DropDownList OnSelectedIndexChanged未觸發（AutoPostBack為“ true”，未發現其他問題）

Question

ListBox1連接到SQL數據庫，並將查詢的數據綁定到ddCountries（DropDownList）。 選擇DropDownList項目時，應該更新頁面上其他位置的標簽，但是由於某些原因，在應用程序運行時根本無法訪問ddCountries_SelectedIndexChanged方法。

是的，AutoPostBack設置為“ true”。

Default.aspx

ListBox1：

protected void ListBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            SqlConnection countriesConnection = new SqlConnection();

            countriesConnection.ConnectionString =
                System.Web.Configuration.WebConfigurationManager.ConnectionStrings["CSharpClass1ConnectionString"].ConnectionString;

            SqlCommand cmd = countriesConnection.CreateCommand();

            int ContID = Convert.ToInt32(ListBox1.SelectedValue);

            try
            {
                string query = "SELECT * FROM Country WHERE ContinentId=" + ContID + ";";

                SqlDataAdapter adpt = new SqlDataAdapter(query, countriesConnection);
                DataTable dt = new DataTable();
                adpt.Fill(dt);
                ddCountries.DataSource = dt;
                ddCountries.DataBind();
                ddCountries.DataTextField = "CountryName";
                ddCountries.DataValueField = "ContinentId";
                ddCountries.DataBind();
            }
            catch (Exception ex)
            {

            }
            finally
            {
                cmd.Dispose();
                countriesConnection.Close();
            }
        }

下拉列表：

<asp:DropDownList ID="ddCountries" runat="server" Height="16px" Width="238px" OnSelectedIndexChanged="ddCountries_SelectedIndexChanged" AutoPostBack="True">
        <asp:ListItem Text="None" value=""></asp:ListItem>
    </asp:DropDownList>

Default.aspx.cs

protected void ddCountries_SelectedIndexChanged(object sender, EventArgs e)
        {
            lblThankYou.Visible = true;
            lblThankYou.Text = "You have selected " + ddCountries.SelectedValue.ToString() + "!";
        }

沒有錯誤消息，標簽（lblThankYou）根本不會被更新。 根據調試，該方法永遠不會被訪問。

Answer 1

您應該使用UpdatePanle和ScriptManager：

<asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Always" ChildrenAsTriggers="true" >      
  <ContentTemplate>
    <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="True"  onselectedindexchanged="DropDownList1_SelectedIndexChanged">
      <asp:ListItem>item 1</asp:ListItem>
      <asp:ListItem>item 2</asp:ListItem>
    </asp:DropDownList>
  </ContentTemplate>
</asp:UpdatePanel>



protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
    throw new NotImplementedException();
}