What causes " Controls collection cannot be modified because the control contains code blocks"?

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What causes " Controls collection cannot be modified because the control contains code blocks"?

I recently put some code <% %> code blocks in my Master Page. Note I've read of the "fix" for either moving things out of <head> or using <%# %> but neither of them work well for my application.

Now the weird thing is that I only get this error on one page of mine. All the other pages seem to work fine, so what actually causes this error? There is nothing I can think of that is unique about this page. It uses the script manager as does other working pages and there is just nothing extraordinary about this page. It does have quite a few custom controls on it, so hunting down what is different in this page is more difficult than usual.

So what actually causes the Controls collection cannot be modified because the control contains code blocks exception?

If you have a page or control with <% %> and ever dynamically update the control collection (add a control to the page that isn't defined in the .aspx/.ascx) this error will trigger. To get around this I have used an <ASP:Literal/> to inject data instead of <% %>

Pete Amundson 2010-08-11 21:30:49

My other pages do this and have no problems though.

Earlz 2010-08-11 21:51:54

If your other pages don't have dynamically added controls, then they won't have issues - also, you might not have any control over the dynamic controls (depending on UI frameworks/helpers you are using)

Pete Amundson 2010-08-11 22:04:33

You could also use a `PlaceHolder` control, or any number of others, for that matter. If it's `UserControl`s causing a problem, I usually end up rendering them server-side and injecting their HTML into the page, seeing as most of what I do is AJAX-enabled interactivity and whatnot.

Cory Larson 2010-08-12 02:56:59

I'm using regular ASP.Net WebForms and I mean I have stuff with `Controls.Add(...)` in other pages that do work. I'm trying to figure out why those pages work and my other page doesn't

Earlz 2010-08-12 05:50:23

Things can go wrong when some code tries to add controls to the tag containing the <% ... %> or <%= ... %> code block (in this case your <head> tag).

For instance, when you're using themes, the Page class will automatically add <link> tags to the <head> for every CSS file in your theme's directory. But it could also be triggered by setting the Page.Title.

But there are many more ways that can cause modifications to the <head> tag, so without further information (such as a stacktrace) it's hard to give a definitive answer.

Ruben 2010-08-11 21:44:21

If you have themes enabled it could cause it to do that.

geekydevjoe 2010-08-11 21:52:08

I do not ` ` ` `

Earlz 2010-08-12 05:48:37

ansaurus

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What causes " Controls collection cannot be modified because the control contains code blocks"?

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