What's the difference between <%# %> and <%= %>?

Question

Pardon my ASP ignorance, but what\'s the difference?

Answer 1

The # version is used while data binding. <%= is just a simple Response.Write

Answer 2

See http://weblogs.asp.net/leftslipper/archive/2007/06/29/how-asp-net-databinding-deals-with-eval-and-bind-statements.aspx

As Albert says, it's all to do with parsing databinding statements.

Answer 3

These are somewhat informally referred to as "bee stings". There are 4 types:

<%# %> is invoked during the DataBinding phase.

<%= %> is used to get values from code to the UI layer. Meant for backward compatibility with ASP applications. Shouldn't use in .NET.

<%@ %> represents directives and allow behaviors to be set without resorting to code.

<%: %> (introduced in ASP.NET 4) is the same as %=, but with the added functionality of HtmlEncoding the output. The intention is for this to be the default usage (over %=) to help shield against script injection attacks.

Directives specify settings that are used by the page and user-control compilers when the compilers process ASP.NET Web Forms pages (.aspx files) and user control (.ascx) files.

ASP.NET treats any directive block (<%@ %>) that does not contain an explicit directive name as an @ Page directive (for a page) or as an @ Control directive (for a user control).

@Esteban - Added a msdn link to directives. If you need...more explanation, please let me know.

Answer 4

javascript in .aspx that uses a master page.

var e = document.getElementById('<%= lblDescription.ClientID %>');
e.innerHTML = 'getElementById(\'lblDescription\') will be null';

Answer 5

Not entirely related to the question, there's another related notation in asp.net called Expression Builder:

<asp:SqlDataSource ... Runat="server"
 ConnectionString="<%$ ConnectionStrings:Northwind %>"
/>

<asp:Literal Runat="server"
  Text="<%$ Resources:MyResources, MyText %>"
/>

and it's extensible, see http://msdn.microsoft.com/en-us/magazine/cc163849.aspx#S4