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knowledge-base/common-resolving-controls-collection-modified-exception.md
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| title: Resolving The Controls collection cannot be modified Exception in ASP.NET Web Forms | ||
| description: Learn how to resolve the "The Controls collection cannot be modified because the control contains code blocks" exception in ASP.NET Web Forms | ||
| type: troubleshooting | ||
| page_title: Resolving "The Controls collection cannot be modified" Exception in ASP.NET Web Forms | ||
| slug: common-resolving-controls-collection-modified-exception | ||
| tags: exception, ASP.NET, Telerik, controls collection, code blocks | ||
| res_type: kb | ||
| --- | ||
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| ## Environment | ||
| | Product | Progress® Telerik® UI for ASP.NET AJAX | | ||
| | ------- | ------------------------------------ | | ||
| | Version | 2023.3.1010 | | ||
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| ## Description | ||
| I am encountering the following exception when running an ASP.NET Web Forms application: | ||
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| ``` | ||
| 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) +8677431 | ||
| Telerik.Web.SkinRegistrar.RegisterCssReference(Page _page, Control _control, String _url) +257 | ||
| ``` | ||
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| ## Solution | ||
| To resolve the "The Controls collection cannot be modified" exception in ASP.NET Web Forms when adding a Telerik control, follow these steps: | ||
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| 1. Check for any `<% ... %>` code blocks in the `<head>` tag of the `default.aspx` or related master pages and user controls. | ||
| 2. If you find any code blocks, wrap the `<link>` and `<script>` tags in `<telerik:RadScriptBlock>` tags. | ||
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| Example: | ||
| ```html | ||
| <head runat="server"> | ||
| <telerik:RadScriptBlock ID="RadScriptBlock1" runat="server"> | ||
| <link rel="stylesheet" href="<%=Page.ResolveUrl("~/Styles/global.css")%>" type="text/css" /> | ||
| <link rel="stylesheet" href="<%=Page.ResolveUrl("~/Styles/styles2.css")%>" type="text/css" /> | ||
| </telerik:RadScriptBlock> | ||
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| <telerik:RadScriptBlock ID="RadScriptBlock2" runat="server"> | ||
| <script src="<%=Page.ResolveUrl("~/Scripts/jquery-3.5.1.min.js")%>" type="text/javascript"></script> | ||
| <script src="<%=Page.ResolveUrl("~/Scripts/dialog.js")%>" type="text/javascript"></script> | ||
| </telerik:RadScriptBlock> | ||
| </head> | ||
| ``` | ||
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| If using `<telerik:RadScriptBlock>` does not solve the problem, replace it with `<telerik:RadCodeBlock>` tags. | ||
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| 3. Alternatively, you can try moving the code block(s) out of the `<head>` tag and place them inside the `<body>` tag. | ||
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| By following these steps, you should be able to resolve the "The Controls collection cannot be modified" exception in ASP.NET Web Forms when adding a Telerik control. |