输入一个年月日，判断是这一年的第几天？
Console.WriteLine("请输入年：");
int year = int.Parse(Console.ReadLine());
Console.WriteLine("请输入月：");
int month = int.Parse(Console.ReadLine());
Console.WriteLine("请输入日：");
int day = int.Parse(Console.ReadLine());
int sum=0;
if ((year % 400 == 0) || ((year % 100 != 0) && (year % 4 == 0)))
{
switch (month)
{
case 1:
sum =day;
break;
case 2:
sum =day + 31;
break;
case 3:
sum =day + 31 + 29;
break;
case 4:
sum =day + 31 + 29 + 31;
break;
case 5:
sum =day + 31 + 29 + 31 + 30;
break;
case 6:
sum =day + 31 + 29 + 31 + 30 + 31;
break;
case 7:
sum =day + 31 + 29 + 31 + 30 + 31 + 30;
break;
case 8:
sum =day + 31 + 29 + 31 + 30 + 31 + 30 + 31;
break;
case 9:
sum =day + 31 + 29 + 31 + 30 + 31 + 30 + 31 + 31;
break;
case 10:
sum =day + 31 + 29 + 31 + 30 + 31 + 30 + 31 + 31 + 30;
break;
case 11:
sum =day + 31 + 29 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31;
break;
case 12:
sum =day + 31 + 29 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30;
break;
default:
Console.WriteLine("输入错误");
break;
}
}
else
{
switch (month)
{
case 1:
sum =day;
break;
case 2:
sum =day + 31;
break;
case 3:
sum = day + 31 + 28;
break;
case 4:
sum =day + 31 + 28 + 31;
break;
case 5:
sum =day + 31 + 28 + 31 + 30;
break;
case 6:
sum =day + 31 + 28 + 31 + 30 + 31;
break;
case 7:
sum =day + 31 + 28 + 31 + 30 + 31 + 30;
break;
case 8:
sum =day + 31 + 28 + 31 + 30 + 31 + 30 + 31;
break;
case 9:
sum =day + 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31;
break;
case 10:
sum =day + 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30;
break;
case 11:
sum =day + 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31;
break;
case 12:
sum =day + 31 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 + 31 + 30;
break;
default:
Console.WriteLine("输入错误");
break;
}
}
Console.WriteLine("这是{0}年的第{1}天",year,sum);
原文：http://www.cnblogs.com/yangyue/p/4158470.html