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Jan 22, 2013 · The async is used with a function to makes it into an asynchronous function. The await keyword is used to invoke an asynchronous function synchronously. The await keyword holds the JS engine execution until promise is resolved. We should use async & await only when we want the result immediately.
If your async method needs to return int you'd mark the return type of the method as Task<int> and you'll return plain int not the Task<int>. Compiler will convert the int to Task<int> for you. private async Task<int> MethodName() {. await SomethingAsync(); return 42;//Note we return int not Task<int> and that compiles.
- Overview
- Async improves responsiveness
- Async methods are easy to write
- What happens in an async method
- API async methods
- Threads
- async and await
- Return types and parameters
- Naming convention
- See also
You can avoid performance bottlenecks and enhance the overall responsiveness of your application by using asynchronous programming. However, traditional techniques for writing asynchronous applications can be complicated, making them difficult to write, debug, and maintain.
C# supports simplified approach, async programming, that leverages asynchronous support in the .NET runtime. The compiler does the difficult work that the developer used to do, and your application retains a logical structure that resembles synchronous code. As a result, you get all the advantages of asynchronous programming with a fraction of the effort.
Asynchrony is essential for activities that are potentially blocking, such as web access. Access to a web resource sometimes is slow or delayed. If such an activity is blocked in a synchronous process, the entire application must wait. In an asynchronous process, the application can continue with other work that doesn't depend on the web resource until the potentially blocking task finishes.
The following table shows typical areas where asynchronous programming improves responsiveness. The listed APIs from .NET and the Windows Runtime contain methods that support async programming.
Asynchrony proves especially valuable for applications that access the UI thread because all UI-related activity usually shares one thread. If any process is blocked in a synchronous application, all are blocked. Your application stops responding, and you might conclude that it has failed when instead it's just waiting.
When you use asynchronous methods, the application continues to respond to the UI. You can resize or minimize a window, for example, or you can close the application if you don't want to wait for it to finish.
The async and await keywords in C# are the heart of async programming. By using those two keywords, you can use resources in .NET Framework, .NET Core, or the Windows Runtime to create an asynchronous method almost as easily as you create a synchronous method. Asynchronous methods that you define by using the async keyword are referred to as async methods.
The following example shows an async method. Almost everything in the code should look familiar to you.
You can find a complete Windows Presentation Foundation (WPF) example available for download from Asynchronous programming with async and await in C#.
You can learn several practices from the preceding sample. Start with the method signature. It includes the async modifier. The return type is Task (See "Return Types" section for more options). The method name ends in Async. In the body of the method, GetStringAsync returns a Task . That means that when you await the task you'll get a string (contents). Before awaiting the task, you can do work that doesn't rely on the string from GetStringAsync.
Pay close attention to the await operator. It suspends GetUrlContentLengthAsync:
•GetUrlContentLengthAsync can't continue until getStringTask is complete.
The most important thing to understand in asynchronous programming is how the control flow moves from method to method. The following diagram leads you through the process:
The numbers in the diagram correspond to the following steps, initiated when a calling method calls the async method.
1.A calling method calls and awaits the GetUrlContentLengthAsync async method.
2.GetUrlContentLengthAsync creates an HttpClient instance and calls the GetStringAsync asynchronous method to download the contents of a website as a string.
3.Something happens in GetStringAsync that suspends its progress. Perhaps it must wait for a website to download or some other blocking activity. To avoid blocking resources, GetStringAsync yields control to its caller, GetUrlContentLengthAsync.
GetStringAsync returns a Task , where TResult is a string, and GetUrlContentLengthAsync assigns the task to the getStringTask variable. The task represents the ongoing process for the call to GetStringAsync, with a commitment to produce an actual string value when the work is complete.
You might be wondering where to find methods such as GetStringAsync that support async programming. .NET Framework 4.5 or higher and .NET Core contain many members that work with async and await. You can recognize them by the "Async" suffix that's appended to the member name, and by their return type of Task or Task . For example, the System.IO.Stream class contains methods such as CopyToAsync, ReadAsync, and WriteAsync alongside the synchronous methods CopyTo, Read, and Write.
The Windows Runtime also contains many methods that you can use with async and await in Windows apps. For more information, see Threading and async programming for UWP development, and Asynchronous programming (Windows Store apps) and Quickstart: Calling asynchronous APIs in C# or Visual Basic if you use earlier versions of the Windows Runtime.
Async methods are intended to be non-blocking operations. An await expression in an async method doesn't block the current thread while the awaited task is running. Instead, the expression signs up the rest of the method as a continuation and returns control to the caller of the async method.
The async and await keywords don't cause additional threads to be created. Async methods don't require multithreading because an async method doesn't run on its own thread. The method runs on the current synchronization context and uses time on the thread only when the method is active. You can use Task.Run to move CPU-bound work to a background thread, but a background thread doesn't help with a process that's just waiting for results to become available.
If you specify that a method is an async method by using the async modifier, you enable the following two capabilities.
•The marked async method can use await to designate suspension points. The await operator tells the compiler that the async method can't continue past that point until the awaited asynchronous process is complete. In the meantime, control returns to the caller of the async method.
The suspension of an async method at an await expression doesn't constitute an exit from the method, and finally blocks don't run.
•The marked async method can itself be awaited by methods that call it.
An async method typically contains one or more occurrences of an await operator, but the absence of await expressions doesn't cause a compiler error. If an async method doesn't use an await operator to mark a suspension point, the method executes as a synchronous method does, despite the async modifier. The compiler issues a warning for such methods.
async and await are contextual keywords. For more information and examples, see the following topics:
An async method typically returns a Task or a Task . Inside an async method, an await operator is applied to a task that's returned from a call to another async method.
You specify Task as the return type if the method contains a return statement that specifies an operand of type TResult.
You use Task as the return type if the method has no return statement or has a return statement that doesn't return an operand.
You can also specify any other return type, provided that the type includes a GetAwaiter method. ValueTask is an example of such a type. It is available in the System.Threading.Tasks.Extension NuGet package.
The following example shows how you declare and call a method that returns a Task or a Task:
Each returned task represents ongoing work. A task encapsulates information about the state of the asynchronous process and, eventually, either the final result from the process or the exception that the process raises if it doesn't succeed.
By convention, methods that return commonly awaitable types (for example, Task, Task , ValueTask, ValueTask ) should have names that end with "Async". Methods that start an asynchronous operation but do not return an awaitable type should not have names that end with "Async", but may start with "Begin", "Start", or some other verb to suggest this method does not return or throw the result of the operation.
You can ignore the convention where an event, base class, or interface contract suggests a different name. For example, you shouldn't rename common event handlers, such as OnButtonClick.
Sep 4, 2015 · However, some semantics of an async void method are subtly different than the semantics of an async Task or async Task<T> method. Async void methods have different error-handling semantics. When an exception is thrown out of an async Task or async Task<T> method, that exception is captured and placed on the Task object.
In this article. You can implement the Task-based Asynchronous Pattern (TAP) in three ways: by using the C# and Visual Basic compilers in Visual Studio, manually, or through a combination of the compiler and manual methods. The following sections discuss each method in detail.
Jan 28, 2022 · async, await, and Task. Use async along with await and Task if the async method returns a value back to the calling code. We used only the async keyword in the above program to demonstrate the simple asynchronous void method. The await keyword waits for the async method until it returns a value. So the main application thread stops there until ...
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Mar 16, 2023 · There are two await s in the async method: one for a Task<int> returned by ReadAsync, and one for a Task returned by WriteAsync. Task.GetAwaiter () returns a TaskAwaiter, and Task<TResult>.GetAwaiter () returns a TaskAwaiter<TResult>, both of which are distinct struct types.
