Garbage collection

The garbage collector is .NET’s answer to memory management, and in particular to the question of what
to do about reclaiming memory that running applications ask for. Up until now there have been two
techniques used on the Windows platform for de-allocating memory that processes have dynamically
requested from the system:
❑ Make the application code do it all manually.
❑ Make objects maintain reference counts.
Having the application code responsible for de-allocating memory is the technique used by lower-level,
high-performance languages such as C++. It is efficient, and it has the advantage that (in general)
resources are never occupied for longer than unnecessary. The big disadvantage, however, is the frequency
of bugs. Code that requests memory also should explicitly inform the system when it no longer
requires that memory. However, it is easy to overlook this, resulting in memory leaks.
Although modern developer environments do provide tools to assist in detecting memory leaks, they
remain difficult bugs to track down, because they have no effect until so much memory has been leaked
that Windows refuses to grant any more to the process. By this point, the entire computer may have
appreciably slowed down due to the memory demands being made on it.
Maintaining reference counts is favored in COM. The idea is that each COM component maintains a
count of how many clients are currently maintaining references to it. When this count falls to zero, the
component can destroy itself and free up associated memory and resources. The problem with this is
that it still relies on the good behavior of clients to notify the component that they have finished with it.
It only takes one client not to do so, and the object sits in memory. In some ways, this is a potentially
more serious problem than a simple C++-style memory leak, because the COM object may exist in its
own process, which means that it will never be removed by the system (at least with C++ memory leaks,
the system can reclaim all memory when the process terminates).
The .NET runtime relies on the garbage collector instead. This is a program whose purpose is to clean up
memory. The idea is that all dynamically requested memory is allocated on the heap (that is true for all
languages, although in the case of .NET, the CLR maintains its own managed heap for .NET applications
to use). Every so often, when .NET detects that the managed heap for a given process is becoming full
and therefore needs tidying up, it calls the garbage collector. The garbage collector runs through variables
currently in scope in your code, examining references to objects stored on the heap to identify
which ones are accessible from your code—that is to say which objects have references that refer to
them. Any objects that are not referred to are deemed to be no longer accessible from your code and can
therefore be removed. Java uses a similar system of garbage collection to this.
Garbage collection works in .NET because IL has been designed to facilitate the process. The principle
requires that you cannot get references to existing objects other than by copying existing references and
that IL is type safe. In this context, what we mean is that if any reference to an object exists, then there is
sufficient information in the reference to exactly determine the type of the object.
It would not be possible to use the garbage collection mechanism with a language such as unmanaged
C++, for example, because C++ allows pointers to be freely cast between types.

One important aspect of garbage collection is that it is not deterministic. In other words, you cannot
guarantee when the garbage collector will be called; it will be called when the CLR decides that it is
needed (unless you explicitly call the collector). Though it is also possible to override this process and
call up the garbage collector in your code.

2007-07-14 20:03:00


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