Combobox的使用，日期选择器

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Threading;

namespace Combobox
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
}

private void Form1_Load(object sender, EventArgs e)
{
int year = DateTime.Now.Year;

for (int i = 0; i <= 20; i++ )
{
cboYear.Items.Add((year - i)+"年");
}

}

private void button1_Click(object sender, EventArgs e)
{
cboYear.Items.Remove("2");

}

private void button2_Click(object sender, EventArgs e)
{
cboYear.Items.Clear();
}

private void cboYear_SelectedIndexChanged(object sender, EventArgs e)
{
if (cboMonth.Items.Count != 0)
return;
for(int i = 1; i <= 12; i++)
{
cboMonth.Items.Add(i + "月");
}
}

private void cboMonth_SelectedIndexChanged(object sender, EventArgs e)
{
cboDay.Items.Clear();
int day = 0;

//cboMonth.SelectedItem.ToString() 返回1月，要去除月
//Split(new char[] {'月'},StringSplitOptions.RemoveEmptyEntries) 返回数组
string strMonth = cboMonth.SelectedItem.ToString().Split(new char[] {'月'},StringSplitOptions.RemoveEmptyEntries)[0];
string strYear = cboYear.SelectedItem.ToString().Split(new char[] {'年'},StringSplitOptions.RemoveEmptyEntries)[0];

int year = Convert.ToInt32(strYear);
int month = Convert.ToInt32(strMonth);

switch(month)
{
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12: day = 31;
break;
case 2:
if ((year % 400 == 0) || year % 4 == 0 && year % 100 != 0)
day = 29;
else
day = 28;
break;
default:
day = 30;
break;
}

for (int i = 1; i <= day; i++)
{
cboDay.Items.Add(i + "日");
}
}
}
}