create 2d barcode vb.net INTRODUCTION TO ASYNCHRONOUS PROGRAMMING in Visual C#.NET

Generation Data Matrix in Visual C#.NET INTRODUCTION TO ASYNCHRONOUS PROGRAMMING

CHAPTER 22 INTRODUCTION TO ASYNCHRONOUS PROGRAMMING
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This code produces the following output: After BeginInvoke Not Done Inside Sum Not Done Not Done Done Result: 8
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CHAPTER 22 INTRODUCTION TO ASYNCHRONOUS PROGRAMMING
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The Callback Pattern
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In the previous two patterns, wait-until-done and polling, the initial thread continues on with its flow of control only after it knows that the spawned thread has completed. It then retrieves the results and continues. The callback pattern is different in that once the initial thread spawns the asynchronous method, it goes on its way without synchronizing with it again. When the asynchronous method call completes, the system invokes a user-supplied method to handle its results, and to call the delegate s EndInvoke method. This user-defined method is called a callback method, or just callback. The two extra parameters at the end of the BeginInvoke parameter list are used with the callback method as follows: The first of the two parameters, the callback parameter, is the name of the callback method. The second parameter, the state parameter, can be either null or a reference to an object you want passed into the callback method. You ll be able to access this object through the method s IAsyncResult parameter using its AsyncState property. The type of this parameter is object.
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The Callback Method
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The signature and return type of the callback method must be of the form described by the AsyncCallback delegate type. This form requires that the method take a single parameter of type IAsyncResult and have a void return type, as shown here: void AsyncCallback( IAsyncResult iar ) There are several ways you can supply the callback method to the BeginInvoke method. Since the callback parameter in BeginInvoke is a delegate of type AsyncCallback, you can supply it as a delegate, as shown in the first code statement that follows. Or you can just supply the name of the callback method and let the compiler create the delegate for you. Both forms are semantically equivalent. Create a delegate with the callback method. IAsyncResult iar1 = del.BeginInvoke(3, 5, new AsyncCallback(CallWhenDone), null); Just use the callback method s name. IAsyncResult iar2 = del.BeginInvoke(3, 5, CallWhenDone, null); The second BeginInvoke parameter is used to send an object to the callback method. It can be an object of any type, but the parameter is of type object, so inside the callback method you will have to cast it to the correct type.
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CHAPTER 22 INTRODUCTION TO ASYNCHRONOUS PROGRAMMING
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Calling EndInvoke Inside the Callback Method
Inside the callback method, your code should call the delegate s EndInvoke method and take care of handling the output results of the asynchronous method execution. To call the delegate s EndInvoke method, though, you need a reference to the delegate object, which is in the initial thread not here in the spawned thread. If you re not using BeginInvoke s state parameter for anything else, you can use it to send the delegate reference to the callback method, as shown here: Delegate object Send delegate object as state param IAsyncResult iar = del.BeginInvoke(3, 5, CallWhenDone, del); Otherwise, you can extract the delegate s reference from the IAsyncResult object sent into the method as the parameter. This is shown in the following code and illustrated in Figure 22-3. The single parameter to the callback method is a reference to the IAsyncResult interface of the asynchronous method that has just completed. Remember that the IAsyncResult interface object is inside the AsyncResult class object. Although the IAsyncResult interface doesn t have a reference to the delegate object, the AsyncResult class object enclosing it does have a reference to the delegate object. So the first line inside the example method body gets a reference to the class object by casting the interface reference to the class type. Variable ar now has a reference to the class object. With the reference to the class object, you can now call the AsyncDelegate property of the class object and cast it to the appropriate delegate type. This gives you the delegate reference, which you can then use to call EndInvoke. using System.Runtime.Remoting.Messaging; void CallWhenDone( IAsyncResult iar ) { AsyncResult ar = (AsyncResult) iar; MyDel del = (MyDel) ar.AsyncDelegate; long Sum = del.EndInvoke( iar ); ... } // Contains AsyncResult class
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