Datasheet

We won’t go into the details of the CLS specifications here. In general, the CLS won’t affect your C# code
very much, because there are very few non–CLS-compliant features of C# anyway.
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 two techniques have
been used on the Windows platform for deallocating 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 deallocating 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 necessary. 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 compo-
nent 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 vari-
ables 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 system of garbage collection similar 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.
13
.NET Architecture
05_575341 ch01.qxd 10/4/05 7:03 PM Page 13