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 a

Answer 1

As other answers have noted, the per-byte cost of copying a structure which is larger than a certain threshold (which was 16 bytes in earlier versions of .NET, but has since grown to 20-24) is significantly greater than the per-byte cost of a smaller structure. It's important to note, however, that copying a structure of any particular size once will be a fraction of the cost creating a new class object instance of that same size. If a struct would be copied many times in its lifetime, and the value-type semantics are not particularly required, a class object may be preferable. If, however, a struct would end up being copied only once or twice, such copying would likely be cheaper than the creation of a new class object. The break-even number of copies where a class object would become cheaper varies with the size of the struct/object in question, but is much higher for things that are below the "cheap copying" threshold, than for things above.

BTW, another point worth mentioning is that the cost of passing a struct as a ref parameter is independent of the size of the struct. In many cases, optimal performance may be achieved by using value types and passing them by ref. Once must be careful to avoid using properties or readonly fields of structure types, however, since accessing either of those will create an implicit temporary copy of the struct in question.