这个怎么修改啊。初学者。

Console.WriteLine("请输入想要查询的年份：");
int year = Convert.ToInt32(Console.ReadLine());
Console.WriteLine("请输入想要查询的月份：");
int month = Convert.ToInt32(Console.ReadLine());
int day;
bool flag=false;
switch (month)
{
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:
day = 31;
break;
case 4:
case 6:
case 9:
case 11:
day = 30;
break;
case 2:
if ((year % 400 == 0) || (year % 4 == 0) && (year % 100 != 0))
day = 29;
else
day = 28;
break;
default:
Console.WriteLine("你输入的月份有误！");
flag = true;
break;
}
if (flag == false)
Console.WriteLine("你所查询的月份为{0}天。",day);
Console.ReadKey();
问题是 最后运行说使用了未赋值的局部变量 day