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If (rnd.Next(100) > 80) Then notify(name + ": """ + WhichAction() + """ ") Else notify(name + ": ""Zzz"" ") Thread.Sleep(sleepTime) End If Next End Sub Private Sub Finished(ByVal ar As IAsyncResult) notify(name + ": Finished") End Sub Public Sub Go(ByVal howMuch As Integer) Dim state As Object callback = New AsyncCallback(AddressOf Finished) async = New DoStuff(AddressOf DoCatStuff) async.BeginInvoke(howMuch, callback, state) End Sub End Class
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Creates an instance of the DoStuff delegate and then calls BeginInvoke
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To take advantage of asynchronous delegate execution, we need a delegate to associate with the method we wish to execute. The DoStuff delegate is private to the ClassCat class. We need an instance of the DoStuff delegate to utilize. The async private data member is used to store the reference to the instance. The public method Go is used to create an instance of the DoStuff delegate. Part of the creation process is to associate the delegate with a method to execute. Visual Basic uses the AddressOf keyword to differentiate between a method and a reference to that method. Once the async variable contains a reference to a new DoStuff delegate, we can use the BeginInvoke method to start the asynchronous execution of the DoCatStuff method. Listing 3.3 contains code of the main module that utilizes the ClassCat class.
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Listing 3.3 The main module that uses ClassCat (VB.NET)
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Module Module1 Sub Main() Dim notify As ClassCat.DidSomething notify = New ClassCat.DidSomething(AddressOf OutputLine) Dim oTiger As ClassCat oTiger = New ClassCat("Tiger", notify) oTiger.Go(10) Dim oGarfield As ClassCat oGarfield = New ClassCat("Garfield", notify) oGarfield.Go(10) Console.WriteLine("Press enter to exit") Console.ReadLine() End Sub
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One instance of ClassCat named Tiger
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Another named Garfield
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Sub OutputLine(ByVal message As String) Console.WriteLine(message) End Sub End Module
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This example is very similar to that in chapter 1. One major difference is that the execution of the DoCatStuff method occurs on a different thread. The oTiger variable is a reference to an instance of the ClassCat. When the Go method of the oTiger method is invoked, an instance of the DoStuff delegate is created and BeginInvoke is called on it. The Go method returns as soon as it has invoked the BeginInvoke method. This will be before the DoCatStuff method has completed. To see that the execution occurs on different timelines it helps to have two instances of ClassCat. The second instance has a different name but shares the same DidSomething delegate. The program produces the following output:
Press enter to exit Tiger: "Zzz" Garfield: "Zzz" Tiger: "Zzz" Garfield: "Drink" Garfield: "Zzz" Tiger: "Zzz" Garfield: "Zzz" Tiger: "Zzz" Garfield: "Zzz" Tiger: "Take a bath" Tiger: "Zzz" Garfield: "Zzz" Tiger: "Zzz" Garfield: "Zzz" Tiger: "Zzz" Garfield: "Zzz" Tiger: "Meow" Tiger: "Zzz" Garfield: "Zzz" Tiger: Finished Garfield: "Zzz" Garfield: Finished
Notice that the two cats output is commingled. If the execution were occurring on the same thread, the Tiger output would be separate from the Garfield output. Asynchronous delegates are an easy way to execute methods on different threads. There are limitations on how this should be used. The source of these limitations is related to asynchronous delegates using a thread pool to do their asynchronous execution. Thread pools are limited in size. Because the number of threads that can exist in a thread pool is restricted, methods invoked in thread pools should be short-lived. This BENEFITS OF .NET TO MULTITHREADING 47
restriction is often too severe for many problems. Because not every problem can be solved using asynchronous delegates, it s important to understand how to do multithreading using the Thread class in the Threading namespace. That s where we pick up in the next chapter.
SUMMARY
This chapter has introduced some fundamental elements of multithreading in the .NET framework. Application domains provide the framework with a way of determining the boundaries of an application. Application domains are very similar to the Win32 process within which they live. Each application domain contains one or more logical threads that execute a series of instructions. Logical threads are represented in the .NET framework using the System.Threading.Thread class. This class is used to create, control, and manage logical threads. There are times that it is necessary to manipulate physical threads; to do so the .NET framework includes the System.Diagnostics.Process and System.Diagnostics.ProcessThread classes. These classes allow access to all physical threads on a system, not just those related to the .NET framework. .NET provides many benefits to developers. This is especially true with regards to multithreaded development. Since the .NET framework is object-oriented, all methods needed to manipulate a logical thread are contained in the System.Threading.Thread class. This grouping makes it very easy to find the methods to manipulate a thread. The System.Threading.Thread class is not the only way to execute a method on a different thread. We discussed the asynchronous execution of delegates. While this approach is simpler to implement than using the System.Threading.Thread class, it lacks flexibility. In the next chapter we dig into the means of creating, destroying, and interacting with logical threads.
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