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 Deciding When to Implement the Even...
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Microsoft Visual Studio 2008/.NET Framework 3.5

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.NET Framework Developer's Guide
Deciding When to Implement the Event-based Asynchronous Pattern

The Event-based Asynchronous Pattern provides a pattern for exposing the asynchronous behavior of a class. With the introduction of this pattern, the .NET Framework defines two patterns for exposing asynchronous behavior: the Asynchronous Pattern based on the System..::.IAsyncResult interface, and the event-based pattern. This topic describes when it is appropriate for you to implement both patterns.

For more information about asynchronous programming with the IAsyncResult interface, see Asynchronous Programming Design Patterns.

In general, you should expose asynchronous features using the Event-based Asynchronous Pattern whenever possible. However, there are some requirements that the event-based pattern cannot meet. In those cases, you may need to implement the IAsyncResult pattern in addition to the event-based pattern.


It is rare for the IAsyncResult pattern to be implemented without the event-based pattern also being implemented.

The following list describes the guidelines for when you should implement Event-based Asynchronous Pattern:

  • Use the event-based pattern as the default API to expose asynchronous behavior for your class.

  • Do not expose the IAsyncResult pattern when your class is primarily used in a client application, for example Windows Forms.

  • Only expose the IAsyncResult pattern when it is necessary for meeting your requirements. For example, compatibility with an existing API may require you to expose the IAsyncResult pattern.

  • Do not expose the IAsyncResult pattern without also exposing the event-based pattern.

  • If you must expose the IAsyncResult pattern, do so as an advanced option. For example, if you generate a proxy object, generate the event-based pattern by default, with an option to generate the IAsyncResult pattern.

  • Build your event-based pattern implementation on your IAsyncResult pattern implementation.

  • Avoid exposing both the event-based pattern and the IAsyncResult pattern on the same class. Expose the event-based pattern on “higher level” classes and the IAsyncResult pattern on “lower level” classes. For example, compare the event-based pattern on the WebClient component with the IAsyncResult pattern on the HttpRequest class.

    • Expose the event-based pattern and the IAsyncResult pattern on the same class when compatibility requires it. For example, if you have already released an API that uses the IAsyncResult pattern, you would need to retain the IAsyncResult pattern for backward compatibility.

    • Expose the event-based pattern and the IAsyncResult pattern on the same class if the resulting object model complexity outweighs the benefit of separating the implementations. It is better to expose both patterns on a single class than to avoid exposing the event-based pattern.

    • If you must expose both the event-based pattern and IAsyncResult pattern on a single class, use EditorBrowsableAttribute set to Advanced to mark the IAsyncResult pattern implementation as an advanced feature. This indicates to design environments, such as Visual Studio IntelliSense, not to display the IAsyncResult properties and methods. These properties and methods are still fully usable, but the developer working through IntelliSense has a clearer view of the API.

While the Event-based Asynchronous Pattern has many benefits under the previously mentioned scenarios, it does have some drawbacks, which you should be aware of if performance is your most important requirement.

There are three scenarios that the event-based pattern does not address as well as the IAsyncResult pattern:

You can address these scenarios by using the event-based pattern, but doing so is more cumbersome than using the IAsyncResult pattern.

Developers often use the IAsyncResult pattern for services that typically have very high performance requirements. For example, the polling for completion scenario is a high-performance server technique.

Additionally, the event-based pattern is less efficient than the IAsyncResult pattern because it creates more objects, especially EventArgs, and because it synchronizes across threads.

The following list shows some recommendations to follow if you decide to use the IAsyncResult pattern:

  • Only expose the IAsyncResult pattern when you specifically require support for WaitHandle or IAsyncResult objects.

  • Only expose the IAsyncResult pattern when you have an existing API that uses the IAsyncResult pattern.

  • If you have an existing API based on the IAsyncResult pattern, consider also exposing the event-based pattern in your next release.

  • Only expose IAsyncResult pattern if you have high performance requirements which you have verified cannot be met by the event-based pattern but can be met by the IAsyncResult pattern.

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