ref or value class for a list of 20000 objects

Question

I would need to maintain a list of about 20000 objects of a class. This class would be a managed wrapper for a c++ one. I would like to know if a ref class or a value class is they way to go.

I was told the following for pros/cons w.r.t GC

In the case of a value class, there is a single point to garbage collect vs reference class with a pointer, where there would be 20000 objects to garbage collect. Also there is no finalizer or destructor for value class and hence these objects wont end up in the finalization queue which is single threaded.

Can someone elaborate on the functioning of the GC in both these cases for large lists of objects.

What kind of class is it? It is important to know the "size" of the class before thinking about using a ref class or a value class — Yet Another Geek, Apr 14 '11 at 09:42
@Hans: I'm in agreement with that 16 bytes, but I've not seen an authoritative reference on exactly why. Do you have one? — Johann Gerell, Apr 14 '11 at 10:58
@Johann - it doesn't fit in available cpu registers anymore beyond this on the x86 jitter. Copying gets expensive. — Hans Passant, Apr 14 '11 at 11:00
@Hands: `rep movsd` doesn't use the registers. It instructs the memory controller to transfer the data. (The contents are actually never in the CPU registers). I'm sure the mem controller will saturate cachelines (depending on architecture really) otherwise available for CPU, but the '16 bytes because of register capacity' argument fails to ... _register_ with me — sehe, Apr 14 '11 at 12:26
@Johann Gerell : 16 bytes is the long-standing recommendation of Rico Mariani, who was the original CLR performance guru at Microsoft. If you look through his blog you'll find the original citation. — ildjarn, Apr 14 '11 at 13:46
@ildjarn: He's always been my reference on this since I read it many years ago, but I don't remember him explaining *why* and also, a lot has happened to .NET since then. — Johann Gerell, Apr 14 '11 at 13:59
@Johann Gerell : Here's a *relatively* recent explanation, sounds plausible: http://stackoverflow.com/q/2437925/ — ildjarn, Apr 15 '11 at 04:15
@Sriram: Is the .NET wrapper 100 bytes (why so big), or is that the size of the native structure it points to? — Ben Voigt, Apr 15 '11 at 23:59

ali_bahoo · Accepted Answer · 2011-04-14T11:02:33.527

2

In an answer of a question which I could not find at SO, @Hans Passant suggested that a .net struct (value class of C++ Cli) should be 16 bytes atmost.

So do not hesitate, use ref class.

Here is a useful link.

edited Apr 14 '11 at 11:02

answered Apr 14 '11 at 10:54

ali_bahoo

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score 0 · Answer 2 · answered Apr 14 '11 at 12:32

It's definitely easiest to use reference types - the 'Don't make me think' balance really tips this way

FWIW, I can perfectly see myself allocating a buffer of roughly 2Mb to contain an array of valuetype objects.

The thing is, how many times will these have to be transferred. If it just for a lookup table, the array of value objects will win a bit of performance, and perhaps other benefits[1]. This could be useful in a 'FlyWeight' kind of pattern

Just take care to only pass indexes to such objects around or the copying will soon make life miserable on your application

[1] e.g. quick serialization; the array indexes are under the control of the app developer, in contrast to the identity of a reference value. This can make for very efficient 'shallow' serialization of such lookup items by index value, which is useful in a marshalling type of use case (instead of actually marshalling the full reference object, you can refer to it by using the 'ubiquitous constant ID' that you have designed for that purpose).

ref or value class for a list of 20000 objects

2 Answers2