What is the memory overhead of an Object in .NET? I'm talking about an arbitrary bare-bones object.... the overhead of the internal .NET workings or references:
var obj = new System.Object();
How much space does obj occupy in the heap?
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1Heap and stack are implementation details.asawyer– asawyer2012-05-18 15:53:13 +00:00Commented May 18, 2012 at 15:53
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3@ThePower: Not a duplicate. That other question (and its answers) don't talk about the basic memory overhead of a .NET object at all.Robert Harvey– Robert Harvey2012-05-18 15:55:43 +00:00Commented May 18, 2012 at 15:55
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2@ThePower It doesn't answer my question, I want to know the memory overhead, not how to measure itreach4thelasers– reach4thelasers2012-05-18 15:57:07 +00:00Commented May 18, 2012 at 15:57
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3@asawyer - could you explain your comment? I thought a heap was an abstract data structure in which all objects are stored... why is it wrong to ask "How much space does it occupy in the heap"?reach4thelasers– reach4thelasers2012-05-18 16:02:24 +00:00Commented May 18, 2012 at 16:02
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3How does: "You could use a memory profiler like .NET Memory Profiler...." answer my question????reach4thelasers– reach4thelasers2012-05-18 16:03:50 +00:00Commented May 18, 2012 at 16:03
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1 Answer
I talk about this in a blog post "Of memory and strings". It's implementation-specific, but for the Microsoft .NET CLR v4, the x86 CLR has a per-object overhead of 8 bytes, and the x64 CLR has a per-object overhead of 16 bytes.
However, there are minimum sizes of 12 and 24 bytes respectively - it's just that you get the first 4 or 8 bytes "free" when you start storing useful information :)
(See the blog post for more information.)
4 Comments
RBT
There may be padding for member variables to fit on address boundaries as well. This padding concept further complicates the calculation of memory overhead of an object. No wonder C# never introduced sizeof operator for instance variables for types.
JohnF
I ran a few tests in a .net console app to check memory usage. Creating 10 million empty objects resulted in ca 340MB usage (includes the list with references). Adding 1 or 2 integers to the class changed nothing. Only when adding the 3rd integer did I see an increase to ca 430MB. This fully supports your statement of 24 bytes minimum with 8 bytes free for information.
BorisD
Hi @JonSkeet could you add a generic definition for "memory overhead"? I dont really understand what it is. It's like an extra memory allocated for each object? Thank you
Jon Skeet
@BorisD: Yes - it's the memory required in addition to the "obvious" memory required for the data itself. (Housekeeping information etc.)