I have always believed that each collection is by definition IEnumerable. That belief held true until today...
First let me define my loose definition of what a collection is. A collection is a set of objects. In .NET Framework I have noticed a special pattern for naming collection classes - they usually end with a Collection word. Examples include dozens of collection classes in System.CodeDom namespace, AttributeCollection or even ControlCollection. I'm so used to this kind of naming convention that when I see it I automatically know that I can do a foreach loop over it.
There is however at least one exception from this rule and that is a System.Web.UI.CssStyleCollection class. It is a class that derives directly from System.Object and it does not implement any interface which means no IEnumerable!!! When I looked at the public properties, they suggest that it should really be a collection. It has an indexer, it has a collection of keys and values. It even has a Count property not to mention all the usual methods for manipulating collection-like objects.
Strange, but true 