using System;

using System.Linq;

using System.Collections.Generic;

using System.Collections;

public class Program

{

    public static void Main()

    {

        List<(int, int)> myPairs = [(1, 2), (3, 4), (5, 6)];

        Console.WriteLine($"The list [{string.Join(", ", myPairs)}]");

        var secondThings = new ProjectedMutableList<(int, int), int>(myPairs, selector: x => x.Item2, modify: (pair, newYCoord) => (pair.Item1, newYCoord));

        Console.WriteLine($"All the second items of each pair in the list: [{string.Join(", ", secondThings)}]");

        secondThings[1] = 17;

        Console.WriteLine($"All the second items of each pair in the list: [{string.Join(", ", secondThings)}]");

    }

    public class ProjectedMutableList<T, V>(List<T> things, Func<T, V> selector, Func<T, V, T> modify) : IList<V>

    {

       public V this[int i] 

       {

           get { return selector(things[i]); }

           set { things[i] = modify(things[i], value); }

       }

       public int IndexOf(V val) => things.FindIndex(x => Equals(selector(x), val));

       public void RemoveAt(int index) => things.RemoveAt(index);

       public int Count => things.Count;

       public void Clear() => things.Clear();

       public bool Contains(V val) => IndexOf(val) >= 0;

       public IEnumerator<V> GetEnumerator() => things.Select(selector).GetEnumerator();

       IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();     

       public bool Remove(V val)

       {

           int index = IndexOf(val);

           if (index >= 0)

           {

              RemoveAt(index);

              return true;

           }

           return false;

       }

       public bool IsReadOnly => false;

       public void CopyTo(V[] dst, int index) => throw new NotSupportedException();    

       public void Insert(int index, V val) => throw new NotSupportedException();

       public void Add(V val) => throw new NotSupportedException();

    }

}