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There is a limitation to the effectiveness of the garbage collector; it decides when to compact the memory taken up by discarded objects (collect the garbage) based on the amount of available space left, how frequently objects are being created to ll it up and other factors In some systems with a lot of memory, it could take hours for the garbage collector to decide a tidy-up was necessary, and during this time, our object s le will remain open Leaving a le open for a long time beyond the time we need it for is not good practice If the system crashes, someone trips over the computer s power cord or some other spanner falls into the works, the le will be corrupted and probably its contents will be lost for good For similar reasons, there are a number of resources that you might use in a program that it would be best not to trust to the unpredictable operation of the garbage collector Leaving a modem connection open, not freeing the system printer after a printing session or hanging on to a connection to another program after data has been transferred to it are all situations where a program can hog a scarce resource and cause system problems The upshot of this is that, like in a C++ program, we need to take care of our own resources if they are not managed by the NET run-time system for us
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Every object that you create in a program occupies some system memory Objects are created in an area of memory called the heap , which, as its name suggests, is a fairly unorganized section of memory We create objects and assign them to reference variables so that we are able to access their properties and methods, and these reference variables act as our link to objects in heap memory When an object is created, NET must rst request the memory it will occupy from the operating system When the request for memory has been granted, NET will then call the object s constructor which will store the object s data in the memory, and return the reference to the object to be assigned to a reference variable in your program Figure 108 shows a number of objects referred to by reference variables in a program The heap is a block of memory allocated to an application for storing objects in When a new object is created, the CLR allocates some space in the heap by simply taking the next free area beyond the space it allocated to the previous object Pretty obviously if a program runs for long enough and creates new objects periodically as it does, the heap will eventually ll up However, during the period the program has been running, objects will also have been discarded by the program, and the space that was taken up by them will be holes in the area of the heap assumed to be in use Garbage collection takes place periodically so that the CLR can reclaim all of the discarded blocks of memory that were once objects (eg objects 9 and 10 in Figure 108) Once the garbage collector has worked on the heap, all of the objects still in use occupy one contiguous block of memory The memory that was once occupied by discarded objects has been collected into a single block so that the free heap space is also collected together The result is that the heap once again has a large contiguous block of free memory from which it can allocate space for new objects
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104 Managing Scarce Resources
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Program Memory Structured Memory Area: contains Global variables and references to main objects (Forms, etc) RefA RefB RefC RefD 8 7 10 6 4 9 1 2 5 Objects in Heap Memory 3
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