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

一位同事向我展示了这个:

A coworker showed me this:

他在网页上有一个 DropDownList 和一个按钮.这是背后的代码:

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;
    }

在页面加载时，项目的工具提示显示，但在第一次回发时，属性丢失.为什么会这样，是否有任何解决方法?

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?

推荐答案

我遇到了同样的问题，想贡献一下 this 资源，作者在其中创建了一个继承的 ListItem Consumer 以将属性持久化到 ViewState.希望它可以节省我浪费的时间，直到我偶然发现它.

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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