using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var box = new Box();

        foreach( var rw in box.Editable )

            rw.Dummy = "Edited";

        foreach( var ro in box.Readonly )

            Console.WriteLine(ro.Dummy);

    }

}

​

public interface IReadonlyBanana

{

    string Dummy {get;}

}

​

public class Banana : IReadonlyBanana

{

    public string Dummy {get; set;} = "Test";

}

​

public class Box

{

    private readonly List<Banana> _bananas = new(){ new(), new() };

    public IEnumerable<IReadonlyBanana> Readonly =>

        _bananas.Cast<IReadonlyBanana>().AsEnumerable();

    public IEnumerable<Banana> Editable =>

        _bananas.AsEnumerable();

}