From a purely functional perspective, the above code is all we need in order to implement our chat client.  To help improve the design-time experience in Blend, though, we'll also add a constructor that checks whether we are in runtime or design-time mode, and loads up our ChatSession object with "fake data" if it is being hosted in a designer:

Figure 35

We'll see in a moment how this helps make it easier to visualize data-bound data in the designer.