Question

我收到上面的错误，我现在真的陷入困境，这是一个简单的表单，我试图在单击按钮时动态添加控件。我对代码的评论很高兴我收到错误“pnltxtProductDescription.Controls.Add（txt）'这就是我在哪里得到错误”

我的ASP页面

<%@ Page Title="" Language="vb" AutoEventWireup="false" MasterPageFile="~/Pages/master-pages/main.Master" CodeBehind="quotation-new.aspx.vb" Inherits="iconHub.quotation_new1" %>

<title>New Quotation | iconHub</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">

<div id="wrapper" class="wrapper">
    <!-- MAIN CONTENT WRAPPER -->
    <div id="main-content-wrapper" class="content-wrapper ">

            <div class="main-content">

                        <div class="col-md-12" style="padding-bottom: 15px;">
                            <div class="row">
                                <table class="table table-bordered">

                                    <thead>
                                        <tr>
                                            <th class="col-md-1">#</th>
                                            <th class="col-md-2">Item</th>
                                            <th class="col-md-4">Description</th>
                                            <th class="col-md-2">Unit Cost</th>
                                            <th class="col-md-1">Quantity</th>
                                            <th class="col-md-2">Line Total</th>

                                        </tr>
                                    </thead>

                                </table>
                            </div>
                                <div class="col-md-4">
                                    <!--Left Column-->
                                    <div class="form-group">
                                        <input type="text" class="form-control" name="txtProductDescription" runat="server" id="txtProductDescription">
                                    </div>
                                </div>

                            </div>
                            <div class="row">

                                <asp:Panel ID="pnltxtProductDescription" runat="server">
                                </asp:Panel>                                      
                            </div>
                        </div>
                        <div class="col-md-12 pull-right" style="padding-bottom: 15px;">
                            <div class="row">
                                <asp:Button id="cmdAddLine" CssClass="btn btn-default nextBtn btn-sm pull-right" runat="server" onclick="AddTextBox" Text="Add Line" />
                            </div>
                        </div>

                    </div>
                </div>
            </div>
        </div>
    </div>
</div>

我的幕后代码

Public Class quotation_new1 Inherits System.Web.UI.Page

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load

End Sub

Protected Sub AddTextBox(sender As Object, e As EventArgs)
    Dim index As Integer
    'Description
    index = pnltxtProductDescription.Controls.OfType(Of TextBox)().ToList().Count + 1
    CreatetxtProductDescription("txtProductDescription" & index)


End Sub

Private Sub CreatetxtProductDescription(id As String)
    Dim txt As New TextBox
    txt.ID = id

    pnltxtProductDescription.Controls.Add(txt) ' THIS IS WHERE I AM GETTING THE ERROR

    Dim lt As New Literal With {
        .Text = "<br />"
    }
    pnltxtProductDescription.Controls.Add(lt)
End Sub




Protected Sub Page_PreInit(sender As Object, e As EventArgs) Handles Me.PreInit
    Dim keys As List(Of String) = Request.Form.AllKeys.Where(Function(key) key.Contains("txtProductDescription")).ToList()
    Dim i As Integer = 1
    For Each key As String In keys
        CreatetxtProductDescription("txtProductDescription" & i)
        i += 1
    Next
End Sub

结束班

Answer 1

它失败的原因是您从CreatetxtProductDescription()事件中呼叫PreInit。在初始化控件之前会触发PreInit，包括您的Panel控件（请参阅ASP.Net Page Life Cycle）。由于尚未创建Panel控件，因此当您尝试访问NullException属性时，它会抛出.Controls错误。

要解决您的问题，请将PreInit事件中的代码移至Load事件。

System.NullReferenceException：'对象引用未设置为对象的实例。'

1 个答案: