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