C#•11mo ago

`readonly` struct, `in` modifier and defensive copies

Hi people, I just encountered the

in

in

modifier for structs.
As far as I'm concerned the

in

in

modifier can negatively affect performance if the struct I'm passing is NOT

readonly

readonly

, because it might create defensive copies - however, how can I avoid defensive copies without marking my struct

readonly

readonly

? Is there a rule of thumb when defensive copies are created?
Thanks in advance!

OdexOP•2/9/25, 10:36 AM

As far as I know the compiler will create defensive copies whenver I'm trying to access mutable data
What if I have private fields and use

readonly

readonly

ter for that? Will the compiler still create defensive copies?

OOdex Hi people, I just encountered the `in` modifier for structs. As far as I'm conc...

Anton•2/9/25, 12:38 PM

this is not true

Anton•2/9/25, 12:39 PM

it will create defensive copies when you try to mutate the struct by calling a non-readonly method

Anton•2/9/25, 12:39 PM

that's it

Anton•2/9/25, 12:39 PM

so mark all methods that don't mutate the struct as readonly

OdexOP•2/9/25, 12:40 PM

what about fields?

Anton•2/9/25, 12:40 PM

not fields, just methods

Anton•2/9/25, 12:40 PM

and get set properties, because those are methods

Anton•2/9/25, 12:40 PM

you won't be able to set fields

Anton•2/9/25, 12:40 PM

and reading them can't mutate the struct

OdexOP•2/9/25, 12:41 PM

OH! Now everything's clear

OdexOP•2/9/25, 12:42 PM

So it's safe to access fields when passing a struct with

in

in

modifier, since I can't change them (and reading won't mutate)
And it's also safe to access methods as long as they are

readonly

readonly

Anton•2/9/25, 12:42 PM

yes

Anton•2/9/25, 12:43 PM

the whole struct doesn't have to be readonly either

OdexOP•2/9/25, 12:43 PM

Thanks dude! I couldn't find an exact explanation online

Anton•2/9/25, 12:43 PM

you're welcome

OdexOP•2/17/25, 10:08 AM

@Anton Does this apply to only getters as well?
Because the property of arrays it not

readonly

readonly

, so I'm concerned that doing something like this

will slow down performance

OdexOP•2/17/25, 10:09 AM

On second thought, I don't need the

in

in

modifier at all for arrays of structs, since they're still passed by reference because arrays are allocated in the heap, right?

OdexOP•2/17/25, 10:10 AM

So do you recommend me removing the

in

in

modifier when passing arrays (even if it's an array of struct)?

OdexOP•2/17/25, 10:11 AM

Again, my priority is performance and avoid as many useless copies as possible

Anton•2/17/25, 12:56 PM

arrays are references already

Anton•2/17/25, 12:57 PM

use or for arrays (depending on whether you need to use or )

Anton•2/17/25, 12:57 PM

autogetters are readonly

Anton•2/17/25, 12:58 PM

iirc

Anton•2/17/25, 12:58 PM

automatically

Anton•2/17/25, 12:58 PM

regular getters that have code need to be specified as readonly manually

Anton•2/17/25, 12:58 PM

for arrays there's no copy because it's a reference type

Anton•2/17/25, 12:59 PM

in your code you're passing an immutable reference to a reference to your array as a parameter

Anton•2/17/25, 12:59 PM

that's just a pointless level of indirection

Anton•2/17/25, 1:01 PM

when you index the array, assign the result to a variable to avoid a copy