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?
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.
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.
The size figure comes largely from the amount of time it takes to copy the struct on the stack, for example to pass to a method. Anything much larger than this and you are consuming a lot of stack space and CPU cycles just copying data - when a reference to an immutable class (even with dereferencing) could be a lot more efficient.
Here is a scenario where structs can exhibit superior performance:
When you need to create 1000s of instances. In this case if you were to use a class, you would first need to allocate the array to hold the 1000s of instances and then in a loop allocate each instance. But instead if you were to use structs, then the 1000s of instances become available immediately after you allocate the array that is going to hold them.
In addition, structs are extremely useful when you need to do interop or want to dip into unsafe code for performance reasons.
As always there is a trade-off and one needs to analyze what they are doing to determine the best way to implement something.
ps: This scenario came into play when I was working with LIDAR data where there could be millions of points representing x,y,z and other attributes for ground data. This data needed to be loaded into memory for some intensive computation to output all kinds of stuff.
I think the 16 bytes is just a rule of thumb from a performance point of view. An object in .NET uses at least 24 bytes of memory (IIRC), so if you made your structure much larger than that, a reference type would be preferable.
I can't think of any reason why they chose 16 bytes specifically.
