Console.Write("nhap size: ");
            int size = int.Parse(Console.ReadLine());
            int nhan = size;
            Console.WriteLine();
            for (int i = 1; i <= size; i++, nhan--)
            {
                for (int a = 1; a <= i; a++)
                {
                    Console.Write(" ");
                }
                for (int b = nhan; b >= 1; b--)
                {
                    if (b == 1 || b == nhan)
                    {
                        Console.Write(" *");
                    }
                    else
                    {
                        Console.Write("  ");
                    }
                }
                Console.WriteLine();
            }
            Console.ReadKey();

Code của bạn nhé

class Program
    {
        static string userChar = "*";
        static void Main(string[] args)
        {
            V(20);
        }

        public static void V(int width)
        {
            int lines = width / 2;

            for (var i = 1; i < lines; i++)
                Console.WriteLine(userChar.PadLeft(i) + userChar.PadLeft(2 * (lines - i)));
            Console.WriteLine(userChar.PadLeft(lines));            
        }
    }