Asynchronous Cancellation Patterns with .NET 4


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For calling methods asynchronously, since .NET 1.0 the async pattern can be used. .NET 2.0 added the event-based async pattern (also known as async component pattern) that makes async calls easier with Windows applications. Before .NET 4 no standard mechanism has been available to cancel asynchronous method calls. That’s new with .NET 4.0.

In this blog series you can read about these async patterns as well as the new unified model for cancellation.

Part 1 of this series introduced the async patterns and introduced cancellation with the BackgroundWorker class as it existed since .NET 2. In part 2 of this series Christian will show how the new .NET 4 cancellation framework can be used.

Previously to .NET 4, cancellation with async method calls was implemented in different ways if it was supported at all. For example, the BackgroundWorker implements cooperative cancellation by invoking the CancelAsync method, the long-running method needs to verify if it should be canceled by checking the CancellationPending property and needs to cancel by setting the Cancel property of the DoWorkEventArgs. More about this in the previous blog entry.

.NET 4 now supports cooperative cancellation of async methods in a standard way. The heart of this new framework is the CancellationToken struct. The CancellationToken is created by a CancellationTokenSource. This token can then be passed to any activity that should be cancelled. In case of a cancellation, the async call can verify cancellation by checking the IsCancellationRequested property of the CancellationToken. This unified model is now available with several classes offering asynchronous requests.

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