webform的三级联动

  for (int i = DateTime.Now.Year; i >= 1900; i--)
            {
                ListItem li = new ListItem(i.ToString());
                Year_Dr.Items.Add(li);
            }

            for (int i = 1; i <= 12; i++)
            {
                ListItem li = new ListItem(i.ToString());
               Mouth_dr.Items.Add(li);
            }

            Days();
           Mouth_dr.SelectedValue = DateTime.Now.Month.ToString();
           Day_dr.SelectedValue = DateTime.Now.Day.ToString();
        }

        Year_Dr.SelectedIndexChanged +=Year_Dr_SelectedIndexChanged;
        Mouth_dr.SelectedIndexChanged +=Mouth_dr_SelectedIndexChanged;
//方法及事件

 private void Days()
    {
        for (int i = 1; i <= 31; i++)
        {
            int year = Convert.ToInt32(Year_Dr.SelectedValue);
            int mouth = Convert.ToInt32(Mouth_dr.SelectedValue);
            if (mouth == 1 || mouth == 3 || mouth == 5 || mouth == 7 || mouth == 8 || mouth == 10 || mouth == 12)
            {
            }
            else if (mouth == 4 || mouth == 6 || mouth == 9 || mouth == 11)
            {
                if (i == 31)
                {
                    break;
                }
            }
            else if (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0))
            {
                if (i == 30)
                {
                    break;
                }
            }
            else
            {
                if (i == 29)
                {
                    break;
                }
            }
            ListItem li = new ListItem(i.ToString());
            Day_dr.Items.Add(li);
        }
    }

void Mouth_dr_SelectedIndexChanged(object sender, EventArgs e)
{
    Day_dr.Items.Clear();
    Days();
    
}

void Year_Dr_SelectedIndexChanged(object sender, EventArgs e)
{
    Mouth_dr.Items.Clear();
    Day_dr.Items.Clear();
    for (int i = 1; i <= 12; i++)
    {
        ListItem li = new ListItem(i.ToString());
        Mouth_dr.Items.Add(li);
    }

    Days();
}