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KB-Controls.AddAt可能破壞ViewState

 2 years ago
source link: https://blog.darkthread.net/blog/controls-addat-will-spoil-viewstate/
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KB-Controls.AddAt可能破壞ViewState-黑暗執行緒

前些時候，為了解決MatserPage下元件的ClientID會被加註Prefix的問題，我寫了一段彈性化找尋ClientID的Javascript Function取代document.getElementById()，並且為了確保WebControl在產生HTML的同時就可以插入Javascript呼叫它，我利用Page.Form.Control.AddAt(0, Literal)的技巧讓它插隊顯示在最前方。

今天同事回報，這種插隊法會讓下拉選單的選項在PostBack後掉光光，我懷疑是ViewState解析順序被破壞導致，於是寫了以下的Code驗證。以下的寫法，只要呼叫了Page.Form.Controls.AddAt(0, ...), 在按下Button1後，下拉選項就會消失。

排版顯示純文字
<%@ Page Language="C#" AutoEventWireup="true" %>
<html>
<head runat="server">
    <title>ViewState Is Missing</title>
</head>
<body>
    <form id="form1" runat="server">
    <script type="text/C#" runat="server">
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                DropDownList1.Items.Add("Item1");
                DropDownList1.Items.Add("Item2");
                DropDownList1.Items.Add("Item3");
            }
            Literal ltr = new Literal();
            ltr.Text =
                "<script type=\"text/javascript\">function blah() { }</"
                + "script>";
            Page.Form.Controls.AddAt(0, ltr);
        }
        protected void Button1_Click(object sender, EventArgs e)
        {
        }     
    </script>
    <div>
        <asp:DropDownList ID="DropDownList1" runat="server">
        </asp:DropDownList>
        <asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />
    </div>
    </form>
</body>
</html>

微軟有篇文章提到這一點:

When adding a dynamic control c to some parent control p based on some condition (that is, when not loading them on each and every page visit), you need to make sure that you add c to the end of p's Controls collection.

所以囉! 用Controls.AddAt真的挺危險的在Page_Load中用Controls.AddAt真的挺危險的，但我又這麼在意要把<script>擺到最前面，怎麼辦?

改成Page.Header.Controls.Add(...)吧! 搞定收工!

Update @ 2007-01-04
網友大估找到更好的解法，將Control.AddAt()移至Page_Init()事件就可以了，這個測試結果也解釋了Control.AddAt攪亂ViewState的理由:
依Control Execution Lifecyvle中Event的順序，Load被夾在Load View State與Save State之間，因此在Load加入Contorl，會發生Save State時Control存在，下次PostBack Load State時卻Control卻還沒生出來的情況，因此造成了View State錯亂。嚴格來說，新增Control放在Init事件，會比放在Load中好。謝謝大估提供的建議!

Update 2008-01-19
強化版搜尋範圍擴及UserControl，說明在此。


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