c#选择日期

using System;
using System.Windows.Forms;

namespace DateDemo
{
    public partial class Form1 : Form
    {
        private int _currentYearIndex;
        private int _currentMonthIndex;
        private const int StartYear = 1949;
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            DateTime today = DateTime.Today;
            for (var i = today.Year; i >=StartYear; i--)
            {
                cbYear.Items.Add(i);
            }
            for (var i = 1; i < 13; i++)
            {
                cbMonth.Items.Add(i);
            }

            for (var i = 1; i < GetDays(today.Year, today.Month); i++)
            {
                cbDay.Items.Add(i);
            }

            cbYear.SelectedIndex = 0;
            cbMonth.SelectedIndex = today.Month - 1;
            cbDay.SelectedIndex = today.Day - 1;
        }

        private int GetDays(int year, int month)
        {
            var days = 0;
            switch (month)
            {
                case 4:
                case 6:
                case 9:
                case 11:
                    days = 30;
                    break;

                case 2:
                    days = IsLeapYear(year) ? 29 : 28;
                    break;

                default:
                    days = 31;
                    break;
            }

            return days;
        }

        public static bool IsLeapYear(int year)
        {
            return year % 400 == 0 || year % 4 == 0 && year % 100 != 0;
        }

        private void cbYear_SelectedIndexChanged(object sender, EventArgs e)
        {
            _currentYearIndex = cbYear.SelectedIndex;
            int y = DateTime.Today.Year- cbYear.SelectedIndex;
            int m = _currentMonthIndex + 1;
            //            MessageBox.Show(y+"  "+m);
            SettingCbDay(y, m);
            cbDay.SelectedIndex = 0;
        }

        private void SettingCbDay(int y, int m)
        {
            cbDay.Items.Clear();
            for (var i = 1; i <= GetDays(y, m); i++)
            {
                cbDay.Items.Add(i);
            }
        }

        private void cbMonth_SelectedIndexChanged(object sender, EventArgs e)
        {
            _currentMonthIndex = cbMonth.SelectedIndex;
            int y = DateTime.Today.Year - cbYear.SelectedIndex;
            int m = cbMonth.SelectedIndex + 1;
            //            MessageBox.Show(y + "  " + m);
            SettingCbDay(y, m);
            cbDay.SelectedIndex = 0;
        }
    }
}