[Proposal] Extend readonly members to classes and interfaces

[leandromoh]

We already have readonly members, introduced in C# 8, but it works just on structs.

Can we have extend this feature for classes and interfaces take benefits too?

[HaloFour]

The benefit of readonly members on structs is to inform the compiler that it doesn't have to make a defensive copy when calling those members when the struct is in a memory location that should not be modified, like a readonly field or in parameter. That situation doesn't exist for reference types.

[huoyaoyuan]

If we have some concept of readonly instance of reference type in the future, extending readonly members will be useful then.

[Joe4evr]

Duplicate of #3055. Specifically read this post on why this is very likely never happening.

[leandromoh]

I did not know that for structs the target was compiler. What I have in mind was help developers write/read methods without side-effects, or at least, delimit it.

[333fred]

Closing as a duplicate.