Use of IsAssignableFrom and "is" keyword in C#
It's not the same.
if(controllerType is IController)
would always evaluate to false since controllerType is always a Type, and a Type is never an IController.
The is operator is used to check whether an instance is compatible to a given type.
The IsAssignableFrom method is used to check whether a Type is compatible with a given type.
typeof(IController).IsAssignableFrom(controllerType) tests a Type against the interface. The is operator tests an instance against the interface.