Windows 8 async await

Because LINQ uses deferred lazy execution, async calls won't happen immediately as they do in a foreach loop unless you force the generated sequence to iterate with a call to. ToList or. With async programming, there are some details to keep in mind that can prevent unexpected behavior.

This is important to keep in mind. If await is not used in the body of an async method, the C compiler generates a warning, but the code compiles and runs as if it were a normal method. This is incredibly inefficient, as the state machine generated by the C compiler for the async method is not accomplishing anything.

This is the convention used in. NET to more easily differentiate synchronous and asynchronous methods. Certain methods that aren't explicitly called by your code such as event handlers or web controller methods don't necessarily apply. Because they are not explicitly called by your code, being explicit about their naming isn't as important.

Any other use of async void does not follow the TAP model and can be challenging to use, such as:. Lambda expressions in LINQ use deferred execution, meaning code could end up executing at a time when you're not expecting it to.

The introduction of blocking tasks into this can easily result in a deadlock if not written correctly. Additionally, the nesting of asynchronous code like this can also make it more difficult to reason about the execution of the code. Async and LINQ are powerful but should be used together as carefully and clearly as possible. Blocking the current thread as a means to wait for a Task to complete can result in deadlocks and blocked context threads and can require more complex error-handling. The following table provides guidance on how to deal with waiting for tasks in a non-blocking way:.

Returning a Task object from async methods can introduce performance bottlenecks in certain paths. Task is a reference type, so using it means allocating an object. In cases where a method declared with the async modifier returns a cached result or completes synchronously, the extra allocations can become a significant time cost in performance critical sections of code.

It can become costly if those allocations occur in tight loops. For more information, see generalized async return types. A common question is, "when should I use the Task. ConfigureAwait Boolean method? The method allows for a Task instance to configure its awaiter. This is an important consideration and setting it incorrectly could potentially have performance implications and even deadlocks.

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. This topic provides an overview of when and how to use async programming and includes links to support topics that contain details and examples. Asynchrony is essential for activities that are potentially blocking, such as when your application accesses the web.

Access to a web resource sometimes is slow or delayed. If such an activity is blocked within 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 the. 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-based approach adds the equivalent of an automatic transmission to the list of options that you can choose from when designing asynchronous operations. That is, you get all the benefits of traditional asynchronous programming but with much less effort from the developer.

The Async and Await keywords in Visual Basic are the heart of async programming. By using those two keywords, you can use resources in the. NET Framework 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 Async and Await are referred to as async methods. The following example shows an async method. Almost everything in the code should look completely familiar to you.

The comments call out the features that you add to create the asynchrony. If AccessTheWebAsync doesn't have any work that it can do between calling GetStringAsync and awaiting its completion, you can simplify your code by calling and awaiting in the following single statement. The method usually includes at least one await expression, which marks a point where the method can't continue until the awaited asynchronous operation is complete. In the meantime, the method is suspended, and control returns to the method's caller.

The next section of this topic illustrates what happens at the suspension point. In async methods, you use the provided keywords and types to indicate what you want to do, and the compiler does the rest, including keeping track of what must happen when control returns to an await point in a suspended method. Some routine processes, such as loops and exception handling, can be difficult to handle in traditional asynchronous code. In an async method, you write these elements much as you would in a synchronous solution, and the problem is solved.

For more information about asynchrony in previous versions of the. 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:. Something happens in GetStringAsync that suspends its progress. Perhaps it must wait for a website to download or some other blocking activity. 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.

That work is represented by a call to the synchronous method DoIndependentWork. DoIndependentWork is a synchronous method that does its work and returns to its caller.

AccessTheWebAsync next wants to calculate and return the length of the downloaded string, but the method can't calculate that value until the method has the string. The task represents a promise to produce an integer result that's the length of the downloaded string. The expense of suspending and then returning to AccessTheWebAsync would be wasted if the called asynchronous process getStringTask has already completed and AccessTheWebSync doesn't have to wait for the final result.

Inside the caller the event handler in this example , the processing pattern continues. The caller might do other work that doesn't depend on the result from AccessTheWebAsync before awaiting that result, or the caller might await immediately. Ask a question. Quick access. Search related threads. Remove From My Forums. Answered by:. Archived Forums. Sign in to vote. User posted I have a website where we have created a client to consume responses from a web. Friday, May 5, PM. Best Regards, Yuk Ding.

Thursday, May 18, AM. User posted Hi fgormley, I think the limit of concurrent request with IIS lead to the failure of async await. Monday, May 8, AM. User posted Hi Yuk, Thanks for your reply. Monday, May 8, PM.