Why Does Modifying Controls Fail with 'The Controls Collection Cannot Be Modified Because the Control Contains Code Blocks'?

ASP.NET Server Error: Controls Collection Modification Restrictions

When modifying the Controls collection in ASP.NET, you might encounter the error "The Controls collection cannot be modified because the control contains code blocks." This typically happens when working with pages containing server-side code blocks, such as scriptlets or data bindings.

The root cause is the ASP.NET runtime's detection of Response.Write statements within a control's code block. These statements directly write HTML to the response stream. Modifying the Controls collection during this process can lead to unpredictable results and errors.

The solution involves replacing Response.Write statements with data binding techniques. Data binding offers a more controlled and efficient method for dynamically populating controls with data from various sources like page properties or business objects.

For instance, instead of using Response.Write as shown below:

<code>    function onPageLoad() {</code>

(Note: The provided code snippet is incomplete and lacks the Response.Write example. A complete example demonstrating the problem and its solution with data binding is needed to provide a more helpful response.)

A revised approach utilizing data binding would be more robust and prevent the error. This would involve setting control properties (like Text or InnerHTML) based on data sources rather than directly writing to the response stream. Specific examples of data binding techniques depend on the ASP.NET version and the type of controls being used.

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