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

Question

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>

Solution

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

Comments

[Ed Graham]

Also, the "file" attribute allows you to specify files outside the immediate directory tree, which is important for sharing common settings among different sites. Unfortunately, the "configsource" attribute restricts you to files within the currrent tree, so for shared settings you need to specify a virtual directory in IIS.

[Ed Graham]

My above comment was not entirely correct, and I've apparently missed the somewhat arbitrary five-minute window to edit it! You can't set a virtual directory in IIS to allow "configsource" files to live outside the immediate directory tree. So that really is quite limiting. I solved it by using a junction point (or NTFS hard link) but it's not exactly pretty ...

[Leniel Maccaferri]

WAO! Great answer... this really helped me: will not cause web application to restart when modifying the specified file. I do need the app to restart when modifying any setting in the external file for a custom section and so the way to go is with configSource. Interesting is that I was using file and the different sections were working. file also works for sections other than appSettings but there are those gotchas well explained in the answer.

[Piotr Kula]

What if I want a default "connectionstring" for example but use confisection for a local file 100 developers have that is not checked in. With configsections we are forced again to check in the file completely making this exercise redundant?

[frankhommers]

I would like to add to the configSource list: It must refer to a file in the same directory or in a subdirectory as the configuration file.. And also to the file list: It can reside outside the directory of the configuration file itself..

[Tomer Yoskovich]

Is there a way to have that reversed? "will merge (and override) settings in the .config file"

[Steven Liekens]

If calls to Save() writes values to the main .config file, but those values are overridden by the (old) values in the external file, then how is this design not horribly broken? Am I missing something?

[tibx]

Also the file defined in configSection attribute must always exist. It can't be optional. If this file doesn't exist, IIS returns error 500.52 Specified config Source cannot be parsed.

[Jarrod]

Updated link for appSettings element - docs.microsoft.com/en-us/dotnet/framework/configure-apps/…