输入年月日，输出这一天是这一年的多少天

 while (true)
            {
                Console.Write("请输入年份：");
                int a = int.Parse(Console.ReadLine());
                Console.Write("请输入月份：");
                int b = int.Parse(Console.ReadLine());
                Console.Write("请输入几号：");
                int c = int.Parse(Console.ReadLine());
                int d;
                if (a > 9999 || a < 1759 || b <= 0 || b > 12 || c <= 0 || c > 31 || ((b == 4 || b == 6 || b == 9 || b == 11) && c > 30))
                {
                    Console.WriteLine("您输入的日期有误");
                }
                else
                {
                    if (a % 400 == 0 || (a % 100 != 0 && a % 4 == 0))
                    {
                        if (b < 2)
                        {
                            d = c;
                            Console.WriteLine("这是这一年的第" + d + "天");
                        }
                        if (b == 2)
                        {
                            if (c > 29)
                            {
                                Console.WriteLine("您输入的日期有误");
                            }
                            else
                            {
                                d = c + 31;
                                Console.WriteLine("这是这一年的第" + d + "天");
                            }
                        }
                        if (b > 2 && b < 8)
                        {
                            if (b % 2 == 0)
                            {
                                d = b / 2 * 31 + (b / 2 - 2) * 30 + 29 + c;
                                Console.WriteLine("这是这一年的第" + d + "天");
                            }
                            if (b % 2 != 0)
                            {
                                d = (b - 1) / 2 * 31 + ((b - 1) / 2 - 1) * 30 + 29 + c;
                                Console.WriteLine("这是这一年的第" + d + "天");
                            }
                        }
                        if (b >= 8)
                        {
                            if (b % 2 == 0)
                            {
                                d = (b - 8) / 2 * 31 + (b - 8) / 2 * 30 + 31 * 4 + 30 * 2 + 29 + c;
                                Console.WriteLine("这是这一年的第" + d + "天");
                            }
                            else
                            {
                                d = (b - 7) / 2 * 31 + ((b - 7) / 2 - 1) * 30 + 31 * 4 + 30 * 2 + 29 + c;
                                Console.WriteLine("这是这一年的第" + d + "天");
                            }
                        }

                    }

                    else
                    {
                        if (b < 2)
                        {
                            d = c;
                            Console.WriteLine("这是这一年的第" + d + "天");
                        }
                        if (b == 2)
                        {
                            if (c > 28)
                            {
                                Console.WriteLine("您输入的日期有误");
                            }
                            else
                            {
                                d = c + 31;
                                Console.WriteLine("这是这一年的第" + d + "天");
                            }
                        }
                        if (b > 2 && b < 8)
                        {
                            if (b % 2 == 0)
                            {
                                d = b / 2 * 31 + (b / 2 - 2) * 30 + 28 + c;
                                Console.WriteLine("这是这一年的第" + d + "天");
                            }
                            else
                            {
                                d = (b - 1) / 2 * 31 + ((b - 1) / 2 - 1) * 30 + 28 + c;
                                Console.WriteLine("这是这一年的第" + d + "天");
                            }
                        }
                        if (b >= 8)
                        {
                            if (b % 2 == 0)
                            {
                                d = (b - 8) / 2 * 31 + ((b - 8) / 2) * 30 + 31 * 4 + 28 + 30 * 2 + c;
                                Console.WriteLine("这是这一年的第" + d + "天");
                            }
                            else
                            {
                                d = (b - 7) / 2 * 31 + ((b - 7) / 2 - 1) * 30 + 31 * 4 + 28 + 30 * 2 + c;
                                Console.WriteLine("这是这一年的第" + d + "天");
                            }
                        }
                    }
                }
            }