DropDownList控件的问题

DropDownList控件的SelectedIndexChanged时间，该怎样写才有效呢，为什么我写了以后，在改变DropDownList控件的选项时总是无效，下面是我写的部分代码，请朋友们帮忙指正一下，谢谢。
public partial class _Default : System.Web.UI.Page
{
    //连接数据库的字符串
    String myConnetionString = "server=E65B726CAB1141D;Initial Catalog=peoples;User ID=Sa;pwd=19830125;";
    SqlConnection myConnetion = new SqlConnection();
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
            repeaterBind();
    }
    //绑定数据源
    private void repeaterBind()
    {
        myConnetion.ConnectionString = myConnetionString;
        string city = DropDownList1.SelectedItem.ToString();
        string selectCommend = "SELECT * FROM people_1 WHERE usercity='" + city + "'";
        SqlDataAdapter myAdapter = new SqlDataAdapter(selectCommend, myConnetion);
        myConnetion.Open();
        DataSet mySet = new DataSet();
        myAdapter.Fill(mySet, "people_1");
        Repeater1.DataSource = mySet;
        Repeater1.DataBind();
        myConnetion.Close();
    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)  //这个事件到底该怎么写才有效呢？
    {
        repeaterBind();
    }
}