解决Ajax AsyncFileUpload引发的Controls集合无法修改错误

问题概述 #

遇到「The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>)」错误，经排查确认是Ajax AsyncFileUpload控件导致——移除该控件后错误消失。其他页面运行正常，说明母版页本身无问题。

已尝试的无效方案 #

• 将代码块包裹在<asp:PlaceHolder Runat="server">中
• 确保母版页无<%= %>代码块
• 将所有JS文件引用包裹在PlaceHolder中
• 在母版页后台添加Page.Header.DataBind()

母版页相关代码 #

<asp:PlaceHolder runat="server">
    <script src='<%# ResolveUrl("path_to_script") %>'
    ...
</asp:PlaceHolder>

<link rel="stylesheet" href="~/assets/css/whatever.css" />

完整错误信息 #

The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).
[HttpException (0x80004005): The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).]
System.Web.UI.ControlCollection.Add(Control child) +11686804
AjaxControlToolkit.ToolkitResourceManager.RegisterCssReferences(Control control) +893
System.Web.UI.Control.LoadRecursive() +90
System.Web.UI.Control.LoadRecursive() +185
System.Web.UI.Control.LoadRecursive() +185
System.Web.UI.Control.LoadRecursive() +185
System.Web.UI.Control.LoadRecursive() +185
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1602

引发问题的控件代码 #

<ajaxToolkit:AsyncFileUpload runat="server" ID="fuDBData"
    ClientIDMode="Static"
    class="hidden"
    ThrobberID="myThrobber"
    UploaderStyle="Traditional"
    OnUploadedComplete="fuDBData_UploadedComplete"
    OnUploadedFileError="UploadedFileError"
    onClientUploadStart="uploadStart"
    OnClientUploadComplete="uploadComplete" />

内容的提问来源于stack exchange，提问作者NoBullMan