The Composite UI Application Block facilitates the design and implementation of your client applications in three areas:

• It allows your application to be based on the concept of modules or plug-ins.
• It allows developers with shell expertise to build components that hide user interface complexity from the business logic development.
• It facilitates development using patterns for loose coupling between modules.

Reference:

Smart Client – Composite UI Application Block

Consolidating User Interface and Shell Expertise

The Composite UI Application Block provides an architecture around these components and an implementation based on Windows Forms. It includes:

• A consistent way of showing and hiding controls using Workspaces. A shell developer can consistently introduce visual effects, layout strategies, or other behaviors without affecting the business logic components.
• A common way of adding and using UIElements into the shell, such as menu items, status bars, so that the developer of an individual piece of business logic does not need to know how or where that element will be shown.
• A Command architecture that allows a business logic developer to separately define actions a user can take and how they are displayed in a specific shell.

Achieving Modular Design of Your Business Logic

• WorkItems that provide an easy way to scope which collaborating components participate in a use case, share state, events, and common services.
• An Event Broker that provides a many-to-many, loosely coupled event system mechanism between objects in your application.
• Placeholders for sharing State where multiple components can place or retrieve information.