ASP.NET web.config: configSource vs. file attributes

Within an web.config-file in an ASP.NET-application some sections of config, like appSettings and connectionStrings, supports the attributes file and configSource.

What is the difference between using the file-attribute and the configSource-attribute? When should you use which attribute and can you use both?

<?xml version="1.0"?>
<configuration>
  <appSettings file="AppSettings.config">
  </appSettings>
  <connectionStrings configSource="ConnectionStrings.config">      
  </connectionStrings>
  <!-- ... -->
</configuration>

file attribute

• Specifies a relative path to an external file that contains custom application configuration settings
• specific to the appSettings section
• will merge (and override) settings in the .config file
• will not cause web application to restart when modifying the specified file
• http://msdn.microsoft.com/en-US/library/ms228154(v=vs.100).aspx
• Using the Configuration.AppSettings.Settings.Add API will result in all settings being merged back into the main .config on a Configuration.Save call.
• since .NET 1.1
• Exception is not thrown if file does not exist.

configSource attribute

• can apply to most sections of a configuration file, not just appSettings
• will override the entire section with the external file, no merging
• CAN cause web application to restart
• http://msdn.microsoft.com/en-US/library/system.configuration.sectioninformation.configsource(v=vs.100).aspx
• Using the Configuration.AppSettings.Settings.Add API will result in all settings being added to the file specified in configSource on a Configuration.Save call.
• since .NET 2.0
• System.Configuration.ConfigurationErrorsException is thrown if config source file does not exist.

The file attribute specifies an external file containing custom settings like you do in the appSettings entry of the web.config file. Meanwhile, the external file specified in the configSource attribute contains the settings for the section which you declare the configSource for. For example, if you use the configSource attribute of the pages section, then the external file will contain the settings for the pages section.

> The custom settings declared in the external config specified in the > file attribute will be merged with the settings in the appSettings > section in the web.config file. In the meanwhile, the configSource > does not support merging, it means that you'll have to move the entire > section settings into the external file.

http://www.codeproject.com/Messages/1463547/Re-difference-between-configSource-and-file-attrib.aspx