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6: Using Multiple Threads
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You can invoke the method referenced by the delegate instance either synchronously or asynchronously. To invoke it synchronously, use the following code.
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string result = longCalcDelegate( 10000 );
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This code internally uses the Invoke method defined in the delegate type above. Because the Invoke method is a synchronous call, this method returns only after the invoked method returns. The return value is the result of the invoked method. More frequently, to prevent the calling thread from blocking, you will choose to invoke the delegate asynchronously, using the BeginInvoke and EndInvoke methods. Asynchronous delegates use the thread pooling capabilities of the .NET Framework for thread management. The standard Asynchronous Call pattern implemented by the .NET Framework provides the BeginInvoke method to initiate the required operation on a thread, and it provides the EndInvoke method to allow the asynchronous operation to be completed and any resulting data to be passed back to the calling thread. After the background processing completes, you can invoke a callback method within which you can call EndInvoke to retrieve the result of the asynchronous operation. When you call the BeginInvoke method, it does not wait for the call to complete; instead, it immediately returns an IAsyncResult object, which can be used to monitor the progress of the call. You can use the WaitHandle member of the IAsyncResult object to wait for the asynchronous call to complete or use the IsComplete member to poll for completion. If you call the EndInvoke method before the call completes, it will block and return only after the call completes. However, you should be careful not to use these techniques to wait for the call to complete, because they may block the UI thread. In general, the callback mechanism is the best way to be notified that the call has completed.
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To execute a method referenced by a delegate asynchronously
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1. Define a delegate representing the long-running asynchronous operation, as shown in the following example.
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delegate string LongCalculationDelegate( int count );
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2. Define a method matching the delegate signature. The following example method simulates a time-consuming operation by causing the thread to sleep for count milliseconds before returning.
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private string LongCalculation( int count ) { Thread.Sleep( count ); return count.ToString(); }
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3. Define a callback method that corresponds to the AsyncCallback delegate defined by the .NET Framework, as shown in the following example.
private void CallbackMethod( IAsyncResult ar ) { // Retrieve the invoking delegate. LongCalculationDelegate dlgt = (LongCalculationDelegate)ar.AsyncState; // Call EndInvoke to retrieve the results. string results = dlgt.EndInvoke(ar); }
4. Create an instance of a delegate that references the method you want to call asynchronously and create an AsyncCallback delegate that references the callback method, as shown in the following code example.
LongCalculationDelegate longCalcDelegate = new LongCalculationDelegate( calculationMethod ); AsyncCallback callback = new AsyncCallback( CallbackMethod );
5. From your calling thread, initiate the asynchronous call by calling the BeginInvoke method on the delegate that references the code you want to execute asynchronously.
longCalcDelegate.BeginInvoke( count, callback, longCalcDelegate );
The method LongCalculation is called on the worker thread. When it completes, the method CallbackMethod is called, and the results of the calculation retrieved.
Note: The callback method is executed on a non-UI thread. To modify the UI, you need to use techniques to switch from this thread to the UI thread. For more information, see Using Tasks to Handle Interactions Between the UI Thread and Other Threads later in this chapter.
You can use a custom delegate to pass arbitrary parameters to a method to be executed on a separate thread (something you cannot do when you create threads directly using either the Thread object or a thread pool.) Invoking delegates asynchronously is particularly useful when you need to invoke long-running operations in the application UI. If users perform an operation in the UI that is expected to take a long time to complete, you do not want the UI to freeze and not be able to refresh itself. Using an asynchronous delegate, you can return control to your main UI thread to perform other operations. You should use a delegate to invoke a method asynchronously when: You need to pass arbitrary parameters to a method you want to execute asynchronously. You want to use the Asynchronous Call pattern provided by the .NET Framework.
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