Overlapping Server and embedded <% %> tags.

[This was originally posted at http://timstall.dotnetdevelopersjournal.com/overlapping_server_and_embedded_lt_gt_tags.htm]

In ASP.Net, you can run into unexpected problems when combining embedded script and runat=server controls on the same object. Two resulting problems are:

1. Your Html simply won't render (and you'll wonder why)
2. You'll get an Html parse error, without it ever even hitting your code behind.

Let's look at a couple such snippets:

Okay: Embedded script doesn't overlap runat=server control.

<table border="1"> <tr> <td><%="Hello World" %>td> <td><asp:Label id="Label1" runat="server">This is okayasp:Label>td> tr> table>

Problem: This embedded script won't render because it is within the innerHtml of a WebControl (which is obviously runat=server).:

<tr>   <td>     <asp:Label id="Label1" runat="server">       <%="Hello World" %>     asp:Label>   td> tr>

Okay: This does render (within the innerHtml of a runat=server Html control):

<tr>   <td runat="server" id="Td1">     By<%="Hello World" %>   td> tr>

Problem: This won't render, because it is within the attributes of a runat=server control:

<tr>   <td runat="server" id="Td1" height='<%="60" %>' >     By   td> tr>

Big Problem: And this doesn't just render, it gives an Html parse error:

<tr>   <td>     <asp:Label id="Label1" runat="server" Height='<%="30px"%>'>Labelasp:Label>   td> tr>

The principles we see here are:

1. Try to avoid combining embedded script and runat=server controls for the same object.
2. While you may be able to get away with embedded script within the innerHtml of a runat=server HtmlControl, you:
   1. May run into difficulty putting it in the innerHtml of a WebControl (such as the Text of a Label).
   2. Should never put it in the attribute of the control itself. If you do need to put it in the attribute, then don't make the control runat=server.