I got a runtime error after changing a pound code block(<%#statement%>) to equals code block( <%=statement> ) in some legacy ASP.Net code that I was cleaning up . I'd never seen the pound version before and had assumed it was an old obsolete way of doing things. Since this clearly wasn't the case I decided to do a bit of digging.

Eventually I turned up a post on code blocks by Dan Crevier that shed some light on things.

He summarizes the differences as follows:

• The <%= (equal) expressions are evaluated at render time.
• The <%# (pound) expressions are evaluated at DataBind() time and are not evaluated at all if DataBind() is not called.
• <%# (pound) expressions can be used as properties in server-side controls. <%= (equal) expressions cannot.

This helped explain the error I got; unfortunately the post itself was hard for me to find (Google doesn't support searching for non-alphanumeric characters like less-than, percent, pound and equal) so I'm throwing this out there in hopes that it'll show better in search and help some people. All props to Dan though; he did a lot of work to research this.

Thanks Dan.