How to Fix the 'Controls Collection Cannot Be Modified' Error When Using AjaxToolkit SliderExtender?

Troubleshooting the "Controls Collection Cannot Be Modified" Error in ASP.NET

Adding an AjaxToolkit SliderExtender to a user control sometimes results in the frustrating "Controls collection cannot be modified because the control contains code blocks" error. This occurs because code blocks within the control interfere with modifications to the Controls collection.

The solution involves converting code blocks within the user control's markup to data-binding expressions using the <%# ... %> syntax. Data-binding expressions are not treated as code blocks by the Common Language Runtime (CLR), thus resolving the conflict.

For example, if you have code blocks like <...> within your user control, replace them with equivalent data-binding expressions.

In your master page's code-behind:

<code class="language-csharp">protected void Page_Load(object sender, EventArgs e)
{
    Page.Header.DataBind();
}</code>

This ensures that data binding occurs, allowing the SliderExtender to be added without error. Remember to properly handle any data binding within your user control itself.

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