[ASP.NET][Web Form]PostBack時失去Label的Text

2017-05-04

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Web Form
2017-05-05

最近同事在Client-side修改asp.net web form控制項的內容，但postback到Server-Side後抓不到值，同事發生問題的控制項是Label，但我們常使用的TextBox控制項則可以，另外根據MSDN上的說明，使用 Label 控制項可以用來顯示在頁面上設定位置的文字，好，馬上實驗Http Request內容是否有帶postdata?

建立環境:

1.新增ViewStateAndPostData.aspx

分別建立三個會在前端修改值的控制項，依序為Textbox、HiddenField及Label，同時新增兩個按鈕，分別是client端事件按鈕及postback回server side的按鈕。

在client事件中會分別將三個控制項的值依序改為iPhone8、S8 Edge及xz premium。

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="ViewStateAndPostData.aspx.cs" Inherits="WebApplication1.ViewStateAndPostData" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <title></title>
    <script>
        function changeText() {
            document.getElementById("txtInPut").value = "iPhone8";
            document.getElementById("hdfInPut").value = "S8 Edge";
            document.getElementById("lblInPut").innerHTML = "xz premium";
        };
    </script>
</head>
<body>
    <form id="form1" runat="server">
        <div>
            <asp:TextBox ID="txtInPut" runat="server"></asp:TextBox>
            <asp:HiddenField ID="hdfInPut" runat="server"></asp:HiddenField>
            <asp:Label ID="lblInPut" runat="server"></asp:Label>
            <button type="button" onclick="changeText()">手機型號</button>
            <asp:Button ID="btnPost" Text="PostBack的按鈕" runat="server" OnClick="btnPost_Click" />
        </div>
    </form>
</body>
</html>

2.按F7切換到code behind(ViewStateAndPostData.aspx.cs)

a.若是第一次載入頁面，則將三個控制項依序帶入apple、samsung及sony。

b.依序在Page_Init、Page_Load、post_back event及Page_PreRender觀察三個控制項的值

protected void Page_Init(object sender, EventArgs e)
{
    Debug.WriteLine($"Page_Init Text = {this.txtInPut.Text}");
    Debug.WriteLine($"Page_Init HidenField= {this.hdfInPut.Value}");
    Debug.WriteLine($"Page_Init Label = {this.lblInPut.Text}");
}

protected void Page_Load(object sender, EventArgs e)
{
    Debug.WriteLine($"Page_Load Text = {this.txtInPut.Text}");
    Debug.WriteLine($"Page_Load HidenField= {this.hdfInPut.Value}");
    Debug.WriteLine($"Page_Load Label = {this.lblInPut.Text}");
    if (!Page.IsPostBack)
    {
        this.txtInPut.Text = "apple";
        this.hdfInPut.Value = "samsung";
        this.lblInPut.Text = "sony";
    }
}

protected void btnPost_Click(object sender, EventArgs e)
{
    Debug.WriteLine($"btnPost_Click Text = {this.txtInPut.Text}");
    Debug.WriteLine($"btnPost_Click HidenField= {this.hdfInPut.Value}");
    Debug.WriteLine($"btnPost_Click Label = {this.lblInPut.Text}");
}

protected void Page_PreRender(object sender, EventArgs e)
{
    Debug.WriteLine($"Page_PreRender Text = {this.txtInPut.Text}");
    Debug.WriteLine($"Page_PreRender HidenField= {this.hdfInPut.Value}");
    Debug.WriteLine($"Page_PreRender Label = {this.lblInPut.Text}");
}