Typesafe bivariant arrays in C#

BivariantArrays.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace BivariantArrays
{
    public interface IReadable<in T>
    {
        public T this[int i] { get; }
    }

    public interface IWriteable<out T>
    {
        public T this[int i] { set; }
    }

    public interface IReadWrite<T> : IReadable<T>, IWriteable<T>
    {
    }

    public class EString
    {
        protected string contents;
        public EString(string contents) {
            this.contents = contents;
        }
        public char this[int i] {
            get {
                return contents[i];
            }
        }
    }

    public class LengthString : EString
    {
        public LengthString(string contents) : base(contents) { }
        public int Length {
            get {
                return contents.Length;
            }
        }
    }

    public class PadString : EString
    {
        private char pad;
        public PadString(string contents, char pad) : base(contents) {
            this.pad = pad;
        }
        public override PadString(string contents) : this(contents, ' ') {

        }
        public override char this[int i]
        {
            get {
                if (i < 0 || i >= contents.Length) {
                    return pad;
                } else {
                    return contents[i];
                }
            }
        }
    }

    public class BivariantArray<T> : IReadWrite<T>
    {
        private T[] contents;

        public BivariantArray(int capacity) {
            contents = new T[capacity];
        }
        public T this[int i]
        {
            get {
                return contents[i];
            }
            set {
                contents[i] = value;
            }
        }
        public static void Main() {
            BivariantArray<EString> estrings = new BivariantArray<EString>(1);
            BivariantArray<PadString> pstrings = new BivariantArray<PadString>(1);
            BivariantArray<LengthString> lstrings = new BivariantArray<LengthString>(1);

            pstrings[0] = new PadString("dog");
            lstrings[0] = new LengthString("cat");
            estrings[0] = new EString("mammal");
            
            estrings[0] = new PadString("wolf");
     /* XXX pstrings[0] = new EString("quadruped");  Fails typecheck */
     /* XXX pstrings[0] = new LengthString("tiger"); Fails typecheck */
            EString dog2 = pstrings[0]; 
     /* XXX PadString mammal2 = estrings[0];         Fails typecheck */
     /* XXX LengthString dog3 = pstrings[0];         Fails typecheck */
        }
    }
}