ArmedCards.Entities.GameRound.ValidateWinnerSelection C# (CSharp) Method

        public List<Int32> ValidateWinnerSelection(List<Int32> cardIDs)
        {
            List<Int32> invalidCardIDs = new List<Int32>();

            Int32 indexOf = -1;

            foreach (Int32 cardID in cardIDs)
            {
                indexOf = Answers.FindIndex(x => x.Card_CardID == cardID);

                if (indexOf < 0)
                {
                    invalidCardIDs.Add(cardID);
                }
                else
                {
                    Answers[indexOf].Winner = true;
                }
            }

            return invalidCardIDs;
        }