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This chapter is from the book

Using the Web.Config File

ASP.NET applications are configured with Web.Config files, which are standard, human-readable XML files that you can open and modify with any text editor.

The ASP.NET framework uses a hierarchical configuration system. You can use Web.Config files to specify the settings for every ASP.NET application on a machine, a particular ASP.NET application, the ASP.NET pages in a particular directory, or a single ASP.NET page.

At the top of the hierarchy sits the Machine.Config file, which specifies the settings that are global to a particular machine. This file is located at the following path:

\WINNT\Microsoft.NET\Framework\[Framework Version]\CONFIG\machine.config

You can override settings in the Machine.Config file for all the applications in a particular Web site by placing a Web.Config file in the root directory of the Web site as follows:


This optional file applies to all applications (virtual directories) for a particular Web site. If you don't include it, all the settings from the Machine.Config file apply.

You also can place a Web.Config file in the root directory of any particular application—in other words, in the root directory of a particular virtual directory. The following Web.Config file applies to all pages within the application:


Finally, you can place a Web.Config file in any subdirectory of an application. The file then applies to any pages located in the same directory or any subdirectories.


To learn how to apply a Web.Config file to a single file, see the section titled "Setting the Configuration Location" later in this chapter.

You need to understand that child Web.Config files override configuration settings specified by their parents. This means that you do not need to copy the complete contents of a parent Web.Config file when creating a Web.Config lower in the hierarchy. You can specify only the configuration settings that you need to modify.

When you modify the settings in the Web.Config file, you do not need to restart the Web service for the modifications to take effect. The ASP.NET framework caches the settings in the Web.Config file. When you modify the file, the framework automatically detects the file change and reloads the settings from the file into the cache.

The Web.Config file doesn't secretly use the computer Registry or the Metabase to save configuration information. The ASP.NET framework reads all the configuration information directly from the Web.Config files. This means that if you need to move an ASP. NET application to a new server, you can simply copy all the files (including the Web.Config files) to the new server, and all the configuration settings are carried over.

Examining the Configuration Sections

This section provides a brief overview of all the standard sections of the Web.Config (and Machine.Config) configuration files that apply to ASP.NET applications. Because many of the configuration sections are described more fully in other parts of this book, I have also provided references to other parts of this book where you can find more information.


The Machine.Config and Web.Config files are case-sensitive. The section names in these files use a naming convention called camel casing. According to this convention, the first letter of the first word of a section name is lowercase.

The configuration settings that affect ASP.NET applications are contained within the System.Web section of the Machine.Config and Web.Config files. The following is a partial list of the settings found in each of these sections:

  • trace—This section contains configuration settings for page and application tracing. For more information, see Chapter 18, "Application Tracing and Error Handling."

  • globalization—This section specifies the character encoding to use with both requests and responses.

  • httpRunTime—This section specifies the maximum amount of time that a page will execute without timing out, the maximum size of a request, and whether fully qualified URLs should be used for client redirects. (Some mobile devices require fully qualified client redirects.)

  • compilation—This section contains configuration information for compiling pages. You can specify the default language to use for compiling ASP.NET pages, such as Visual Basic, C#, or JavaScript. (If you are fond of C#, you might want to specify that language as the default.)

  • This section can also be used to indicate whether pages should be compiled in debug mode so that you can view error information.

    You can also use this section to specify the assemblies available in an application. By default, all assemblies located in the /bin directory are available. However, you can modify this configuration section to make only select assemblies available.

  • pages—This section specifies configuration information for ASP.NET pages. For example, you can use this section to disable page buffering, session state, or view state.

  • customErrors—This section specifies how error information should be displayed. To learn more information, see Chapter 18.

  • authentication—This section specifies information for authenticating users. For more information, see Chapter 19, "Using Forms-Based Authentication," and Chapter 20, "Using Windows-Based Authentication."

  • identity—This section configures user account impersonation. For more information, see Chapter 20.

  • authorization—This section specifies the users and roles authorized to access files. For more information, see Chapters 19 and 20.

  • machineKey—This section is used for sharing a standard encryption key across machines in a Web farm. For more information, see Chapter 19.

  • trust—This section is used for setting security policies. For more information, see Chapter 20.

  • securityPolicy—This section contains a list of available security policies. For more information, see Chapter 20.

  • sessionState—This section contains configuration information for session state. You can use this section to enable in-process, out-of-process, or cookieless sessions. For more information, see Chapter 16, "Tracking User Sessions."

  • httpHandlers—This section associates a particular HTTP handler with a particular page path and request verb. For more information, see "Using HTTP Handlers and Modules" later in this chapter.

  • httpModules—This section lists the modules that are involved in every page request. For more information, see "Using HTTP Handlers and Modules."

  • processModel—This section configures the process model settings on Internet Information Server. For more information, see Chapter 18.

  • webControls—This section specifies the location of the client-side script library used with Web controls, such as the validation controls. For more information, see Chapter 3, "Performing Form Validation with Validation Controls."

  • clientTarget—This section lists the values that you can use with the ClientTarget property. See Chapter 3.

  • browserCaps—This section lists information on the capabilities of different browsers. The HttpBrowserCapabilities class uses this information to report on the features of different browsers.

  • webServices—This section contains configuration information for Web services. For more information, see Part VI of this book, "Building ASP.NET Web Services."

Modifying Configuration Settings

You can modify configuration settings directly in the Machine.Config file. However, by modifying the Machine.Config file, you modify the configuration settings for every application on your machine. For this reason, typically, you should add a new Web. Config file to a particular subdirectory, rather than modify the global Machine.Config file.

Suppose that you are developing a new ASP.NET application, and you want all the pages in the application to display error information. In that case, you can add the Web.Config file in Listing 15.9 to the root directory of the application. (This file is located in the ConfigErrors subdirectory on the CD that accompanies this book.)

Listing 15.9 ConfigErrors/Web.Config

  <compilation debug="true" />

The file in Listing 15.9 overrides a single configuration setting in the Machine.Config file; it sets debug mode to true for the application. If you request an ASP.NET page that contains a runtime error, the error message is displayed along with the source code listing that indicates the exact line where the error occurred.

You can test the Web.Config file in Listing 15.9 by requesting the DisplayError.aspx page from the ConfigErrors subdirectory. This page intentionally generates a runtime error.


Because Web.Config and Machine.Config are XML files, they are case sensitive. You therefore receive an error if you create a Web.Config file that changes the Debug mode, rather than the debug mode.

Setting the Configuration Location

By default, the Web.Config file applies to all the pages in the current directory and its subdirectories. You can modify this default behavior by adding a location section to a configuration file.

You can use a location section to specify a path for configuration settings. The path can be used to specify an application, a particular directory, or even an individual file.

The Web.Config file in Listing 15.10, for example, disables view state for all files in a subdirectory named NoViewState. (You can find this file in the ConfigLocationDir subdirectory on the CD that accompanies this book.)

Listing 15.10 ConfigLocationDir/Web.Config

 <location path="NoViewState">
  <pages enableViewState="false" />

The configuration settings in Listing 15.10 apply to a directory named NoViewState. They do not modify the settings for the current directory.

You can test the Web.Config file by requesting the DisplayViewState.aspx file located in both the ConfigLocationDir subdirectory and the ConfigLocationDir/NoViewState directory.

You also can use the <location> tag to modify the configuration settings for a particular page. For example, the Web.Config file in Listing 15.11 denies authorization on a single page named Secret.aspx. (You can find this file in the ConfigLocationFile subdirectory on the CD that accompanies this book.)

Listing 15.11 ConfigLocationFile/Web.Config

  <authentication mode="Forms" />

 <location path="secret.aspx">
   <deny users="*" />

Because the Web.Config file in Listing 15.11 modifies the authentication mode, you must place it in the root directory of your application.

The <location> tag denies access to the Secret.aspx page. If you request the Secret. aspx page, you are automatically redirected to the Login.aspx page.

The location tag applies to only a single file in the directory. You can request any file other than the Secret.aspx page.

Locking Configuration Settings

You can use the <location> tag to lock configuration settings in the Web.Config file so that they cannot be overridden by a Web.Config file located below it. You can use the allowOverride attribute to lock configuration settings. This attribute is especially valuable if you are hosting untrusted applications on your server.

The Web.Config file in Listing 15.12, for example, locks the identity configuration setting for two subdirectories. The Web application runs under the app1 identity in the directory named application1, and the Web application runs under the app2 identity in the directory named application2. (You can find the Web.Config file in Listing 15.12 in the ConfigLock directory on the CD that accompanies this book.)

Listing 15.12 ConfigLock/Web.Config

 <location allowOverride="false" path="application1">
  <identity userName="app1" password="secret" />
 <location allowOverride="false" path="application2">
  <identity userName="app2" password="secret" />

Because the Web.Config file in Listing 15.12 locks the settings with the allowOverride attribute, you cannot place a Web.Config file that alters the identity settings in either the application1 or application2 subdirectory. If you attempt to modify these settings, you get an error.

You can test this by attempting to retrieve the Default.aspx file from the /ConfigLock/Application1 directory. This directory contains a Web.Config file that attempts to override the locked identity setting. When you attempt to request the Default.aspx file, an error is generated.

Typically, you lock configuration settings by placing a Web.Config file similar to the one in Listing 15.12 in the Web site root directory, or by modifying the Machine.Config file.

Adding Custom Configuration Information

You can add your own application configuration information within the appSettings section of the Web.Config file. Adding information to this section is useful for storing such information as database connection strings.

The Web.Config file in Listing 15.13, for example, contains a database connection string that connects to the Northwind database. (You can find this file in the ConfigCustom directory on the CD that accompanies this book.)

Listing 15.13 ConfigCustom/Web.Config

  <add key="conString"
   value="Server=localhost;UID=sa;PWD=secret;Database=Northwind" />

You can retrieve configuration information from the Config.Web file with the AppSettings property of the ConfigurationSettings class. For example, the page in Listing 15.14 uses the value conString to open a database connection.

Listing 15.14 Default.aspx

<%@ Import Namespace="System.Data.SqlClient" %>

<Script Runat="Server">

Sub Page_Load
 Dim strConString As String
 Dim conNorthwind As SqlConnection
 Dim cmdSelect As SqlCommand

 strConString = ConfigurationSettings.AppSettings( "conString" )
 conNorthwind = New SqlConnection( strConString )
 cmdSelect = New SqlCommand( "select * From Products", conNorthwind )
 dgrdProducts.DataSource = cmdSelect.ExecuteReader()
End Sub



 Runat="Server" />


Notice that the value of conString is retrieved from the Web.Config file with the following single line of code:

strConString = ConfigurationSettings.AppSettings( "conString" )
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