Why Can't I Modify the Controls Collection in My User Control Due to Code Blocks?

Troubleshooting User Control Modifications: Addressing the Controls Collection Error

Problem Summary

Adding controls (like an AjaxToolkit SliderExtender) to a custom user control dynamically often results in an error: the Controls collection cannot be modified because of code blocks (e.g., <%$ %>). This happens even when using placeholders.

Solution: Convert Code Blocks to Data Binding Expressions

The root cause is the use of Response.Write-style code blocks within the user control's markup. These need to be converted to data binding expressions.

Detailed Steps

Updating User Control Markup

Find all code blocks beginning with <%$ and change them to start with <%#. For instance:

<code>```  should become  ```</code>

This crucial change transforms the code from server-side Response.Write statements to data binding expressions, resolving the conflict with dynamic control additions.

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