DropDownList的错误 - OnSelectedIndexChanged火灾只有当DDL填入HTML [英] DropDownList bug - OnSelectedIndexChanged fires only when DDL is filled in HTML

问题描述

的构想

步骤1. DropDownList中充满了对的Page_Load项目。

步骤2.我选择一个项目，它发起回传。在这回发刷新页面，然后再OnSelectedIndexChanged火灾和ddl1_Select功能运行。

问题

如果我填静态与.aspx的（HTML）文件数据的DropDownList，它工作正常。
但是，如果我填它code-背后的OnSelectedIndexChanged事件永远不会触发和DDL1_Select程序不启动。页面只是回发，并跳过我的步骤。为什么没有这个事件火灾和如何使它工作？

ASPX：

 ＆LT;％@页面语言=C＃的MasterPageFile =〜/ Master.masterAutoEventWireup =真
codeFILE =mypage.aspx.cs继承='我的空间标题=我的空间的EnableViewState =假％GT;...＆LT; ASP：DropDownList的ID =ddl1=服务器OnSelectedIndexChanged =ddl1_Select的AutoPostBack =真/＆GT;
 

ASPX.CS：

 保护无效的Page_Load（对象发件人，EventArgs的发送）
{
    ddl1.Items.Add（1）;
    ddl1.Items [0]。价值=1;
    ddl1.Items.Add（2）;
    ddl1.Items [1]。价值=2;
    ddl1.Items.Add（3）;
    ddl1.Items [2]。价值=3;    如果（会话[NewSelection]！= NULL）//注1
    {
        。字符串itemValue =会话[NewSelection]的ToString（）;
        ddl1.SelectedIndex = ddl1.Items.IndexOf（ddl1.Items.FindByValue（itemValue））;
        会话[NewSelection] = NULL; //见注1
    }
}保护无效DDL1_Select（对象o，EventArgs的发送）
{
    会话[NewSelection] = ddl1.SelectedValue;
    Page.Response.Redirect（？mypage.aspx测试=+会话[NewSelection]的ToString（））;
}
 

注1 即可。这些指令从会话变量取旧索引用于prevent ddl1。所以，当我在ddl1选择一个项目，首先进入回发。在这回发ddl1从会话变量需要旧索引和我的新选择后丢失。这些指示我prevent它。但它并没有任何区别，除非主要问题是解决了。

解决方案

 保护无效的Page_Load（对象发件人，EventArgs的发送）
{
  如果（！的IsPostBack）
{
    ddl1.Items.Add（1）;
    ddl1.Items [0]。价值=1;
    ddl1.Items.Add（2）;
    ddl1.Items [1]。价值=2;
    ddl1.Items.Add（3）;
    ddl1.Items [2]。价值=3;    如果（会话[NewSelection]！= NULL）//注1
    {
        。字符串itemValue =会话[NewSelection]的ToString（）;
        ddl1.SelectedIndex = ddl1.Items.IndexOf（ddl1.Items.FindByValue（itemValue））;
        会话[NewSelection] = NULL; //见注1
    }
}
}
 

THE CONCEPTION

Step 1. The DropDownList is filled with items on Page_Load.

Step 2. I select an item and it initiates postback. During this postback Page refreshes first, then OnSelectedIndexChanged fires and "ddl1_Select" function runs.

THE PROBLEM

If I statically fill the DropDownList with data in Aspx (HTML) file, it works properly. But, if I fill it in code-behind, the OnSelectedIndexChanged event never fires and DDL1_Select procedure doesn't start. Page just posts back and skips my procedure. Why doesn't this event fire and how to make it work?

ASPX:

<%@ Page Language="C#" MasterPageFile="~/Master.master" AutoEventWireup="true" 
CodeFile="mypage.aspx.cs" Inherits="mypage" Title="mypage" EnableViewState="False" %>

...

<asp:DropDownList ID="ddl1" runat="server" OnSelectedIndexChanged="ddl1_Select" AutoPostBack="True"/>

ASPX.CS:

protected void Page_Load(object sender, EventArgs e)
{
    ddl1.Items.Add("1");
    ddl1.Items[0].Value = "1";
    ddl1.Items.Add("2");
    ddl1.Items[1].Value = "2";
    ddl1.Items.Add("3");
    ddl1.Items[2].Value = "3";

    if (Session["NewSelection"] != null) // see note 1
    {
        string itemValue = Session["NewSelection"].ToString();
        ddl1.SelectedIndex = ddl1.Items.IndexOf(ddl1.Items.FindByValue(itemValue));
        Session["NewSelection"] = null; // see note 1
    }
}

protected void DDL1_Select(object o, EventArgs e)
{
    Session["NewSelection"] = ddl1.SelectedValue;
    Page.Response.Redirect("mypage.aspx?test=" + Session["NewSelection"].ToString());
}

Note 1. Those instructions are used to prevent ddl1 from taking old index from session variable. So, after I select an item in ddl1, first goes postback. During this postback ddl1 takes old index from session variable and my new selection is lost after that. With these instructions I prevent it. But it doesn't make any difference unless the main problem is resolved.

解决方案

protected void Page_Load(object sender, EventArgs e)
{
  if (!IsPostBack)
{ 
    ddl1.Items.Add("1");
    ddl1.Items[0].Value = "1";
    ddl1.Items.Add("2");
    ddl1.Items[1].Value = "2";
    ddl1.Items.Add("3");
    ddl1.Items[2].Value = "3";

    if (Session["NewSelection"] != null) // see note 1
    {
        string itemValue = Session["NewSelection"].ToString();
        ddl1.SelectedIndex = ddl1.Items.IndexOf(ddl1.Items.FindByValue(itemValue));
        Session["NewSelection"] = null; // see note 1
    }
}
}

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