i think you are asking for the website application in asp.net.
then,
you can use...
<link rel="stylesheet" type="text/css" href="~/Full_path_of_the_File/file.css" />
here ~ indicates the root path of the solution.
or you can just use this in your asp.net application like..
add your site url path to the web.config file..
<appSettings>
<add key="Site_Url" value="http://localhost:12018/"/>
</appSettings>
and used it like..
<link href="<%=ConfigurationManager.AppSettings[" site_url="] %>css/base.css" rel="stylesheet" />