System.Collections.ObjectModel.Collection does not contain a definition for "Items"?

I'm following this article on how to build a Binary Tree in C# from msdn:

http://msdn.microsoft.com/en-US/libr...(v=vs.80).aspx

But in Unity my code is producing the following error:

error CS0117: 'System.Collections.ObjectModel.Collection >' does not contain a definition for 'Items'.

The offending code:

 using System.Collections.ObjectModel; 
 
 public class NodeList<T> : Collection<Node<T>>
 {
     public NodeList() : base() { } 
 
     public NodeList(int initialSize)
     {
         for (int i = 0; i < initialSize; i++)
             base.Items.Add(default(Node<T>));
     }
 
     ....
 }

Its the last line, which doesn't recognize Items from the Collection class, even though it has Items: http://msdn.microsoft.com/en-us/library/ms132397.aspx

Also its not producing an error in visual studio... only the Unity editor. Is this a bug in the Unity editor or something of my own? Could someone try to reproduce this for me?