我试图用C＃建立一个网站。我有一个主页，有一个DevExpress DateEdit，4个DevExpress DropDownLists，一个按钮和一个标签。我有这样的方法：
protected void dropdownNolar_SelectedIndexChanged(object sender, EventArgs e)
{
var halisaha = (from veri in veriler.tum_halisahalars
where veri.Il == dropdownIl.Text &&
veri.Ilce == dropdownIlce.Text &&
veri.Mahalle == dropdownMahalle.Text &&
veri.No == dropdownNolar.Text
select veri).FirstOrDefault();
if (halisaha != null)
if (halisaha.description != null)
lblDescription.Text = halisaha.description;
}
和masterpage的aspx代码：
<%@ Master Language="C#" AutoEventWireup="true" EnableViewState="true" CodeBehind="~/masterPage.master.cs" Inherits="HaliSahaKiralama.masterPage" %>
<%@ Register assembly="DevExpress.Web.v14.1, Version=14.1.6.0, Culture=neutral, PublicKeyToken=b88d1754d700e49a" namespace="DevExpress.Web.ASPxEditors" tagprefix="dx" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head id="Head1" runat="server">
<title>Untitled</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta name="generator" content="Web Page Maker">
<style type="text/css">
#wrapper {
position:relative;
width: 1127px;
margin-top: 20px;
margin-left: auto;
margin-right: auto;
text-align:left;
top: -2px;
left: 55px;
height: 1000px;
background-color:sandybrown;
}
.auto-style1 {
width: 100%;
height: 912px;
border: 5px solid #FFFFFF;
background-color: #66FF33;
}
.auto-style2 {
height: 792px;
}
.auto-style4 {
height: 130px;
border:thick none white;
}
.auto-style5 {
height: 792px;
width: 237px;
}
</style>
</head>
<body style="margin:0px; height: 1093px; background-color:sandybrown" onload="">
<script type="text/javascript" language="javascript">
</script>
<form id="form1" runat="server">
<div id="wrapper">
<table class="auto-style1">
<tr>
<td class="auto-style4" colspan="2" style="background-color:lightseagreen">
<div style="border-style: none; border-color: inherit; border-width: thick; z-index: 1; left: 8px; top: 9px; position: absolute; height: 130px; width: 1111px;">
<img src="hs2.jpg" style="position:absolute; top: 6px; left: 33px; width: 157px;" alt="" />
<dx:ASPxLabel ID="ASPxLabel6" runat="server" Font-Size="36pt" style="z-index: 1; left: 226px; top: 31px; position: absolute; height: 82px; width: 505px" Text="Bamyacılar Halısaha" Theme="Youthful">
</dx:ASPxLabel>
</div>
</td>
</tr>
<tr>
<td class="auto-style5" style="background-color:darkgreen">
<div style="position:absolute; top: -13px; left: 8px; width: 236px; height: 639px; background-color:transparent">
<asp:ContentPlaceHolder ID="ContentPlaceHolder2" runat="server">
<dx:ASPxDateEdit ID="date_editTarih" runat="server" Date="2015-03-24" Font-Size="16pt" style="z-index: 1; left: 18px; top: 196px; position: absolute; height: 21px; width: 170px" Theme="Youthful">
</dx:ASPxDateEdit>
<dx:ASPxLabel ID="ASPxLabel1" runat="server" Font-Size="12pt" style="z-index: 1; left: 18px; top: 165px; position: absolute" Text="İstediğiniz Tarihi Seçiniz:" Theme="Youthful" ForeColor="White">
</dx:ASPxLabel>
<dx:ASPxComboBox ID="dropdownIl" runat="server" style="z-index: 1; left: 18px; top: 280px; position: absolute; height: 29px; width: 170px" Theme="Youthful" OnSelectedIndexChanged="dropdownIl_SelectedIndexChanged1" NullText="İl Seçiniz!">
</dx:ASPxComboBox>
<dx:ASPxLabel ID="ASPxLabel2" runat="server" Font-Size="12pt" style="z-index: 1; left: 18px; top: 478px; position: absolute" Text="Stat Numarasını Seçiniz:" Theme="Youthful" ForeColor="White">
</dx:ASPxLabel>
<dx:ASPxComboBox ID="dropdownIlce" runat="server" style="z-index: 1; left: 17px; top: 353px; position: absolute; height: 29px; width: 170px; bottom: 727px;" Theme="Youthful" OnSelectedIndexChanged="dropdownIlce_SelectedIndexChanged1" NullText="İlçe Seçiniz! ">
</dx:ASPxComboBox>
<dx:ASPxLabel ID="ASPxLabel3" runat="server" Font-Size="12pt" style="z-index: 1; left: 18px; top: 249px; position: absolute" Text="İlinizi Seçiniz:" Theme="Youthful" ForeColor="White">
</dx:ASPxLabel>
<dx:ASPxComboBox ID="dropdownMahalle" runat="server" style="z-index: 1; left: 17px; top: 431px; position: absolute; height: 29px; width: 170px" Theme="Youthful" OnSelectedIndexChanged="dropdownMahalle_SelectedIndexChanged1" NullText="Mahalle Seçiniz! ">
</dx:ASPxComboBox>
<dx:ASPxLabel ID="ASPxLabel4" runat="server" Font-Size="12pt" style="z-index: 1; left: 18px; top: 322px; position: absolute" Text="İlçenizi Seçiniz:" Theme="Youthful" ForeColor="White">
</dx:ASPxLabel>
<dx:ASPxComboBox ID="dropdownNolar" runat="server" style="z-index: 1; left: 18px; top: 510px; position: absolute; height: 29px; width: 170px" Theme="Youthful" OnSelectedIndexChanged="dropdownNolar_SelectedIndexChanged" NullText="Numara Seçiniz!">
</dx:ASPxComboBox>
<dx:ASPxLabel ID="ASPxLabel5" runat="server" Font-Size="12pt" style="z-index: 1; left: 18px; top: 400px; position: absolute" Text="Mahallenizi Seçiniz:" Theme="Youthful" ForeColor="White">
</dx:ASPxLabel>
<dx:ASPxButton ID="BtnMasterPageDevamEt" runat="server" Font-Size="12pt" style="z-index: 1; left: 18px; top: 555px; position: absolute" Text="Devam Et" Theme="PlasticBlue" OnClick="btnClickEski01">
</dx:ASPxButton>
</asp:ContentPlaceHolder>
</div>
<br />
</td>
<td class="auto-style2">
<div style="z-index: 1; left: 257px; top: 144px; position: absolute; height: 784px; width: 862px;background-color:#9ACD32">
<asp:ContentPlaceHolder ID="ContentPlaceHolder1" runat="server">
<p>
<br />
</p>
<p>
</p>
<p>
</p>
<p>
<dx:ASPxBinaryImage ID="ASPxBinaryImage1" runat="server" style="z-index: 1; left: -237px; top: 443px; position: absolute; height: 109px; width: 155px">
</dx:ASPxBinaryImage>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p>
</p>
<p style="margin-left: 0px">
</p>
<asp:Label ID="lblDescription" runat="server" ForeColor="White" Font-Names="Comic Sans MS" style="z-index: 1; left: -226px; top: 583px; position: absolute; height: 14px" Text="Açıklama">
</asp:Label>
<p>
</p>
<p>
</p>
<p>
</p>
</asp:ContentPlaceHolder>
</div>
</td>
</tr>
</table>
</div>
</form>
</body>
</html>
DropDownLists不为空，但我的标签是。为什么会这样？
答案 0 :(得分：2)
查看您的代码示例。您处于“Postback事件处理”阶段。在那个阶段，所有控件都应该可以使用了。
问题是什么可能导致这种意外行为以及如何找出问题？这是MasterPage运行时行为：
问题是该流程出了什么问题。为什么MasterPage中的控件设置为null？ *您在MasterPage上的占位符中添加了代码，该代码将替换为Content控件，并且最终可能为空值。
希望它会对你有所帮助。