Master Page Basics in ASP.NET

The best websites don’t look like a series of web pages—instead, they give the illusion of a continuously running application. For example, try ordering a book on Amazon. While you search, click through the links, and then head to your shopping cart, you’ll always see a continuous user interface with a common header at the top and a set of navigation links on the left.

Creating something that polished with ASP.NET is possible, but it isn’t as easy as it seems. For example, what if you want a navigation bar on every web page? Not only do you need to copy the same user interface markup to each page, you also need to make sure it ends up in the same place. An offset of a couple of pixels will completely ruin the illusion, making it obvious that the pages aren’t really integrated. And even if you copy your markup perfectly, you’re still left with an extremely brittle design. If you decide to update your navigation bar later, you’ll need to modify every web page to apply the same change.

So how can you deal with the complexity of different pages that need to look and act the same? One option is to subdivide the page into frames. Frames are an HTML feature that lets the browser show more than one web page alongside another. Unfortunately, frames have problems of their own, including that each frame is treated as a separate document and requested separately by the browser. This makes it difficult to create code that communicates between frames. A better choice is to use ASP.NET’s master pages feature, which allows you to define page templates and reuse them across your website.

Master pages are similar to ordinary ASP.NET pages. Like ordinary pages, master pages are text files that can contain HTML, web controls, and code. However, master pages have a different file extension (.master instead of .aspx), and they can’t be viewed directly by a browser. Instead, master pages must be used by other pages, which are known as content pages. Essentially, the master page defines the page structure and the common ingredients. The content pages adopt this structure and just fill it with the appropriate content. For example, if a website such as www.amazon.com were created using ASP.NET, a single master page might define the layout for the entire site. Every page would use that master page, and as a result, every page would have the same basic organization and the same title, footer, and so on. However, each page would also insert its specific information, such as product descriptions, book reviews, or search results, into this template.