Why are application settings read-only in app.config?

The real complete answer:

The app.config settings are read-only because there are 2 types of settings:

1. Application Settings
2. User Settings

The first won’t change unless the application publisher publishes a new version of it. The second is not stored in the app.config, but in a user.config file. In the abscence of this user.config file the app.config provides the default value.

If MySetting is a User Setting:

Settings.Default.MySetting = MyNewValue;
Settings.Default.Save();

It will create a user.config file at [User Local Settings Application Data]\[company name]\[application].exe[hash string]\[version] with the new settings, and those settings will prevail over the settings in the app.config file.