Inherits EventArgs. End Class. End Namespace. Public Class DelegatesModule. End NameSpace. IsCancellationRequested Then. End If. Dim operations As New Operations. AddHandler operations. If Not operations. FolderExists Then. ElseIf foundFiles. Show "No matches". Catch oce As OperationCanceledException. Show "Operation cancelled".
Catch ex As Exception. Show ex. End Try. Imports System. Namespace Extensions. Public Module StreamExtensions. Dim amountRead As Integer. ReadAsync buffer, amountRead, bufferSize - amountRead. Exit Do. Await destination. WriteAsync buffer, 0, amountRead. If progress IsNot Nothing Then. End Module. Imports CopyFileAsync. Public Class Form1. Handles CopyFileButton. 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.
GetStringAsync completes and produces a string result. The string result isn't returned by the call to GetStringAsync in the way that you might expect. Remember that the method already returned a task in step 3. Instead, the string result is stored in the task that represents the completion of the method, getStringTask. The await operator retrieves the result from getStringTask.
The assignment statement assigns the retrieved result to urlContents. When AccessTheWebAsync has the string result, the method can calculate the length of the string. Then the work of AccessTheWebAsync is also complete, and the waiting event handler can resume.
In the full example at the end of the topic, you can confirm that the event handler retrieves and prints the value of the length result. If you are new to asynchronous programming, take a minute to consider the difference between synchronous and asynchronous behavior.
A synchronous method returns when its work is complete step 5 , but an async method returns a task value when its work is suspended steps 3 and 6. When the async method eventually completes its work, the task is marked as completed and the result, if any, is stored in the task. You might be wondering where to find methods such as GetStringAsync that support async programming. You can recognize these members by the "Async" suffix that's attached to the member name and a return type of Task or Task Of TResult.
For example, the System. The Windows Runtime also contains many methods that you can use with Async and Await in Windows apps. 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 multi-threading because an async method doesn't run on its own thread. The original example has the statement Await sourceStream. WriteAsync encodedText, 0, encodedText. Length , which is a contraction of the following two statements:.
The first statement returns a task and causes file processing to start. The second statement with the await causes the method to immediately exit and return a different task. When the file processing later completes, execution returns to the statement that follows the await. The following example reads text from a file.
The text is buffered and, in this case, placed into a StringBuilder. Unlike in the previous example, the evaluation of the await produces a value. The following example demonstrates parallel processing by writing 10 text files. For each file, the WriteAsync method returns a task that is then added to a list of tasks. The Await Task. WhenAll tasks statement exits the method and resumes within the method when file processing is complete for all of the tasks.
The example closes all FileStream instances in a Finally block after the tasks are complete. If each FileStream was instead created in a Imports statement, the FileStream might be disposed of before the task was complete.
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