Master Pages

A master page, saved with a .master extension, defines the shared chrome of a site — header, navigation, footer, and CSS/script references — and declares one or more asp:ContentPlaceHolder controls marking the regions that individual content pages are allowed to fill. A content page then sets its @ Page directive's MasterPageFile attribute and wraps its own markup in asp:Content controls, each one targeting a specific ContentPlaceHolderID, so the final rendered page is the master's layout with the content page's fragments merged into the matching placeholders.

At runtime, ASP.NET parses the content page, locates its MasterPageFile, and merges the two control trees: everything inside an asp:Content block is injected into the ContentPlaceHolder with the matching ContentPlaceHolderID, while any markup in the master page outside a placeholder is rendered as-is on every page that uses it. A content page cannot place markup outside of asp:Content tags — anything outside those tags in a page that specifies MasterPageFile is simply ignored — and a master page can itself reference another master page to build nested layouts for sections of a site that need a shared sub-layout, such as an admin area within a public site.

Rather than hard-coding MasterPageFile in the @ Page directive, a site can select a master page at runtime in the PreInit event, the earliest point in the page lifecycle where the master can still be changed: Page.MasterPageFile = "~/AltSite.master" set inside Page_PreInit lets an application swap layouts per user, per theme, or per section of the site (for example, a slimmer master page for a mobile-optimized area). Content pages can also expose strongly typed properties or public members that the master page reads, and, in the other direction, a content page can access members on its master via the Page.Master property cast to the specific master page type, enabling two-way communication between page and layout.