If IDictionary is a “more generic” type than Dictionary then it makes sense to use the more generic type in declaring variables. That way you don’t have to care as much about the implementing class assigned to the variable and you can change the type easily in the future without having to change a lot of following code. For example, in Java it’s often considered better to do
List<Integer> intList=new LinkedList<Integer>();
than it is to do
LinkedList<Integer> intList=new LinkedList<Integer>();
That way I’m sure all following code treats the list as a List and not a LinkedList, making it easy in the future to switch out LinkedList for Vector or any other class which implements List. I’d say this is common to Java and good programming in general.