using System;

using System.Linq;

​

namespace Playground

{

    class Program

    {

        static void Main(string[] args)

        {

            // Local variable - mutates ✓

            var val = new Thing();

            Console.WriteLine(val); // 0

            val.Increment();

            Console.WriteLine(val); // 1

​

            // Nested in class - mutates ✓

            var bigThing = new BigThing

            {

                Thing = new Thing(),

            };

            Console.WriteLine(bigThing); // 0

            bigThing.Thing.Increment();

            Console.WriteLine(bigThing); // 1

​

            // In list element - does not mutate the one in the list ✗

            var list = Enumerable.Range(0, 3).Select(_ => new Thing()).ToList();

            Console.WriteLine(string.Join(", ", list)); // 0, 0, 0

            list[0].Increment();

            Console.WriteLine(string.Join(", ", list)); // 0, 0, 0

        }

    }

​

    struct Thing

    {

        public int Number;

​

        public void Increment() => Number++;

        public override string ToString() => $"{Number}";

    }

​

    class BigThing

    {

        public Thing Thing;

​

        public override string ToString() => $"{Thing.Number}";

    }

}