Foundations of Programming - Karl Seguin

Chapter 7 - Back to Basics: Memory
Some prefer to call the type of
pointers found in C#, VB.NET and
Java References or Safe Pointers,
because they either point to a valid
object or null. There is no such
guarantee in languages such as C
or C++ since developers are free to
manipulate pointers directly.
Interestingly, when we leave our method, the boss variable
will pop off the stack, but the subordinate, which is defined in
a parent scope, won't. This means the garbage collector won't
have anything to clean-up because both heap values will still
be referenced (one directly from the stack, and the other
indirectly from the stack through a referenced object).
As you can see, pointers most definitely play a significant part
in both C# and VB.NET. Since pointer arithmetic isn't available
in either language, pointers are greatly simplified and
hopefully easily understood.
Memory Model in Practice
We'll now look at the actual impact this has on our applications. Keep in mind though that
understanding the memory model in play won't only help you avoid pitfalls, but it will also help you
write better applications.
Boxing
Boxing occurs when a value type (stored on the stack) is coerced onto the heap. Unboxing happens
when these value types are placed back onto the stack. The simplest way to coerce a value type, such as
an integer, onto the heap is by casting it:
int x = 5;
object y = x;
A more common scenario where boxing occurs is when you supply a value type to a method that
accepts an object. This was common with collections in .NET 1.x before the introduction of generics. The
non-generic collection classes mostly work with the object type, so the following code results in boxing
and unboxing:
ArrayList userIds = new ArrayList(2);
userIds.Add(1);
userIds.Add(2);;
int firstId = (int)userIds[0];
The real benefit of generics is the increase in type-safety, but they also address the performance penalty
associated with boxing. In most cases you wouldn't notice this penalty, but in some situations, such as
large collections, you very well could. Regardless of whether or not it's something you ought to actually
concern yourself with, boxing is a prime example of how the underlying memory system can have an
impact on your application.
Foundations of Programming Copyright © Karl Seguin www.codebetter.com
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