Recipe 1.4. Changing Which Master Page Is Used Without Modifying All Affected Application Pages

Problem

You want to use the same master page for all pages in a section of your application, but you want the ability to change which master page is used without having to modify all of the affected pages, something you cannot do with the more traditional master/content page approaches described in the previous recipes in this chapter.

Solution

Implicitly assign a master page to a content page as follows:

Create a new folder within your application.
Create the master page your application will be using.
Place all pages that will be using the master page in the folder.
Create a web.config file that contains a <pages> element with the masterPageFile attribute set to the name of the .master file and place it in the folder.

Example 1-5 shows the web.config file used to set the master page implicitly, and Examples 1-6 and 1-7 show the .master and .aspx files for our example.

Discussion

In some applications, it is desirable to use the same master page for a large number of content pages and, at the same time, to be able to change to another master page without having to edit all of the pages in the application. ASP.NET 2.0 provides the ability to assign the master page implicitly to content pages by using the new masterPageFile attribute of the <pages> element in the web.config file. Setting the masterPageFile attribute to the name of a .master file assigns the master page to all content pages in the folder where the web.config file is located. This assigns the master page to all content pages in all subfolders unless another web.config file in a subfolder overrides the setting.

ASP.NET provides a lot of flexibility when using this approach. Any content page can still explicitly set the MasterPageFile attribute in the @ Page directive, as described in Recipe 1.1, which will override the setting in the web.config file. This is convenient when you have a small number of pages that need to be handled differently; however, this can become confusing when the assignment of a master page is changed in the web.config file and the developer is expecting it to affect all pages.

Implicitly assigning a master page using this approach will not work for nested master pages. If a nested master page is included without explicitly setting the MasterPageFile attribute of the @ Master directive, you will get an error indicating that content controls are only allowed in pages that reference a master page.

In our example, we have created a new folder and placed in it the web.config file shown in Example 1-5 along with the .master and .aspx files shown in Example 1-6 and Example 1-7. The primary difference between this example and the example shown in Recipe 1.1 is the removal of the MasterPageFile attribute from the @ Page directive of the content page and the presence of the web.config file.

Using this approach can cause problems referencing images and stylesheets. This is particularly an issue if a master page uses images, is located in the root directory, and is then used in a subfolder. The page is rendered using the path to the subfolder resulting in the URL for images pointing to the subfolder. If the images exist only in the root folder, the images referenced by the master page will not be displayed.

 <%@ Master Language="VB"     AutoEventWireup="false" %>  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"              "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">  <html xmlns="http://www.w3.org/1999/xhtml" >  <head  runat="server">      <title>ASP.NET Cookbook 2nd Edition</title>  <link rel="Stylesheet" href="../css/ASPNetCookbook.css" />  </head>  <body>      <form  runat="server">           <div align="center" >                <img src="/books/1/505/1/html/2/../images/ASPNETCookbookHeading_blue.gif" />           </div>           <div>                <asp:ContentPlaceHolder  Runat="server" >                     <div align="center">                           <br />                           <br />                           <h4>Default Content Displayed When No Content Is Provided In                                    Content Pages</h4>                     </div>                </asp:ContentPlaceHolder>           </div>      </form>  </body>  </html>

Example 1-7. Implicitly assigning a master page to a content page (.aspx)

 <%@ Page Language="VB"    AutoEventWireup="false"    title="Attaching Master Pages Using Web.Config" %>  <asp:Content  ContentPlaceHolder Runat="Server">      <div align="center" >           Attaching Master Pages Using Web.Config (VB)      </div>  <br />  <p align="center">The content for your pages is placed here.</p>  </asp:Content>

Problem

Solution

Discussion

See Also

Example 1-5. web.config file for implicitly assigning a master page to a content page

Example 1-6. Implicitly assigning a master page to a content page (.master)

Example 1-7. Implicitly assigning a master page to a content page (.aspx)