A master page is useless without a content page that uses it. Generally, you’ll have only a few
master pages, whereas you can have many content pages in your site. To base a content page
on a master page, check the Select Master Page option at the bottom right of the Add New Item
dialog box when you add a new Web Form to your site. Alternatively, you can set the
MasterPageFile attribute on the page directly in the Markup View of the page. You saw this @
Page directive earlier in this chapter when master and content pages were introduced.
Content pages can only directly contain Content controls that each map to a ContentPlaceHolder
control in the master page. These content controls in turn can contain standard markup like HTML
and server control declarations. Because the entire markup in a content page needs to be wrapped
by tags, it’s not easy to turn an existing ASPX page into a content page. Usually the
easiest thing to do is copy the content you want to keep to the clipboard, delete the old page, and
then add a new page based on the master page to the website. Once the page is added, you can paste
the markup within the tags. You see how this works in the following exercise.