It’s an easy mistake to have your logic take a configuration option straight from the configuration file, for example using a piece of code like:

var section = ConfigurationManager.GetSection("appSettings");
var hostname = section.Get("hostname");

There’s couple of reasons to not do that:

• It gets harder to unit test the piece of code: you’re going to need a configuration file to property run the test. It’s hard to control the property’s value from the setup of the unit test.
• Your logic demands something from it’s runtime container, which is bad for reuse.
• You’re mixing responsibilities, namely the logic your class is responsible for and the Configuration concern. If your logic needs a parameter it should be passed to your logic from the outside, either through the contructor, through the method usage (e.g. a method call), or by requesting the value through some interface.