When looking at ASP.NET MVC examples on the web almost all action methods return ActionResult, even methods that could return a specific subclass. Here is an example from the NerdDinner source code:
This method always returns a ViewResult, still they declare the return type as the ActionResult base class. This practice often leads to test code like the following:
This also makes it immediately clear for people reading the code, what this action method returns.
The only time I would return ActionResult is when different paths of the action method returns different subclasses. In these cases, you should of course write tests that verify the return types of the different paths.
In the book Pro ASP.NET MVC Framework Steven Sanderson also recommends returning specific types. Take a look at the quote below:
“This action method specifically declares that it returns an
instance of ViewResult. It would work just the same if instead the
method return type was ActionResult (the base class for all action
results). In fact, some ASP.NET MVC programmers declare all their
action methods as returning a nonspecific ActionResult, even if they
know for sure that it will always return one particular subclass.
However, it’s a well-established principle in object-oriented
programming that methods should return the most specific type they can
(as well as accepting the most general parameter types they can).
Following this principle maximizes convenience and flexibility for code
that calls your method, such as your unit tests.”
BTW: This blog post is not meant to criticize the NerdDinner tutorial. It is a great free tutorial, and is probably one of the best ways to get introduced to the ASP.NET MVC framework