DropDownList中的ListItems属性在回发时丢失？ [英] ListItems attributes in a DropDownList are lost on postback?

问题描述

一个同事告诉我：

他在网页上有一个DropDownList和一个按钮。以下是代码：

  protected void Page_Load（object sender，EventArgs e）
 {
 if（ ！IsPostBack）
 {
 ListItem item = new ListItem（1）; 
 item.Attributes.Add（title，A）; 
 
 ListItem item2 = new ListItem（2）; 
 item2.Attributes.Add（title，B）; 
 
 DropDownList1.Items.AddRange（new [] {item，item2}）; 
 string s = DropDownList1.Items [0] .Attributes [title]; 
} 
} 
 
 protected void Button1_Click（object sender，EventArgs e）
 {
 DropDownList1.Visible =！DropDownList1.Visible; 
} 
  

在页面加载中，项目的工具提示显示，但在第一个回发，属性丢失。为什么是这种情况，有没有解决方法？

解决方案

我有同样的问题，想贡献这个资源，作者创建一个继承的ListItem Consumer将属性持久化到ViewState。希望它会节省某人我被浪费的时间，直到我偶然发现。

 保护覆盖对象SaveViewState（）
 { 
 //创建Item count的对象数组+ 1 
 object [] allStates = new object [this.Items.Count + 1]; 
 
 // +1保存基本信息
 object baseState = base.SaveViewState（）; 
 allStates [0] = baseState; 
 
 Int32 i = 1; 
 //现在循环并保存List 
 foreach的每个Style属性（在this.Items中为ListItem li）
 {
 Int32 j = 0; 
 string [] [] attributes = new string [li.Attributes.Count] []; 
 foreach（li.Attributes.Keys中的字符串属性）
 {
 attributes [j ++] = new string [] {attribute，li.Attributes [attribute]}; 
} 
 allStates [i ++] =属性; 
} 
 return allStates; 
} 
 
 protected override void LoadViewState（object savedState）
 {
 if（savedState！= null）
 {
 object [] myState =（object []）savedState; 
 
 //恢复基本
如果（myState [0]！= null）
 base.LoadViewState（myState [0]）; 
 
 Int32 i = 1; 
 foreach（ListItem li in this.Items）
 {
 //循环并恢复每个样式属性
 foreach（string [] attribute in（string [] []）myState [i ++]）
 {
 li.Attributes [attribute [0]] = attribute [1]; 
} 
} 
} 
} 
  

A coworker showed me this:

He has a DropDownList and a button on a web page. Here's the code behind:

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            ListItem item = new ListItem("1");
            item.Attributes.Add("title", "A");

            ListItem item2 = new ListItem("2");
            item2.Attributes.Add("title", "B");

            DropDownList1.Items.AddRange(new[] {item, item2});
            string s = DropDownList1.Items[0].Attributes["title"];
        }
    }

    protected void Button1_Click(object sender, EventArgs e)
    {
        DropDownList1.Visible = !DropDownList1.Visible;
    }

On the page load, the items' tooltips are showing, but on the first postback, the attributes are lost. Why is this the case, and are there any workarounds?

解决方案

I had the same problem and wanted to contribute this resource where the author created an inherited ListItem Consumer to persist attributes to ViewState. Hopefully it will save someone the time I wasted until I stumbled on it.

protected override object SaveViewState()
{
    // create object array for Item count + 1
    object[] allStates = new object[this.Items.Count + 1];

    // the +1 is to hold the base info
    object baseState = base.SaveViewState();
    allStates[0] = baseState;

    Int32 i = 1;
    // now loop through and save each Style attribute for the List
    foreach (ListItem li in this.Items)
    {
        Int32 j = 0;
        string[][] attributes = new string[li.Attributes.Count][];
        foreach (string attribute in li.Attributes.Keys)
        {
            attributes[j++] = new string[] {attribute, li.Attributes[attribute]};
        }
        allStates[i++] = attributes;
    }
    return allStates;
}

protected override void LoadViewState(object savedState)
{
    if (savedState != null)
    {
        object[] myState = (object[])savedState;

        // restore base first
        if (myState[0] != null)
            base.LoadViewState(myState[0]);

        Int32 i = 1;
        foreach (ListItem li in this.Items)
        {
            // loop through and restore each style attribute
            foreach (string[] attribute in (string[][])myState[i++])
            {
                li.Attributes[attribute[0]] = attribute[1];
            }
        }
    }
}

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