This is almost always exclusively ‘owned’ by the list, so the relationship is fairly simple: The total size of the list is the size of the small initial object and the large array it references. However, one of these is the array of objects in the list: this can be quite large if the list has many entries. For instance, the memory used by a List object is quite small, as the List class only has a few fields. This can make it difficult to determine the amount of memory used by a particular object. NET forms a complicated, knotted graph of references and cross-references. This means that these objects usually require more than one garbage collection to be removed from memory, as they will survive the first time they are found to be unused. Instead, it becomes a special kind of root until. If an object has a finalizer, it is not immediately removed when the garbage collector decides it is no longer ‘live’. This is because COM+ doesn’t do garbage collection: It uses, instead, a reference counting system once the COM+ library finishes with the object by setting the reference count to 0 it ceases to be a GC root and can be collected again. If a managed object is passed to an unmanaged COM+ library through interop, then it will also become a GC root with a reference count. Variables declared as ‘thread static’ will only last for as long as that thread is running. The objects they reference can be accessed at any time by the class that declared them (or the rest of the program if they are public), so. Static variables are also always considered GC roots. This strategy isn’t always used and can be turned off, for example, by running the program in a debugger. In release builds, the JIT is able to look at the program structure to work out the last point within the execution that a variable can be used by the method and will discard it when it is no longer required. In debug builds, a local variable lasts for as long as the method is on the stack. The lifetime of these roots can depend on the way the program was built. The objects referenced by these variables can always be accessed immediately by the method they are declared in, and so they must be kept around. NET:Ī local variable in a method that is currently running is considered to be a GC root. Any object referenced by a GC root will automatically survive the next garbage collection. GC roots are not objects in themselves but are instead references to objects. Once all of these live objects are known, any remaining objects can be discarded and the space re-used for new objects.NET compacts memory so that there are no gaps (effectively squashing the discarded objects out of existence) – this means that free memory is always located at the end of a heap and makes allocating new objects very fast. It marks anything that they reference as also being used until it can find no further objects.Īn object is identified, by the Garbage Collector, as referencing another object if it, or one of its superclasses, has a field that contains the other object. It continues in this manner, iterating through all of the objects it knows are ‘live’. It marks these objects as ‘live’ and then looks at any objects that they reference it marks these as being ‘live’ too. These are memory locations that are designated to be always reachable for some reason, and which contain references to objects created by the program. When a garbage collection starts, it looks at a set of references called the ‘GC roots’. How, then, does the garbage collector achieve its magic? The basic idea is pretty simple: it examines how objects are laid out in memory and identifies all those objects that can be ‘reached’ by the running program by following some series of references. Before fixing these problems, you need some understanding of how the collector works. It also solves the related problem of memory being released too soon, but the way in which this is solved can lead to memory leaks when the garbage collector has a different opinion to the developer about whether or not an object is still ‘live’ and able to be used. The garbage collector certainly solves the most common leaks in unmanaged programs – those caused by developers forgetting to release memory when they have finished with it. The reality is more complicated than this, however. NET’s garbage collector has been sold to us as the end of explicit memory management, and of memory leaks, in Windows applications: the idea is that, with a garbage collector running in the background, developers no longer need to worry about the need to manage the life-cycle of the objects they create – the garbage collector will take care of them once the application has finished with them.
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