protected void Page_Load(object sender, EventArgs e)
...{
if (!IsPostBack)
...{ DateTime dt = DateTime.Now; DropDownList2.Items.Add(dt.Year.ToString()); DropDownList3.Items.Add(dt.Month.ToString()); DropDownList4.Items.Add(dt.Day.ToString()); DropDownList4.Items.Add("23");
} 
}//加减乘除 protected void Button1_Click(object sender, EventArgs e) ...{ int a = int.Parse(TextBox1.Text); int b = int.Parse(TextBox2.Text); int c = 0; switch(DropDownList1.SelectedValue) ...{ case "+": c = a + b; Label2.Text = c.ToString(); break; case "-": c = a - b; Label2.Text = c.ToString(); break; case "*": c = a * b; Label2.Text = c.ToString(); break; case "/": c = a / b; Label2.Text = c.ToString(); break; } }//DropDownList 改变index时发生的事件 protected void DropDownList4_SelectedIndexChanged(object sender, EventArgs e) ...{ Response.Write(DropDownList2.SelectedValue+"-"+ DropDownList3.SelectedValue+"-"+DropDownList4.SelectedValue); } //CheckBoxList 改变index时发生的事件 protected void CheckBoxList1_SelectedIndexChanged(object sender, EventArgs e) ...{ for (int i = 0; i < CheckBoxList1.Items.Count; i++) ...{ if (CheckBoxList1.Items[i].Selected) //注意启用postback，这样可以实时观测 ...{ Response.Write(CheckBoxList1.Items[i].Text); } } }