When should I use # and = in ASP.NET controls?

Question

I have been using ASP.NET for years, but I can never remember when using the # and = are appropriate.

For example:

<%= Grid.ClientID %>

or

<%# Eval("FullName")%>

Can someone explain when each should be used so I can keep it straight in my mind? Is # only used in controls that support databinding?

Answer 1

<%= %> is the equivalent of doing Response.Write("") wherever you place it.

<%# %> is for Databinding and can only be used where databinding is supported (you can use these on the page-level outside a control if you call Page.DataBind() in your codebehind)

Databinding Expressions Overview

Answer 2

There are a couple of different 'bee-stings':

• <%@ - page directive
• <%$ - resource access
• <%= - explicit output to page
• <%# - data binding
• <%-- - server side comment block

Also new in ASP.Net 4:

• <%: - writes out to the page, but with HTML encoded

Also new in ASP.Net 4.5:

• <%#: - HTML encoded data binding

Answer 3

Here's a great blog post by Dan Crevier that walks through a test app he wrote to show the differences.

In essence:

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