Why dealing with Value is faster than with Reference for Struct?
It maybe unclear for me but, when I read the msdn doc and I try to understand deeply Struct behaviour.
From msdn
Dealing with Stack :
This will yield performance gains.
and :
Whenever you have a need for a type that will be used often and is mostly just a piece of data, structs might be a good option.
I don't understand because, I guess when I pass a Struct in parameter of a method the "copy value" process must be slower than "copy reference" process?
The cost of passing a struct is proportional to its size. If the struct is smaller than a reference or the same size as a reference then passing its value will have the same cost as passing a reference.
If not, then you are correct; copying the struct might be more expensive than copying the reference. That's why the design guidelines say to keep a struct small.
(Note that when you call a method on a struct, the "this" is actually passed as a reference to the variable that contains the struct value; that's how you can write a mutable struct.)
There are potential performance gains when using structs, but as you correctly point out, there are potential performance losses as well. Structs are cheap (in both memory and time) to allocate and cheap to deallocate (in time), and cheap to copy if they are small. References are slightly more expensive in both memory and time to allocate, more expensive to deallocate, and cheap to copy. If you have a large number of small structs -- say, a million Point structs -- then it will be cheaper to allocate and deallocate an array with a million structs in it than an array with a million references to a million instances of a Point class.
But if the struct is big, then all that additional copying might be more expensive than the benefit you get from the more efficient allocation and deallocation. You have to look at the whole picture when doing performance analysis; don't make the "struct vs class" decision on the basis of performance without empirical data to back up that decision.
There is much misinformation on the internet, in our own documentation, and in many books, about how memory management works behind the scenes in C#. If you are interested in learning what is myth and what is reality, I recommend reading my series of articles on the subject. Start from the bottom:
http://blogs.msdn.com/b/ericlippert/archive/tags/memory+management/
Another recommendation for structs is that they should be small; not larger than 16 bytes. That way they can be copied with a single instruction, or just a few instructions.
Copying a reasonably small amount of data will be almost as fast as copying a reference, and then it will be faster for the method to access the data as there is no redirection needed.
If the struct is smaller than a pointer (ie 32 or 64 bits), it will even be faster to copy the value than to copy a reference.
Even if a structure is a bit larger than a reference, there is still some overhead involved with creating objects. Each object has some overhead and has to be allocated as a separate memory block. A byte as a value type takes up just a single byte, but if you box the byte as an object, it will take up 16 or 24 bytes on the heap, plus another 4 or 8 bytes for the reference.
Anyhow, a decision to use a struct or a class should normally be about what kind of data they represent, and not just about performance. A structure works well for data that represent a single entity, so that you can treat it as a single value.
It is true about what you said regarding copy process as in copying a reference takes lesser time than copying a struct as all struct and references are stored on the stack. But the reason why msdn suggests using struct would give a performance gain is the time takes to access the stack and the heap.
If you need a type that contains mostly static data and does is not huge (meaning it does not contain huge arrays, multi dimensional or otherwise, of value types) then it would be wiser to use struct rather than reference types as the access for stack is much lower than the managed heap.
Along with that, the time taken for allocation and deallocation, or you can say in short, the management of heap is somewhat time consuming as compared to the stack.
You can have a better understanding of this topic here and here as this has been explained in detail.
Hope it helps.
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