Console.WriteLine("请输入菱形的边长");
            int bianChang = int.Parse(Console.ReadLine());
             int zuo = bianChang, you = bianChang;
            for (int hang = 1; hang <= bianChang; hang++)
            {
                for (int lie = 1; lie <= bianChang * 2 - 1; lie++)
                {
                    if (lie == zuo || lie == you)
                    {
                        Console.Write("*");
                    }else
                    {
                        Console.Write(" ");
                    }
                }
                if (hang != bianChang)
                {
                    Console.WriteLine();
                }
                zuo--;
                you++;
            }
            zuo++;
            you--;
            for (int hang = 1; hang <= bianChang; hang++)
            {
                for (int lie = 1; lie <= bianChang * 2 - 1; lie++)
                {
                    if (hang == 1)
                    {
                        continue;
                    }if (lie == zuo || lie == you)
                    {
                        Console.Write("*");
                    }
                    else
                    {
                        Console.Write(" ");
                    }
                }
                Console.WriteLine();
                zuo++;
                you--;
            }

此处我自己想到的方法（不代表别人没想到过啊）。
思想:先分上下，再分左右，用yuo、you标记要打印菱形的位置。