An example Sitecore Multilist with an external datasource

CryptoMultilist.cs

namespace FluxDigital.Sitecore.Extensions.Fields
{
    public class CryptoMultilist : BaseExternalMultiList
    {
        private List<CryptoCoin> _coinList;

        protected override void InitRendering()
        {
            CryptoService cryptoService = new CryptoService();
            _coinList = cryptoService.CointList();
        }

        protected override IEnumerable<KeyValuePair<string, string>> GetNonSelectedItems()
        {
            var nonSelectedGroups = new List<KeyValuePair<string, string>>();
            _coinList.ForEach(coin => nonSelectedGroups.Add(new KeyValuePair<string, string>(coin.Id, coin.CoinName)));

            return nonSelectedGroups;
        }

        protected override IEnumerable<KeyValuePair<string, string>> GetSelectedItems()
        {
            var selectedCoinIds = Value.Split('|');
            var selectedCoins = new List<KeyValuePair<string, string>>();
            foreach (var selectedGroupId in selectedCoinIds)
            {
                var coin = _coinList.FirstOrDefault(g => g.Id == selectedGroupId);
                if (coin != null) selectedCoins.Add(new KeyValuePair<string, string>(coin.Id, coin.CoinName));
            }

            return selectedCoins;
        }
    }
}