[chipdart]

> C# has this:

This is only syntactic sugar to allow using object initializers to initialize specific member varabiles of a class instance instead of simply using a constructor and/or setting member variables in follow-up statements. It's hardly the feature OP was describing.

[neonsunset]

While mutation of init-only properties is sometimes done by e.g. serializers through private reflection or unsafe accessors, it otherwise can lead to unsound behavior if class implementation does not expect this. You cannot bypass this through normal language means.

Same applies to readonly instance fields.

Where does "syntax sugar" end and "true features" begin?

[bee_rider]

Anyway, trying to actually prevent a program from modifying its own memory is really hopeless, right? So any promises beyond “syntactic sugar” would be resting on a poor foundation, perhaps even dangerously misleading.