I've been including this technique in my ASP.NET Tips/Tricks talks the last year, but given how many people are always surprised by its existence I thought it was worth a dedicated tip/trick post to raise the visibility of it (click here to read other posts in my ASP.NET Tips/Tricks series).
In previous versions of ASP.NET developers imported and used both custom server controls and user controls on a page by adding <%@ Register %> directives to the top of pages like so:
Note that the first two register directives above are for user-controls (implemented in .ascx files), while the last is for a custom control compiled into an assembly .dll file. Once registered developers could then declare these controls anywhere on the page using the tagprefix and tagnames configured.
This works fine, but can be a pain to manage when you want to have controls used across lots of pages within your site (especially if you ever move your .ascx files and need to update all of the registration declarations.
ASP.NET 2.0 makes control declarations much cleaner and easier to manage. Instead of duplicating them on all your pages, just declare them once within the new pages->controls section with the web.config file of your application:
You can declare both user controls and compiled custom controls this way. Both are fully supported by Visual Studio when you use this technique -- and both VS 2005 Web Site Projects and VS 2005 Web Application Projects support them (and show the controls in WYSIWYG mode in the designer as well as for field declarations in code-behind files).
One thing to note above is the use of the "~" syntax with the user-controls. For those of you not familiar with this notation, the "~" keyword in ASP.NET means "resolve from the application root path", and provides a good way to avoid adding "..\" syntax all over your code. You will always want/need to use it when declaring user controls within web.config files since pages might be using the controls in different sub-directories - and so you always need to resolve paths from the application root to find the controls consistently.
Once you register the controls within the web.config file, you can then just use the controls on any page, master-page or user control on your site like so (no registration directives required):
Hope this helps,
P.S. Special thanks to Phil Haack who blogged about this technique as well earlier this month (for those of you who don't know Phil, he helps build the very popular SubText blog engine and has a great blog).