What Caused This Error

This is can be seen when you do BOTH of the following on the same page/user control:

1. Include in-line 'Code Blocks' into your page/user control. E.G. <% Response.Write("Hello"); %>
2. Add A user control to the page/user control in a programmatic manner. E.G. Using the LoadControl() Method in the code behind.

What is happening?

The in-line 'Code Block' has modified the Control Collection of the base control that you are working with. Also you are trying to add a control using server code, But you are trying to add that control to the same collection that you have already modified with the in-line 'Code Block'. ASP.NET complains and a error is thrown.

How To Fix

To fix this I have found all I have to do is to put a server control around my in-line 'Code Blocks'. This means that my in-line 'Code Blocks' are only modifying the Control Collection of the new Server Control that contains the code block and not the main page or control that I am working on. You can use a div tag with runat="server" as your server control. All Server Controls should also have an ID, so add that too.

E.G.

<div id="Div2" runat="server">

    <% Response.Write("Hello"); %>

</div>

This simple action of moving the offending code block into a server control fixes the error.

The Server control does not have to be a div. You could use any HTML control with the runat="server" added or any asp control.