Why should a .NET struct be less than 16 bytes?

Question

I've read in a few places now that the maximum instance size for a struct should be 16 bytes.

But I cannot see where that number (16) comes from.

Browsing around the net, I've found some who suggest that it's an approximate number for good performance but Microsoft talk like it is a hard upper limit. (e.g. MSDN )

Does anyone have a definitive answer about why it is 16 bytes?

Answer 1

It is just a performance rule of thumb.

The point is that because value types are passed by value, the entire size of the struct has to be copied if it is passed to a function, whereas for a reference type, only the reference (4 bytes) has to be copied. A struct might save a bit of time though because you remove a layer of indirection, so even if it is larger than these 4 bytes, it might still be more efficient than passing a reference around. But at some point, it becomes so big that the cost of copying becomes noticeable. And a common rule of thumb is that this typically happens around 16 bytes. 16 is chosen because it's a nice round number, a power of two, and the alternatives are either 8 (which is too small, and would make structs almost useless), or 32 (at which point the cost of copying the struct is already problematic if you're using structs for performance reasons)

But ultimately, this is performance advice. It answers the question of "which would be most efficient to use? A struct or a class?". But it doesn't answer the question of "which best maps to my problem domain".

Structs and classes behave differently. If you need a struct's behavior, then I would say to make it a struct, no matter the size. At least until you run into performance problems, profile your code, and find that your struct is a problem.

your link even says that it is just a matter of performance:

If one or more of these conditions are not met, create a reference type instead of a structure. Failure to adhere to this guideline can negatively impact performance.

Answer 2

If a structure is not larger than 16 bytes, it can be copied with a few simple processor instructions. If it's larger, a loop is used to copy the structure.

As long as the structure is not larger than 16 bytes, the processor has to do about the same work when copying the structure as when copying a reference. If the structure is larger, you lose the performance benefit of having a structure, and you should generally make it a class instead.