Accessing SQL CE from WP7 Mango error

I'm running through the sql access example on the MSDN website (http://msdn.microsoft.com/en-us/library/hh202876(v=VS.92).aspx) and i can't understand why the following code isn't working.

MSDN Example: http://msdn.micro开发者_运维百科soft.com/en-us/library/hh202876(v=VS.92).aspx

My Code: http://pastebin.com/MxWtGvPw

The Error: The type or namespace name 'ObservableCollection' could not be found (are you missing a using directive or an assembly reference?)

I'm really struggling to get my head around this so if anyone has any pointers, that would be great.

Thanks!

---

You seem to be missing:

using System.Collections.ObjectModel;