By using the file= attribute of the <appSettings> section of the solution's main config file, developers can override settings for the solution locally. This is especially useful for projects where each developer keeps their own copy of a database locally. Changing the app.config or web.config file to the local setting and checking it in would change the setting for all of the developers on that team.

Below is an example where the developer used the developer.config file to point their local project to a different database than the one specified in the app.config or web.config file.

<?xml version="1.0"?>
<appSettings>
   <add key="database:Server" value="MyDBServer"/>
</appSettings>

Note: Since the developer.config file is not a standard .NET .config file, it is not copied to the output directory by default and therefore, won't be found by the application when run. If you add the developer.config file to your project, you should set the "Copy To Output directory" property on the file to "Copy always". Otherwise, you must manually copy the file to the folder where the main .config file resides.

See the "How-To" help document for a list of all configuration settings