Microsoft & .NET.NETSmart Client: Composite UI Application Block

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In this article you’ll look at another Smart Client technology. Specifically, you’ll explore the Composite UI Application Block (CAB) from the Microsoft Patterns and Practices group. It allows you to build complex Windows Forms-based solutions using proven techniques that make it simple. This article won’t have sample code as the articles normally do because it assumes you’ve downloaded the CAB file that contains full source code for your use.

Application Block Background

The CAB is designed to make it easier for you to build enterprise-ready Windows Forms applications. It is based on the Composite design pattern, which involves creating simple independent user interface parts and assembling them into a solution. A shell Windows Forms-based solution hosts the various parts that act as plug-ins. It allows you to build clients composed of independent yet cooperating modules. For those familiar with Windows SharePoint Services or SharePoint Server, it is similar in concept to SharePoint for Windows applications.

The power of CAB lies in the ability to separate development concerns to target resource expertise as well as to allow for a fluid application that is ready for change. By modularizing components and logic, a CAB application can handle modifications to individual pieces of the application with ease. It also enables out-of-the-box runtime UI customization capabilities based on user roles, which you will see later in this article.

The application block is a component built on the Microsoft .NET Framework 2.0. The full source code is available from the Microsoft Patterns and Practices site. It is available in Visual Basic .NET or C# formats.

The Basics

There are two classes that are needed initially when building a CAB application. First, the ShellApplication is the class that encapsulates the entire application. Second, the WorkItem is a container for all of the classes that work together. A form will provide the real estate for the application. It holds what are known as workspace controls that will in turn contain the user interface parts known as SmartParts. There are several types of workspaces:

  • WindowWorkspace: Show and hide controls inside a framed window.
  • TabWorkspace: Show and hide controls and SmartParts inside a tab of pages.
  • ZoneWorkspace: Create a tiled Workspace where you use splitter controls to create multiple column or row displays. It allows you to create columned and flexible displays similar to that of Microsoft Outlook.
  • DeckWorkspace: Show and hide controls and SmartParts with no visible frame.
  • MDIWorkspace: Show and hide controls and SmartParts in MDI child forms.

The following wireframe depicts an example of the ZoneWorkspace filled with various SmartParts.

Main menu


Search Form SmartPart

Search Results SmartPart


Sharing Information with WorkItems

A CAB application can, and usually does, have multiple WorkItems, but it must have at least one (referred to as the root WorkItem). A WorkItem can be thought of as an implementation of one complete use case for the business logic of an application. Architected in this way, the application would have one WorkItem per use case that it defines. WorkItems hold state for the components contained within them. This allows the components to talk to each other and work together to accomplish the goal of the WorkItem. For example, components can customize (or extend) UIExtensionSites. An example of a UIExtensionSite is a MenuStrip. Because a UIExtensionSite is created at the shell level, the components within the WorkItems then can extend the MenuStrip with the additional items they need. This provides an easy way to manage shared components of the application’s interface.

Building the Parts

SmartParts are the visual components of the application. They are hosted by a Workspace or SmartPartPlaceHolder control. You can create SmartParts by simply customizing a standard user control. They can be created in advance at design time or even created programmatically at runtime. They are designed to be pluggable and reusable within the same or other applications. To return to the SharePoint analogy mentioned earlier, you could think of these as being similar to WebParts.

Connecting the Parts

SmartParts are conceptually designed to be modular and independent of each other. This helps increase application stability and allows the application to be flexible. However, the parts must be able to communicate to cooperate. They can communicate by publishing and subscribing to events. Additionally, there is the state class, which is a dictionary of information that provides notification when items change. You should also be aware that CAB employs a “dependency injection” technique for allowing components to reference instances of other components in a highly fluid, pluggable system. The details of this topic, however, are beyond the scope of this article.

Managing the Parts

Part of the appeal of the CAB is being able to adapt the application to fit the needs of the user. SmartParts can be loaded programmatically. The CAB will load modules based on the contents of the ProfileCatalog.xml file. A module is a collection of components (WorkItems, services, and so forth) that are needed when performing a certain task or assuming a specific role in the application. It is also possible to enable the application to load SmartParts based on custom configuration settings in an XML or other means of configuration. This allows you to adapt the UI and load modules based on criteria such as user roles. For example, the UI could load a shop floor console, manager console, and administrator console based on the current user’s assigned permissions.

Items of Interest

The following additional information should assist you in your endeavors with the Composite UI Application Block:

  • Hands-on labs: Free training content for the CAB. This contains in-depth detail about the contents of the CAB, including descriptions of the sample applications that are included.
  • Community Site: Public site targeted at helping CAB users.
  • Smart Client Software Factory: See below for more information.

Smart Client Software Factory (SCSF)

The Smart Client Software Factory is a great resource for getting started with the CAB. In addition to documentation, examples, and source code, it provides a “guidance package” that automates portions of the design process that are often repeated by developers of CAB-like applications. The link provided above provides extensive information about setting up the SCSF and making the most of its functionality.


Although we didn’t show source code, you have learned about the CAB at a very high level that has been so graciously shared by the Microsoft Patterns and Practices group. It allows you to create Windows Forms-based applications that have sophisticated capability without being as sophisticated to create.

Future Columns

The topic of the next column is yet to be determined. If you have something in particular that you would like to see explained here, you could reach me at

About the Authors

Mark Strawmyer, MCSD, MCSE, MCDBA is a Senior Architect of .NET applications for large and mid-size organizations. Mark is a technology leader with Crowe Chizek in Indianapolis, Indiana. He specializes in architecture, design, and development of Microsoft-based solutions. Mark was honored to be named a Microsoft MVP for application development with C# for the third year in a row. You can reach Mark at

Rachel Wireman is a developer of applications for large and mid-size organizations. She specializes in the development of Smart Client applications. Rachel is with Crowe Chizek in Oak Brook, Illinois and contributed to the examples within the article. You can reach Rachel at

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