Use of IsAssignableFrom and "is" keyword in C#

Question

While trying to learn Unity, I keep seeing the following code for overriding GetControllerInstance in MVC:

if(!typeof(IController).IsAssignableFrom(controllerType)) { ... }

this seems to me a pretty convoluted way of basically writing

if(controllerType is IController) { ... }

I appreciate there are subtle differences between is and IsAssignableFrom, ie IsAssignableFrom doesn't include cast conversions, but I'm struggling to understand the implication of this difference in practical scenarios.

When is it imporantant to choose IsAssignableFrom over is? What difference would it make in the GetControllerExample?

if (!typeof(IController).IsAssignableFrom(controllerType))
      throw new ArgumentException(...);
return _container.Resolve(controllerType) as IController;

Answer 1

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.