Something

public void Megkeres(Eloleny e)
        {
            for (int i = 0; i < N; i++)
            {
                for (int j = 0; j < M; j++)
                {
                    if(matrix[i,j] == e)
                    {
                        return new Koord(i,j);
                    }
                }
            }
            throw new Exception("Nincs ilyen!");
        }

        public List<Koord> uresCellak()
        {
            List<Koord> ures = new List<Koord>();
            Koord k = Megkeres();
            for (int i = -1; i < 2; i++)
            {
                for (int j = -1; j < 2; j++)
                {
                    if (matrix[k.x+i,k.y+j] == null)
                    {
                        ures.Add(new Koord(i,j));
                    }
                }
            }
        }