isnt just about asynchronous operations that you initiate and then asynchronously wait for to complete. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, In addition, there is msdn example, but it is a little bit more verbose, How Intuit democratizes AI development across teams through reusability. 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. There isnt a built-in type for this, but Stephen Toub developed an AsyncLazy that acts like a merge of Task and Lazy. // or 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. As a general rule, async lambdas should only be used if theyre converted to a delegate type that returns Task (for example, Func). but using it in an asynchronous context, for example. After answering many async-related questions on the MSDN forums, Stack Overflow and e-mail, I can say this is by far the most-asked question by async newcomers once they learn the basics: Why does my partially async code deadlock?. RunThisAction(() => Console.WriteLine("Test")); RunThisAction(async () => await Task.Delay(1000)); Give feedback. One consequence of this decision is that the System.Diagnostics.ConditionalAttribute cannot be applied to a lambda expression. Consider the following declaration: The compiler can't infer a parameter type for s. When the compiler can't infer a natural type, you must declare the type: Typically, the return type of a lambda expression is obvious and inferred. Disconnect between goals and daily tasksIs it me, or the industry? It's essentially generating an async void method, IE: Also in your specific example you should be getting a warning: warning CS1998: This async method lacks 'await' operators and will run synchronously. Beta }. RunThisAction(async delegate { await Task.Delay(1000); }); RunThisAction(async () => Figure 1 Summary of Asynchronous Programming Guidelines. { Suppose I have code like this. 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. Is there a proper earth ground point in this switch box? Trying to understand how to get this basic Fourier Series. 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>). Variables introduced within a lambda expression aren't visible in the enclosing method. Continue with Recommended Cookies. It is possible to have an event handler that returns some actual type, but that doesn't work well with the language; invoking an event handler that returns a type is very awkward, and the notion of an event handler actually returning something doesn't make much sense. 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. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. Asking for help, clarification, or responding to other answers. Both TPL Dataflow and Rx have async-ready methods and work well with asynchronous code. Whether turtles or zombies, its definitely true that asynchronous code tends to drive surrounding code to also be asynchronous. How would I run an async Task method synchronously? Event handlers naturally return void, so async methods return void so that you can have an asynchronous event handler. Manage Settings The table above ignores async void methods, which you should be avoiding anyway.Async void methods are tricky because you can assign a lambda like async => { await Task.Yield(); } to a variable of type Action, even though the natural type of that lambda is Func<Task>.Stephen Toub has written more about the pitfalls of async void lambdas.. As a closing note, the C# compiler has been updated in . It will immediately yield, returning an incomplete task, but when it resumes it will synchronously block whatever thread is running. It will still run async so don't worry about having async in the razor calling code. When you don't need any argument or when Blazor can auto add it then you can follow @MisterMagoo's answer. Thank you! doSomething(); The body of an expression lambda can consist of a method call. 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. An expression lambda returns the result of the expression and takes the following basic form: C#. 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. Stephen Toub works on the Visual Studio team at Microsoft. The return type of the delegate representing lambda function should have one of the following return types: Task; Task<T> . For some expressions that doesn't work: Beginning with C# 10, you can specify the return type of a lambda expression before the input parameters. With this function, if I then run the following code: static void Main() { double secs = Time(() => { Thread.Sleep(1000); }); Console.WriteLine(Seconds: {0:F7}, secs); }. Async void methods will notify their SynchronizationContext when they start and finish, but a custom SynchronizationContext is a complex solution for regular application code. : Task LogicMethodAsync (int id) { return _dataAcess.DoActionAsync (id) } c# blazor avoid using 'async' lambda when delegate type returns 'void', How Intuit democratizes AI development across teams through reusability. 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: Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. It seems to me that, in this case, the callback is not awaited, and it just runs in a separate thread. 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. Async Task methods enable easier error-handling, composability and testability. It will still run async so don't worry about having async in the razor calling 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. He has worked with multithreading and asynchronous programming for 16 years and has used async support in the Microsoft .NET Framework since the first CTP. . Huh? For example, a lambda expression that has two parameters and returns no value can be converted to an Action delegate. As far as async/await keywords it depends. 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. You can provide a tuple as an argument to a lambda expression, and your lambda expression can also return a tuple. Is there an easier way to determine that a Blazor App (PWA) has an update available? I get the following warning in JetBrains Rider and I can't find a way to workaround it. If you're querying an IEnumerable, then the input variable is inferred to be a Customer object, which means you have access to its methods and properties: The general rules for type inference for lambdas are as follows: A lambda expression in itself doesn't have a type because the common type system has no intrinsic concept of "lambda expression." Consider this simple example: This method isnt fully asynchronous. . 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. 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. Asynchronous code works best if it doesnt synchronously block. One of the really useful capabilities of the new async methods feature in C# and Visual Basic is the ability to write async lambdas and anonymous methods (from here on in this post, Ill refer to both of these as async lambdas, since the discussion applies equally to both). Mixed async and blocking code can cause deadlocks, more-complex error handling and unexpected blocking of context threads. You can always hover over the method name (like the Run in Task.Run) and Visual Studio will tell you which overload it has inferred: Yeah, it is evaluated to async Task because Task.Delay(n) has return type of Task. This exception includes methods that are logically event handlers even if theyre not literally event handlers (for example, ICommand.Execute implementations). Get only the string of the error from ValidationMessage in blazor? 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. Try to create a barrier in your code between the context-sensitive code and context-free code, and minimize the context-sensitive code. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. In Figure 8, I recommend putting all the core logic of the event handler within a testable and context-free async Task method, leaving only the minimal code in the context-sensitive event handler. Consider applying the 'await' operator to the result of the call." privacy statement. The method returns all the elements in the numbers array until it finds a number whose value is less than its ordinal position in the array: You don't use lambda expressions directly in query expressions, but you can use them in method calls within query expressions, as the following example shows: When writing lambdas, you often don't have to specify a type for the input parameters because the compiler can infer the type based on the lambda body, the parameter types, and other factors as described in the C# language specification. @PathogenDavid I'm saying that I'm getting no warning at all, not now nor before the refactoring, I think you misunderstood me. 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. 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. The problem statement here is that an async method returns a Task that never completes. How do I avoid using a client secret or certificate for Blazor Server when using MSAL? You are correct to return a Task from this method. Match ( Succ: _ => Foo (), Fail: _ => Bar ()); Also, avoid using async without await. 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. For more information, see Using async in C# functions with Lambda. 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". Since your actual code has an await in the lambda, there's warning. 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. This can be beneficial to other community members reading this thread. This code will work just fine in a console application but will deadlock when called from a GUI or ASP.NET context. Avoid event delegate recreation for async methods, When using Blazor WebAssembly with Azure Function in "local mode" accessed via Http.GetStringAsync using IP I get an "Failed to fetch error", Blazor - When to use Async life cycle methods, Blazor await JSRuntime.InvokeAsync capturing image src in C# returns null when I can observe in JS value being captured, NullReferenceException on page initialization if I use OnInitializedAsync method. Console applications cant follow this solution fully because the Main method cant be async. For example, the delegate type is synthesized if the lambda expression has ref parameters. However, some semantics of an async void method are subtly different than the semantics of an async Task or async Task method. . If you're gonna go all-in on reading the spec, I should point out that the newer language features are in separate documents. For asynchronous streams, you can use either TPL Dataflow or Reactive Extensions (Rx). Some events also assume that their handlers are complete when they return. doSomething(); An outer variable must be definitely assigned before it can be consumed in a lambda expression. The method is able to complete, which completes its returned task, and theres no deadlock. Unfortunately, they run into problems with deadlocks. In C#6, it can also be an extension method. You can, however, define a tuple with named components, as the following example does. Lambda expressions are invoked through the underlying delegate type. An example of data being processed may be a unique identifier stored in a cookie. LINQ to Objects, among other implementations, has an input parameter whose type is one of the Func family of generic delegates. Instead of forcing you to declare a delegate type, such as Func<> or Action<> for a lambda expression, the compiler may infer the delegate type from the lambda expression. But what is the best practice here to fix this? The documentation for expression lambdas says, An expression lambda returns the result of the expression. Figure 3 shows a simple example where one method blocks on the result of an async method. Theres a lot to learn about async and await, and its natural to get a little disoriented. But if you have a method that is just a wrapper, then there's no need to await. Figure 7 Having an Async Event Handler Disable and Re-Enable Its Control. To summarize this first guideline, you should prefer async Task to async void. WriteLine ("Item added with instance add method: "+ item);} public IEnumerator GetEnumerator {// Some implementation . For example, the following Windows Forms example contains an event handler that calls and awaits an async method, ExampleMethodAsync.