I'm trying to make the switch from Java to .NET.
I've noticed a number of ASP.NET pages have <%$ sometext %> in them. Can someone explain what this does in a couple of sentences, or point me to a reference on the syntax?
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5 Answers
It's expression builder syntax, and it's used commonly to access settings in the web.config. Here's an example using expression builder syntax to get a connection string:
ConnectionString="<%$ ConnectionStrings:sqlconnection %>"
Here's a good article that explains all of the inline expressions:
http://support.microsoft.com/kb/976112
The expression builder is used to set values of control properties based on the information that is contained in an application's configuration or resource files. The following is the basic syntax of the expression builder:
<%$ Expression Prefix: Expression Value %>The dollar sign ($) indicates to ASP.NET that the following expression is an expression builder. The expression prefix defines the kind of expression, such as AppSettings, ConnectionStrings, or Resources. Additionally, you can create and define your own expression builder. The expression value that follows the colon (:) is what ASP.NET will actually use as the value of a certain property.
4 Comments
InfinitiesLoop
It's not only about web.config. That's just what some expression builders do. See my answer for an example.
James Johnson
@InfinitiesLoop: Yes, I amended my answer. I think that's just what it's used most commonly for.
Jay
Thanks! If I'd known what it was called, I could have googled it. But searching on "<%$" found me pretty much every ASP page in the world!
James Johnson
Yea, it took me a few queries to get the details too ;)
It references what is called an "Expression Builder". It's just a component that can plug into the parsing mechanism. The expression builder is fed the content of the expression, and it is responsible for returning CodeDOM expressions that describe how to get the actual value.
I've implemented a generic expression builder that lets you put any code in it:
http://weblogs.asp.net/infinitiesloop/archive/2006/08/09/The-CodeExpressionBuilder.aspx
Comments
It's inline codebehind.
Here's a link to some more info
Comments
<%$ expressionPrefix: expressionValue %> is used for expressions. Where the expressionPrefix is the expression builder it uses, and the expressionValue is the actual expression that gets passed to the expression builder.
An example usage: <%$ AppSettings: greeting %> which would read out the greeting from the application configuration. Various expression builders are supplied by default such as:
- AppSettings
- Resources
- ConnectionStrings
It is also possible to create your own custom expression builder(s).
This page gives a nice overview of various available ASP.NET tags. Although it is missing <%: %> which HTML encodes the supplied contents.
Comments
It is called expression and is used for various things including reading from web.config, app settings and resource files for localizations. Resource expressions are probably the most used form of expressions. Instead of putting the static text in the controls, this expression can be used and the ASP.NET runtime would pick the resource file for the current culture and extract the value from it.
