how to Implement the IReadOnlyList interface to let other classes read from your internal list.

        
        public class PokemonParty : IReadOnlyList<Pokemon>
        {
            private List<Pokemon> internalList = new List<Pokemon>();

            public PokemonParty()
            {
            }

            Pokemon IReadOnlyList<Pokemon>.this[int index] => internalList[index];

            public int Count => internalList.Count;

            public IEnumerator GetEnumerator()
            {
                return internalList.GetEnumerator();
            }

            IEnumerator<Pokemon> IEnumerable<Pokemon>.GetEnumerator()
            {
                return internalList.GetEnumerator();
            }
        }

        
        public class PokemonParty : IReadOnlyList<Pokemon>
        {
            private List<Pokemon> internalList = new List<Pokemon>();

            public PokemonParty()
            {
            }

            Pokemon IReadOnlyList<Pokemon>.this[int index] => internalList[index];

            public int Count => internalList.Count;

            public IEnumerator GetEnumerator()
            {
                return internalList.GetEnumerator();
            }

            IEnumerator<Pokemon> IEnumerable<Pokemon>.GetEnumerator()
            {
                return internalList.GetEnumerator();
            }
        }

public void Add(Pokemon pokemon) {
  internalList.Add(pokemon);
}

public void Remove(Pokemon pokemon) {
  internalList.Remove(pokemon);
}

public void Add(Pokemon pokemon) {
  internalList.Add(pokemon);
}

public void Remove(Pokemon pokemon) {
  internalList.Remove(pokemon);
}

public bool IsPokemonInParty(IReadOnlyList<Pokemon> party, Pokemon pokemon)
{
  foreach(var partyMember in party) 
  {
    if (partyMember == pokemon)
      return true;
  }

  return false;
}

public bool IsPokemonInParty(IReadOnlyList<Pokemon> party, Pokemon pokemon)
{
  foreach(var partyMember in party) 
  {
    if (partyMember == pokemon)
      return true;
  }

  return false;
}

var party = new PokemonParty();
var pikachu = new Pokemon( /* Omitted: parameters for creating a new Pokemon */ );

party.Add(pikachu);
// Omitted: Add more Pokemon

var isInParty = IsPokemonInParty(party, pikachu);
Console.WriteLine(isInParty);      // TRUE

var party = new PokemonParty();
var pikachu = new Pokemon( /* Omitted: parameters for creating a new Pokemon */ );

party.Add(pikachu);
// Omitted: Add more Pokemon

var isInParty = IsPokemonInParty(party, pikachu);
Console.WriteLine(isInParty);      // TRUE