4 replies to this topic

[SonicD007]

Hey everyone,

I was refreshing on my pointer skills by reading about linked lists and it got me wondering how C# handles it's lists. Lets say I have a global list of type Foo

List<Foo> globalList = new List<Foo>();

Now lets say I create an instantiate a Foo object within a local scope of a function.

void LocalFunction()
{
	 Foo myLocalObject = new myLocalObject();
	 //Do stuff with myLocalObject
	 globalList.Add(myLocalObject);
}

From what I understand with c++, globalList.Add(myLocalObject); is passing the object by address (which is how c# passes classes anyway) so even though myLocalObject is no longer in scope, I can still access it and it's information by using globalList[whateverIndexItsat].whateverMethod();

I'm able to do this because C# has a pointer internally somewhere in which it is pointing to the memory created by that localObject. Is my understanding of this correct? Also, does the garbage collector manage the pointers by reference counting?

I'm just curious as to how everything is working here and the linked list datastructure in C++ is helping me understand where to use pointers a little better.

Thanks, hopefully I'm understanding this correctly.

[SiCrane]

.NET Garbage Collection is significantly more sophisticated than just reference counting.

[Serapth]

Pointers internally are pretty complicated too.

The linked article is rather dated, it covers .NET 1.1 after all, but the basics of memory management are all there.

[Nick Gravelyn]

Here is some more good reading on the subject of references and pointers in C#. Not necessarily directly related to your question, but good reading on the topics, nonetheless.

http://blogs.msdn.com/b/ericlippert/archive/2009/02/17/references-are-not-addresses.aspx
http://blogs.msdn.com/b/ericlippert/archive/2011/03/07/references-and-pointers-part-one.aspx
http://blogs.msdn.com/b/ericlippert/archive/2011/03/10/references-and-pointers-part-two.aspx

A slight aside, List<T> in C# is more analogous to std::vector in C++. It's not a linked list; it's just an indexable collection of objects. Generally speaking a List<T> in C# is just implemented as a raw T[] under the hood, handling resizing the array as necessary. If you wanted a linked list there is the aptly named LinkedList<T> class.

[GnomeTank]

They aren't "pointers" in C#, they are references. This sounds like a non-important distinction, but it is important because C# has actual pointers as well.

.NET does not use reference counting, it uses a generational garbage collector where objects are moved along the generations as they are no longer rooted to another non-collected object. You'll hear the terms "anchored" or "rooted" a lot when dealing with this kind of GC. You'll also hear terms like "gen1 object" or "gen2 object", and this just has to do with where in the GC process the object is. Note that this is all true for the reference implementation of the .NET framework. It's quite possible that projects like Mono have their own implementation, though I am almost positive that Mono uses a very similar generational GC.

That said, your understanding of what's going on from a high level is correct. When you put a reference to your object in to that list, the list now holds it's "root" (or one of them anyway, you may have references in other places). The fact that the variable the reference was assigned to is local means nothing. That local variable will fall out of scope, but the reference it was attached to will not be swept by the GC because you have it anchored in that list.

Edited by GnomeTank, 13 December 2012 - 12:19 PM.