Created attachment 16766 [details]
Sample project solution which demonstrates the problem.
Mono version 4.2.1 (Debian 18.104.22.168+dfsg2-8)
When modifying the App.config file from within an application, if the ServiceModel Client section is referenced in the code, that section will be excluded from the updated config file.
Attached is a project which demonstrates the problem. Execute the "compile-project" shell script under Linux to reproduce the problem.
1) The App.config file contains a section for <system.serviceModel><client>.
2) When the code executes, ConfigurationManager.OpenExeConfiguration is use to retrieve the config file.
3) A key is added to the <appSettings> section.
4) Calling Configuration.Save() updates the App.config file.
The above steps work properly under both .NET and Mono.
However, if the ServiceModelSectionGroup.ClientSection is referenced within the code, under Mono, the <system.serviceModel><client> section will be removed from the updated App.config file.
This does not happen with .NET.
Within the Program.cs file, a "#if true" can be toggled to demonstrate the problem. See how the resulting application config file changes when it flips from true to false.
I can reproduce with the provided test case on OSX 4.6.0.