There’s a little known attribute which was introduced in the .Net framework 2.0 called configSource, which allows you to externalize sections of the configuration file.
It works just like the optional file attribute found on the appSettings element:
<appSettings file = “relative file name” />
but can be added to any configuration section to specify an external file for that section:
<CustomSection configSource=“relative file name” />
When it comes to WCF though, unfortunately the configSource attribute doesn’t work on the <system.serviceModel> section group, but luckily you can still apply it on the child config sections. See the reference link below for some examples.