The Controls collection cannot be modified because the control contains code blocks

Tim

Friday, August 24 2007

Server Error in '/' Application.

The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).

Source Error:

Line 132:                        metaKey.Name = "keywords";
Line 133:                        metaKey.Content = p.MetaKeywords;
Line 134:                        this.Page.Header.Controls.Add(metaKey);
Line 135:                    }
Line 136:                    if (!String.IsNullOrEmpty(p.MetaDescription))

Source File: a:\xyz\ContentHandler.aspx.cs Line: 134

Stack Trace:

[HttpException (0x80004005): The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).]
   System.Web.UI.ControlCollection.Add(Control child) +2105903
   ContentHandler.Page_Load(Object sender, EventArgs e) in a:\xyz\ContentHandler.aspx.cs:134
   System.Web.Util.CalliHelper.EventArgFunctionCaller(IntPtr fp, Object o, Object t, EventArgs e) +15
   System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +34
   System.Web.UI.Control.OnLoad(EventArgs e) +99
   System.Web.UI.Control.LoadRecursive() +47
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1061

Version Information: Microsoft .NET Framework Version:2.0.50727.832; ASP.NET Version:2.0.50727.832

Another day, another issue ;)

This had me going around in circles for a while until I realised what it was, if you're getting this error you can bet your bottom dollar that you have <%= %> somewhere in your page's header -furthermore I'd hazard a guess that you've got it in some JavaScript to reference an ASP.Net control on the page- and then you're trying to add a control to the header programmatically (or a custom control from someone like Telerik is trying to). Am I right¹?

¹ I'm not allowed to ask you to so I won't, but if I was right, then spend that bottom dollar clicking on one of the Google Ads :P

I can't tell you exactly why this occurs but my understanding of it is that ASP.Net can't re-create the header if it has Response.Write somewhere in the header (<%=) -most likely due to when the header is created it's not available (will look into it). No doubt you want to know the fix?

The Fix
The fix is simple, remove the inline code blocks and JavaScript and move it to your code behind i.e.:

string _manageSearch = String.Format( @"

function ManageSearch(){{

var lbl = document.getElementById(""lblFindAGift"");

var txt = document.getElementById(""{0}"");

var btn = document.getElementById(""{1}"");

.Do Something with it..

}}",

txtSearch.ClientID);

this.Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "ManageSearch", _manageSearch, true);

Remember: You need to escape the curly brackets otherwise you will get a "String.Format- Exception of type System.Web.HttpUnhandledException was thrown"

Update: Thanks to Julian Voelcker for sending me this alternative "fix" for the problem, can't say I like it though ;) basically instead of using <%= ... %> you would write the databinding expression of: <%# ... %>

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The Controls collection cannot be modified because the control contains code blocks

Tim

Server Error in '/' Application.

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