Pre-empting the Garbage Collector in Visual Basic .NET

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10411 Pre-empting the Garbage Collector
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Garbage collection is built into the NET framework It kicks in periodically when an application s heap memory is becoming scarce and new objects are being
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104 Managing Scarce Resources
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allocated at a frequency that will make it run out soon New objects are allocated space from the block of memory that does not contain either current or discarded objects, and as an application runs, this block is continually reducing in size In effect the garbage collector has two jobs to do Its primary job is to gure out which memory blocks are no longer in use and mark these as available for use Once it has done this for every block in heap memory it can get on with its second job, which is to shuf e all of the memory blocks still in use into a single, contiguous area, thereby freeing up the memory for use by new objects Since the garbage collector s work can be intensive, we can assume it will not be brought into action more often than is necessary We can make use of the garbage collector s knowledge of every object s Finalize() method This protected method is de ned in the object class, which is the ultimate ancestor of all classes in NET When we allocate an unmanaged resource such as opening a le or database connection to an object in its constructor, it is recommended that we make sure that it is de-allocated (ie closed) in the Finalize() method However, the Finalize() method should never be called directly Microsoft suggest that if you de ne one you should give it protected scope to prevent other users of your class from calling it directly, and also that you should never call it directly yourself If you do, you run the risk of the garbage collector calling it again to free up resources that have already been freed, and thereby causing a run-time error Here is our dilemma:
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we can de-allocate scarce resources in the Finalize() method, which we are not allowed to call directly, and so must then leave their de-allocation up to when the garbage collector gets around to doing it for us potentially a long time; alternatively we can de-allocate resources in some other sub (eg a public sub called Close() or Deallocate()), and leave it up to the user of the class to call it when resources are no longer needed
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The rst alternative gives us the certainty that resources will be freed, at the price of not knowing when The second would make it certain that resources would be freed in a timely manner, but only if we remembered to use it Ideally, we could do with the best of both
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10412 The Dispose() Method
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The Dispose() method provides us with a way out of this dilemma Finalize() is a protected method and so cannot be called directly If we want to be able to invoke it, we can de ne a Public Dispose() method that can be called by code in our application when we know the object can be dispensed with (see Listing 104)
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Class SomeResourceUsingClass 'Other class code ' Protected Overrides Sub Finalize() 'Put code to release any resources here 'eg Close files, network connections, Database
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'connections, unload forms, or release memory hogging 'resources like graphic bitmaps End Sub Public Sub Dispose() GCSuppressFinalize(Me) Finalize() End Sub End Class Listing 104: The Finalize() and Dispose() methods
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In Sub Dispose(), we need to do two things: rstly we need to tell the garbage collector that it should not call Finalize() because we will do it in our own code, and secondly, we must then call Finalize() The overall result of this is that we will have released scarce resources earlier than the garbage collector would have done most of the time We can now arrange that when an object of a class has done its job, we can release it and any resources it makes use of immediately For example, suppose we needed to implement a Data Logging class that keeps a le open to write data to frequently The Finalize() and Dispose() methods would be responsible for ensuring the le is closed
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Class DataLogger Dim F As IOStreamWriter Public Sub New(ByVal LogFileName As String) F = New IOStreamWriter(LogFileName) End Sub Public Sub WriteLog(ByVal Message As String) FWriteLine(Message) End Sub Protected Overrides Sub Finalize() FClose() End Sub Public Sub Dispose() GCSuppressFinalize(Me) Finalize() End Sub End Class Listing 105: Using the Dispose() method to clear objects out of memory
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The class in Listing 105 makes use of a StreamWriter object that can send text to a le in the computer s hard disk There will be a full explanation of les and the associated classes in 11 For now, you simply need to be aware that while a le is open, it is using a scarce system resource If we remember to call Dispose() once we are done with a DataLogger object, the le will be closed If we don t, the garbage collector will do it for us eventually Note that the space occupied by the DataLogger object will still need to wait for the garbage collector to reclaim it
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