References to objects in heap memory in Visual Basic .NET

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References to objects in heap memory
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The main bene t of garbage collection is that you, the programmer, do not have to worry about the way you use and discard objects because an invisible servant follows you around clearing up the mess you have made Since the collection of discarded memory into easily usable blocks is automatic, your program will generally run more ef ciently without you having to do anything The down-side of this is that the garbage collector takes time to do its work, and when it does your application will be put on-hold while it does it This can appear as an unresponsive user-interface, or even, for some types of resources (eg a database connection), act as a block on further processing However, unless you write memory hogging programs which allocate lots of objects only to discard them shortly afterwards, or which allocate very large objects (eg to store pictures or video clips) frequently or make use of very scarce resources, you are unlikely ever to experience a garbage collection delay From a programming perspective, an object is discarded whenever the link between the object and the reference variable that refers to it is broken There are a number of ways in which the link to an object in heap memory can be broken
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The reference variable referring to the object may be a local variable in a sub or function At the end of the sub or function, the values or references in local variables are discarded Therefore, if a local variable holds a reference to an object in the heap, the link to that object will be lost In Figure 108, if the variable RefA goes out of scope, the link to object 1 in the heap will be lost, eg
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Sub W() Dim RefA As SomeClass = New SomeClass() 'Object 1 'Object is used ' End Sub 'RefA out of scope here, so Object 1 is lost
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We can assign a new object to a reference variable that currently refers to an already existing object in the heap The reference to the new object will replace the reference to the existing object, and so the link to that object will be lost If RefD in Figure 108 has a new object assigned to it, the link to object 8 will be lost, eg
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Sub X() Dim RefD As SomeClass = New SomeClass() 'Object 8 'Object 8 is used RefD = New SomeClass() 'Discards Object 8 'New object is used ' End Sub
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We can deliberately assign the value Nothing to a reference variable that currently refers to an object in the heap, and so the link to the object to be lost If Figure 108, assigning Nothing to any of the reference variables will cause the link to the associated object to be lost Note that the object will only be destroyed if there are no other references to it in the program (for example, if it had been added to a collection, it would not be destroyed), eg
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Sub Y() Dim RefA As SomeClass = New SomeClass() 'Object 1 'Object 1 is used RefA = Nothing 'Discards Object 1 ' End Sub
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If an object in the heap references another object in the heap, and the link to the rst object is destroyed by one of the above methods, then the link to the object it references will also be destroyed
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Sub Z() Dim RefC As SomeClass = New SomeClass() 'Object 6 RefCSomeObject = New SomeOtherClass() 'Object 7 'Combination of objects is used ' RefC = Nothing 'Now Objects 6 and 7 are both 'discarded End Sub
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In all of these cases and many other situations, losing the last reference to an object means that the object can no longer be reached by any program code NET is smart enough to realize whether an object is reachable or not and when its garbage collector runs it clears up these unreachable objects If the garbage collector nds an unreachable object, it marks it for removal and subsequently returns the memory it occupied to the operating system
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