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The Asynchronous Programming Model
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The Asynchronous Programming Model (APM) is a pattern that many asynchronous APIs in the .NET Framework conform to. It defines common mechanisms for discovering when work is complete, for collecting the results of completed work, and for reporting errors that occurred during the asynchronous operation. APIs that use the APM offer pairs of methods, starting with Begin and End. For example, the Socket class in the System.Net.Sockets namespace offers numerous instances of this pattern: BeginAccept and EndAccept, BeginSend and EndSend, BeginConnect and EndConnect, and so on.
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#Asynchronous APIs tend to be used slightly differently in server-side code in web applications. There, they are most useful for when an application needs to communicate with multiple different external services to handle a single request.
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The exact signature of the Begin method depends on what it does. For example, a socket s BeginConnect needs the address to which you d like to connect, whereas BeginReceive needs to know where you d like to put the data and how much you re ready to receive. But the APM requires all Begin methods to have the same final two parameters: the method must take an AsyncCallback delegate and an object. And it also requires the method to return an implementation of the IAsyncResult interface. Here s an example from the Dns class in System.Net:
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public static IAsyncResult BeginGetHostEntry( string hostNameOrAddress, AsyncCallback requestCallback, object stateObject )
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Callers may pass a null AsyncCallback. But if they pass a non-null reference, the type implementing the APM is required to invoke the callback once the operation is complete. The AsyncCallback delegate signature requires the callback method to accept an IAsyncResult argument the APM implementation will pass in the same IAsyncRe sult to this completion callback as it returns from the Begin method. This object represents an asynchronous operation in progress many classes can have multiple operations in progress simultaneously, and the IAsyncResult distinguishes between them. Example 16-16 shows one way to use this pattern. It calls the asynchronous BeginGetHostEntry method provided by the Dns class. This looks up the IP address for a computer, so it takes a string the name of the computer to find. And then it takes the two standard final APM arguments a delegate and an object. We can pass anything we like as the object the function we call doesn t actually use it, it just hands it back to us later. We could pass null because our example doesn t need the argument, but we re passing a number just to demonstrate where it comes out. The reason the APM offers this argument is so that if you have multiple simultaneous asynchronous operations in progress at once, you have a convenient way to associate information with each operation. (This mattered much more in older versions of C#, which didn t offer anonymous methods or lambdas back then this argument was the easiest way to pass data into the callback.)
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class Program { static void Main(string[] args) { Dns.BeginGetHostEntry("oreilly.com", OnGetHostEntryComplete, 42); } Console.ReadKey();
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static void OnGetHostEntryComplete(IAsyncResult iar) { IPHostEntry result = Dns.EndGetHostEntry(iar); Console.WriteLine(result.AddressList[0]);
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Console.WriteLine(iar.AsyncState);
The Main method waits until a key is pressed much like with work items in the thread pool, having active asynchronous requests will not keep the process alive, so the program would exit before finishing its work without that ReadKey. (A more robust approach for a real program that needed to wait for work to complete would be to use the CountdownEvent described earlier.) The Dns class will call the OnGetHostEntryComplete method once it has finished its lookup. Notice that the first thing we do is call the EndGetHostEntry method the other half of the APM. The End method always takes the IAsyncResult object corresponding to the call recall that this identifies the call in progress, so this is how EndGetHostEn try knows which particular lookup operation we want to get the results for.
The APM says nothing about which thread your callback will be called on. In practice, it s often a thread pool thread, but not always. Some individual implementations might make guarantees about what sort of thread you ll be called on, but most don t. And since you don t usually know what thread the callback occurred on, you will need to take the same precautions you would when writing multithreaded code where you explicitly create new threads. For example, in a WPF or Windows Forms application, you d need to use the SynchronizationContext class or an equivalent mechanism to get back to a UI thread if you wanted to make updates to the UI when an asynchronous operation completes.
The End method in the APM returns any data that comes out of the operation. In this case, there s a single return value of IPHostEntry, but some implementations may return more by having out or ref arguments. Example 16-16 then prints the results, and finally prints the AsyncState property of the IAsyncResult, which will be 42 this is where the value we passed as the final argument to BeginGetHostEntry pops out. This is not the only way to use the Asynchronous Programming Model you are allowed to pass null as the delegate argument. You have three other options, all revolving around the IAsyncResult object returned by the Begin call. You can poll the IsCompleted property to test for completion. You can call the End method at any time if the work is not finished this will block until it completes.* Or you can use the Asyn cWaitHandle property this returns an object that is a wrapper around a Win32 synchronization handle that will become signaled when the work is complete. (That last one is rarely used, and has some complications regarding ownership and lifetime of the handle, which are described in the MSDN documentation. We mention this technique only out of a pedantic sense of duty to completeness.)
* This isn t always supported. For example, if you attempt such an early call on an End method for a networking operation on the UI thread in a Silverlight application, you ll get an exception.
You are required to call the End method at some point, no matter how you choose to wait for completion. Even if you don t care about the outcome of the operation you must still call the End method. If you don t, the operation might leak resources.
Asynchronous operations can throw exceptions. If the exception is the result of bad input, such as a null reference where an object is required, the Begin method will throw an exception. But it s possible that something failed while the operation was in progress perhaps we lost network connectivity partway through some work. In this case, the End method will throw an exception. The Asynchronous Programming Model is widely used in the .NET Framework class library, and while it is an efficient and flexible way to support asynchronous operations, it s slightly awkward to use in user interfaces. The completion callback typically happens on some random thread, so you can t update the UI in that callback. And the support for multiple simultaneous operations, possible because each operation is represented by a distinct IAsyncResult object, may be useful in server environments, but it s often just an unnecessary complication for client-side code. So there s an alternative pattern better suited to the UI.
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