In .Net 2.0, they introduced Generic Collections. You are able to put a type of object in an array list. For example , ArrayList <Cat> list = new ArrayList <Cat> In this list, all of objects are the instances of Cat Object. We do not need to casting the item and type safe in this list. But in the past time, I like to put a mixture of object types in an array list. Loop out each item and check its type. I used that a lot, when I parse the object from database and do some data mining tools.(Well, I did it in my spare time, of couse , not at work).I think this is a time to change this habit. This is a kind of hack.