SynchronizedCollection<T> class was introduced first in .NET 2.0 to provide a thread-safe collection class. It does this via locking so that you essentially have a
List<T> where every access is wrapped in a
System.Collections.Concurrent namespace is much newer. It wasn't introduced until .NET 4.0 and it includes a substantially improved and more diverse set of choices. These classes no longer use locks to provide thread safety, which means they should scale better in a situation where multiple threads are accessing their data simultaneously. However, a class implementing the
IList<T> interface is notably absent among these options.
So, if you're targeting version 4.0 of the .NET Framework, you should use one of the collections provided by the
System.Collections.Concurrent namespace whenever possible. Just as with choosing between the various types of collections provided in the
System.Collections.Generic namespace, you'll need to choose the one whose features and characteristics best fit your specific needs.
If you're targeting an older version of the .NET Framework or need a collection class that implements the
IList<T> interface, you'll have to opt for the
This article on MSDN is also worth a read: When to Use a Thread-Safe Collection