<asp:DropDownListID="ddl1"runat="server">
</asp:DropDownList>

protected void Page_Load(object sender, EventArgs e)
{
  BindDropDownList();
}
  
private void BindDropDownList()
{
  ddl1.Items.Clear(); //每次绑定前，先清除所有项
  for (int i = 1; i <= 3; i++)
  {
    ListItem item1 = new ListItem();
    item1.Text = "第" + i.ToString() + "项";
    item1.Value = "第" + i.ToString() + "项";
    ddl1.Items.Add(item1);
  }
}

protected void Page_Load(object sender, EventArgs e)
{
  if (!IsPostBack)
  {
    BindDropDownList();
  }
}

<asp:DropDownList ID="ddl1" runat="server">
</asp:DropDownList>
 <asp:Button ID="btnGet" runat="server" Text="获取" onclick="btnGet_Click" />

protected void Page_Load(object sender, EventArgs e)
{
  if (!IsPostBack)
  {
    BindDropDownList();
  }
}
  
private void BindDropDownList()
{
  ddl1.Items.Clear(); //每次绑定前，先清除所有项
  for (int i = 1; i <= 3; i++)
  {
    ListItem item1 = new ListItem();
    item1.Text = "第" + i.ToString() + "项";
    item1.Value = "";
    ddl1.Items.Add(item1);
  }
}
  
protected void btnGet_Click(object sender, EventArgs e)
{
  string str = ddl1.SelectedItem.Text;
  Page.ClientScript.RegisterStartupScript(this.GetType(), "alert", "<script>alert('" + str + "');</script>");
}