System.Xml readonly annotation

[hrrrrustic]

This PR is a part of #46675

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Issue Details

---

This PR is a part of #46675

Author:	hrrrrustic
Assignees:	-
Labels:	area-System.Xml, new-api-needs-documentation
Milestone:	-

[krwq]

The annotations looks good to me.

[krwq]

Annotations look good to me but I'm not sure that's always a good idea to mark as much as possible as read-only:

• Can caching be added later? I.e. in some scenarios hash code could be cached but caching would modify the state and therefore make it non-readonly and potentially be a breaking change...
• Are we introducing "const correctness" in C#? That's probably one of my most hated features in C++ - I'm not personally in favor of having this in C# - it just makes writing code a pain with small benefit - although this is debatable like "tabs" vs "spaces"

[krwq]

@hrrrrustic we have some offline conversation about this and other similar PRs and we need to figure out the guidelines before we can accept these PRs:

• it would be a breaking change if we try to remove the read-only in the future so this one way and we need to be really careful what we do here
• as mentioned above - caching cannot be added after you mark something read-only
• methods modifying argument but not its own state - they need further thought

likely only subset of the changes can be accepted but we need to figure out what subset exactly. We do agree there is a value in some of those, i.e. marking some structs as read-only but we are not particularly sure about marking methods in general. We will keep you updated.

[jeffhandley]

This PR is being marked as a draft per #46675 (comment).

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Draft Pull Request was automatically closed for inactivity. Please let us know if you'd like to reopen it.