A difference in style: IDictionary vs Dictionary

Question

I have a friend who's just getting into .NET development after developing in Java for ages and, after looking at some of his code I notice that he's doing the following quite often:

IDictionary<string, MyClass> dictionary = new Dictionary<string, MyClass>(); 

He's declaring dictionary as the Interface rather than the Class. Typically I would do the following:

Dictionary<string, MyClass> dictionary = new Dictionary<string, MyClass>(); 

I'd only use the IDictionary interface when it's needed (say, for example to pass the dictionary to a method that accepts an IDictionary interface).

My question is: are there any merits to his way of doing things? Is this a common practice in Java?

Answer 1

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.