C# array of consecutive objects in memory?

Question

So my understanding of C# is that when dealing with an array of objects (rather than simple types) the array will simply be a consecutive array of references to objects, hence the need to call 'new' on each element to actually create the object for it.

Block[] blocks = new Block[10];

foreach(Block block in blocks)
{
    block = new Block();
}

This I would imagine would spread 'Block' instances all over memory.

I want my actual objects to be stored consecutively in memory is there any way to do this in C#?

You imagine wrong. block does not point to an element in the array. It is pretty much assigning to a local variable. (IIRC, C# disallows this too). — leppie
– leppie, Commented Jul 22, 2012 at 11:19
The Garbage Collector can move objects at any time if it feels the need for it. you should not worry about the physical location in memory. — huysentruitw
– huysentruitw, Commented Jul 22, 2012 at 11:21
Ok so I'm actually wanting to implement a similar class to C#'s System.Array for an array of garbage collected objects in C++. However for performance reasons due to cache coherency I want my objects in one particular version to be stored consecutively. I find its usually a good idea in these instances to refer to Microsoft's implementation of such things and I just wanted to know if they had actually implemented an array of consecutive objects. — user176168
– user176168, Commented Jul 22, 2012 at 11:23
@user176168 This sounds like premature optimization. If you really need this, look at the unsafe keyword — Oskar Kjellin
– Oskar Kjellin, Commented Jul 22, 2012 at 11:27

Sergey Kalinichenko · Accepted Answer · 2012-07-22 11:19:11Z

2

If you make your objects value types (i.e. structs), they will be stored next to each other in an array, the same way the instances of primitives (int, double, etc.) are stored. For example, if you declare

DateTime d[] = new DateTime[10];

your DateTime objects will be stored consecutively in a memory block, because DateTime is a value type. Note that you woulnd't need to call a constructor either: all structs in your array will be initialized and ready to be used.

answered Jul 22, 2012 at 11:19

Sergey Kalinichenko

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2 Comments

leppie Over a year ago

You might want to mention the structs will be initialized too, hence no need for new.

Sergey Kalinichenko Over a year ago

@leppie Thanks, I was going to get to that part. I'm a relatively slow typer.

zmbq · Accepted Answer · 2012-07-22 11:27:29Z

1

You're probably worried about L1 cache misses, right?

You're out of luck. Allocating several objects one after the other will most likely result in the objects being consecutive in memory. Unfortunately, when the GC does its job, you might end up with the objects being all over the place.

EDIT: A suggestion

If you really must have them all in one region, you can either make them value types and create an array, or resort to unsafe code and fix them in memory.

edited Jul 22, 2012 at 11:27

answered Jul 22, 2012 at 11:19

zmbq

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2 Comments

user176168 Over a year ago

Well you're entirely right on the L1 cache miss reason its just I'm not actually doing this in C# and am doing this in C++ so I could conceivably get my garbage collector to allocate a single block of objects which is then treated as one object by the GC but externally is treated as an array of referenced of objects. I just wanted to see how MS deals with this sort of thing.

zmbq Over a year ago

You can tell the GC to fix objects in memory (you need unsafe code for that). I think you'd be better off by using structures instead.

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