barcode printing vb.net I/O-Bound Asynchronous Operations in C#

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27 I/O-Bound Asynchronous Operations
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private static UInt64 Sum(UInt64 n) { UInt64 sum = 0; for (UInt64 i = 1; i <= n; i++) { checked { // I use checked code so that an OverflowException gets // thrown if the sum doesn't fit in a UInt64. sum += i; } } return sum; }
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If n is large, Sum could take a long time to execute . To keep the UI of my application responsive or to take advantage of other CPUs in the computer, I d like to execute this method asynchronously . To do this, I use the generic System.Func<T, TResult> delegate that accepts two type parameters; one for the argument and one for the return type:
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public delegate TResult Func<T, TResult>(T arg);
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You ll recall from the delegate discussion in 17, Delegates, that the C# compiler compiles this line of code into a class definition that logically looks like this:
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public sealed class Func<T, TResult> : MulticastDelegate { public Func(Object object, IntPtr method); public TResult Invoke(T arg); public IAsyncResult BeginInvoke(T arg, AsyncCallback callback, Object object); public TResult EndInvoke(IAsyncResult result); }
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When you define a delegate in C# source code, the compiler always produces a class that has BeginInvoke and EndInvoke methods . The BeginInvoke method has the same parameters as the delegate definition, with two additional parameters at the end: AsyncCallback and Object . All BeginInvoke methods return an IAsyncResult . The EndInvoke method has one parameter, an IAsyncResult, and the EndInvoke method returns whatever data type the delegate s signature returns . Now that you understand all of this, using a delegate to execute a compute-bound operation is trivial because it follows the APM pattern we ve been talking about . Here is some code that shows how to call Sum asynchronously:
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public static void Main() { // Initialize a delegate variable to refer to the method we want to call asynchronously Func<UInt64, UInt64> sumDelegate = Sum; // Call the method using a thread pool thread sumDelegate.BeginInvoke(1000000000, SumIsDone, sumDelegate); // Executing some other code here would be useful... // For this demo, I'll just suspend the primary thread Console.ReadLine(); }
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Part V
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Threading
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The sumDelegate variable is first initialized to refer to the method you want to call asynchronously . Then BeginInvoke is called to initiate the asynchronous calling of the method . Internally, the CLR constructs an IAsyncResult object to identify the asynchronous operation . As you know, I/O operations are queued to a Windows device driver; however, a delegate s BeginInvoke method queues compute-bound operations to the CLR s thread pool by internally calling ThreadPool's QueueUserWorkItem . Finally, BeginInvoke returns the IAsyncResult object to its caller (which usually ignores it) . Since BeginInvoke queued the operation to the CLR s thread pool, a thread pool thread will wake, dequeue the work item, and call the compute-bound method (Sum, in this example) . Normally, when a thread pool thread returns from executing a method, the thread returns back to the pool . However, in my example, BeginInvoke was called, passing in the name of a method (SumIsDone) for the second-to-last parameter . Because of this, when Sum returns, the thread pool thread does not return back to the pool; instead, it now calls SumIsDone . In other words, the callback is called when the compute-bound operation has completed, just as it would be called when an I/O-bound operation has completed . Here is what my SumIsDone method looks like:
private static void SumIsDone(IAsyncResult ar) { // Extract the sumDelegate (state) from the IAsyncResult object var sumDelegate = (Func<UInt64, UInt64>) ar.AsyncState; try { // Get the result and display it Console.WriteLine("Sum's result: " + sumDelegate.EndInvoke(result)); } catch (OverflowException) { Console.WriteLine("Sum's result is too large to calculate"); } }
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