Why Can't I Modify the Controls Collection When My Control Contains Code Blocks?-C++-php.cn

Why Can't I Modify the Controls Collection When My Control Contains Code Blocks?

Linda Hamilton

Release： 2025-01-14 22:05:51

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Why Can't I Modify the Controls Collection When My Control Contains Code Blocks?

ASP.NET Controls Collection Modification Error: "The Controls collection cannot be modified..."

This error, "The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>)", occurs when attempting to add controls to a parent control that already includes server-side code blocks. This is particularly common when adding Ajax Toolkit controls (like SliderExtender) to user controls with embedded declarative code.

Understanding the Conflict: Code Blocks and Dynamic Control Additions

ASP.NET code blocks, represented by <% ... %>, execute server-side code. These blocks cannot be dynamically altered after the control's initial creation. Adding controls that require modifying the Controls collection (a frequent requirement for many extensions) clashes with the presence of these static code blocks.

Resolution: Utilizing Data Binding Expressions

The solution involves replacing the problematic <%= ... %> code blocks (which directly output values) with data binding expressions: <%# ... %>. Data binding expressions are processed during runtime, avoiding the conflict with the Controls collection.

Illustrative Example:

Let's assume you have Response.Write statements within the <head> section of your user control. Convert these to data binding expressions:

<code class="language-html"><head>
  <!-- Old Code (Problematic): -->
  <% Response.Write("Some Dynamic Text"); %>

  <!-- New Code (Corrected): -->
  <asp:Literal ID="Literal1" runat="server" Text="<%# GetDynamicText() %>"></asp:Literal>
</head></code>

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Code-Behind Modification (Master Page):

In your master page's code-behind, add this line to ensure the data binding occurs:

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

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The GetDynamicText() method would then be responsible for providing the dynamic text. By employing this technique, the code is no longer treated as a static code block, allowing seamless integration of controls like the SliderExtender. This approach ensures that the control's dynamic nature doesn't conflict with the existing code blocks.

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