Immutability: record class vs record struct

[melanchall]

I wonder why record class (or just record) provides immutable type while record struct mutable one? Is it intended or it's a design fault?

I suppose when records were introduced they were supposed to be immutable types. And there were no thoughts that mutability will be needed later so record struct were introduced as more flexible types. Changing the behavior of record class would cause breaking changes since users are already rely on immutability of such types.

I'm asking because from my point of view class and struct imply just value type/reference type difference. It's not about mutability/immutability.

[333fred]

Is it intended or it's a design fault?

Very intentional. The issue with mutability in reference types is unintentional mutation far from where you are creating an using the record, which can cause many fun issues (such as losing a key in a dictionary). Value types are copied by value, not by reference, so any mutations you make only affect your copy of the data, not anyone else's. Passing a value type by ref is a conscious and visible choice in the code, so mutations from that are obvious and obviously intended.

[TahirAhmadov]

You can make a readonly record struct:

https://sharplab.io/#v2:EYLgxg9gTgpgtADwGwBYA0AXEUCuA7AHwAEAmARgFgAoagMQggAIAzBxgXkbxgHdH6IACgBEAQ2BhhASgDc1VhAB0AZQxQOjYQBMYzYXKpEyATkEKVa2dWqxRWiHgA2AT0axIULYwDOanGAx+BkFVKABLPABzRlDZIA=

[melanchall]

Yes, I know. My question was about default behavior without additional modifiers.

[CyrusNajmabadi]

Right. The defaults match the expected behavior of structs for the language. The other has control of the layout. This is safe as clients can only operate on a copy of it unless the owner explicitly gives them access to it's location (which is not the default for how the language normally works).

As such, this is safe and fast. We followed a similar approach for things like tuples.

When the owner themselves wants protection from themselves, we have the readonly modifier.