Object-Oriented Programming in VS .NET

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Object-Oriented Programming
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Sub TestFinalize() Debug.WriteLine( About to create a Person object. ) Dim aPerson As New Person2( Joe", Doe ) Debug.WriteLine( Exiting the TestFinalize procedure. ) End Sub End Module
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These are the messages that you ll see in the Debug window:
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About to create a Person object. Exiting the TestFinalize procedure. About to terminate the application. Person Joe Doe is being destroyed.
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The sequence of messages makes it apparent that the Person2 object isn t destroyed when the TestFinalize procedure exits as would happen in Visual Basic 6 but only some time later, when the application itself terminates. Here s one important .NET programming guideline: never access any external object from a Finalize procedure. The object might have been destroyed already. In fact, the object is being collected because it can t be reached from the main application, so a reference to another object isn t going to keep that object alive. The garbage collector can reclaim unreachable objects in any order, so the other object might be finalized before the current one. An object that can be safely accessed from a Finalize method is the base object of the current object, using the MyBase keyword. In general, it is safe to invoke static methods from inside the Finalize method, except when the application is shutting down. In the latter case, in fact, the .NET Framework might have already destroyed the System.Type object corresponding to the type that exposes the static method. For example, you shouldn t use the Console.WriteLine method because the Console object might be gone. The Debug object is one of the few objects that are guaranteed to stay alive until the very end, however, and that s why the previous code example uses Debug.WriteLine instead of Console.WriteLine. You can discern the two cases by querying the Environment.HasShutdownStarted method:
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Protected Overrides Sub Finalize() If Not Environment.HasShutdownStarted Then It is safe to access static methods of other types. End If End Sub
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You can force a garbage collection during the lifetime of an application by creating a sufficiently large number of objects. For example, try this code:
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Sub Main() TestFinalize2 End Sub Sub TestFinalize2() NOTE: If no Finalize method is invoked on your system, increment the loop upper limit.
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5:
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For i As Integer = 1 To 10000 TestFinalize_Create() Next Debug.WriteLine( About to terminate the application. ) End Sub Sub TestFinalize_Create() Dim aPerson As New Person2( Joe", Doe ) End Sub
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Inheritance
Here s a much simpler way to force a garbage collection: just ask the garbage collector to do it. The garbage collector is just one .NET object defined in the Framework, and it conveniently exposes a Collect method, which fires a garbage collection. You can see how this works by calling this procedure:
Sub TestFinalize3() Debug.WriteLine( About to create a Person object. ) Dim aPerson As New Person2( Joe", Doe ) aPerson = Nothing Debug.WriteLine( About to fire a garbage collection. ) GC.Collect() GC.WaitForPendingFinalizers() End Sub
The WaitForPendingFinalizers method stops the current thread until all objects are cor rectly finalized; this action is necessary because the garbage collection process might run on a different thread. The sequence of messages in the Debug window is now different:
About to create a Person object.
About to fire a garbage collection.
Person object named Joe Doe is being destroyed.
About to create a Person object.
However, calling the GC.Collect method manually is usually a bad idea. The preceding code example, which uses the GC.Collect method only to fire the object s Finalize method, illustrates what you should never do in a real .NET application. If you run a garbage collection frequently, you re missing one of the most promising performance optimizations that the new .NET Framework offers. You should invoke the GC.Collect method only when the application is idle for example, while it waits for user input and only if you see that unexpected (that is, not explicitly requested) garbage collec tions are slowing the program noticeably during time-critical operations. For example, unrequested GCs might be an issue when your application is in charge of controlling hardware devices that require a short response time. Here s another reason for staying clear of the Finalize method: objects that expose such a method aren t immediately reclaimed and usually require at least another garbage collection before they are swept out of the heap. The reason for this behavior is that the code in the Finalize method might assign the current object (using the Me keyword) to a global variable, a technique known as resurrection, which I talk about later
Part II:
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