Master pages, introduced with ASP.NET 2.0, provide developers an easy way to create a shared layout for a site or portion of a site without having to work around Visual Studio .NET. While many sites will have a master page specified in each page by default, you can also design your site to select a master page dynamically. For example, an application I've created contains two master pages. Which one the user sees depends on which hostname he or she uses to visit the site. I do this so that the graphics and text are appropriate for the domain, but I have a single Web site servicing both domains behind the scenes.

The trick to selecting a master page at runtime is to add code into the OnPreInit event handler of your Web page. This is the point at which you can select the master page to be used for the particular Web page. For instance, you might have code that looks like this:

protected override void OnPreInit(EventArgs e)
{
       base.OnPreInit(e);
       if (Request.Url.Host.Contains("test.com"))
              base.MasterPageFile = Server.MapPath("test.com.master");
       else
              base.MasterPageFile = Server.MapPath("test2.com.master");
}

The MasterPageFile property requires a pathname and not a URL reference. However, you can always use Server.MapPath to convert your URL into an actual filename.

This feature works quite well for the content management system I've built. Use it to allow a page to choose a different look on the fly, based on whatever criteria you choose.