Question

There seem to be certain collection classes in System.Collections.ObjectModel as well. What is the difference between the collections under these two namespaces and in which scenario should we use System.Collections.ObjectModel?

Answer 1

From MSDN:

The System.Collections.ObjectModel
namespace contains classes that can be
used as collections in the object
model of a reusable library. Use these
classes when properties or methods
return collections.

See also The reason why Collection, ReadOnlyCollection, and KeyedCollection were moved to System.Collections.ObjectModel namespace