mesing-around-with-ref-return.cs

using System;

public interface Base {
    int Property { get; }
    
    void Mutate(int someValue);
}

public struct SuperA : Base {
    public int Property { get; private set; }
    
    public void Mutate(int someValue) {
        Property = someValue;
    }
}

public struct SuperB : Base {
    public int Property {get;}
    public void Mutate(int someValue) { /* no-op */ }
}

public abstract class ObjBase {
    public abstract ref readonly Base Value { get; }
}

public class Obj : ObjBase {
    private Base _property = new SuperA();
    
    public override ref readonly Base Value => ref _property;
}

public class Program {
    public static void Main(String[] args) {
        ObjBase o = new Obj();
        o.Value.Mutate(42);
        Console.WriteLine(o.Value.Property); // 42
        
        // This compile when making ObjBase.Value ref-return
        // instead of ref-reaonly-return.
        // o.Value = new SuperB();
        o.Value.Mutate(666);
        Console.WriteLine(o.Value.Property); // 0 if not readonly, 666 otherwise
    }
}