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This chapter presents a brief introduction to Windows Workflow Foundation (WF). Instead of diving deeply into any single workflow topic, it provides you with a sampling of topics that are fully presented in other chapters.

The purpose of this chapter is to provide a grand tour of Windows Workflow Foundation (WF). It is a high-level description of the discrete parts of WF and how they work together. Because of the summary focus of this chapter, it doesn’t contain much actual workflow code. As you dig deeper and deeper into WF in later chapters, you’ll want to refer back to this chapter to see where individual features fit into the big picture.

The focus of this chapter is the primary building block of all workflows: the activity. This chapter provides a high-level review of the standard activities that are provided with Windows Workflow Foundation (WF). You will become acquainted with the available activities, but you will not learn how to use all of them in detail. This is the one chapter that provides an overview of all of the available activities. Many of the individual activities are difficult to understand unless they are discussed as part of a larger subject area. For this reason, the subsequent chapters in this book each focus on a specific subject area and provide additional detail on the activities that are relevant to that subject.

The focus of this chapter is hosting the workflow runtime within your applications. A simple hosting example is used to illustrate the basic requirements for workflow hosting. A set of workflow manager classes are then developed that address some of the problems and limitations of the first example.

One of the great advantages to workflows is that they enable you to declaratively control the flow of execution. No longer are you required to tightly couple the business logic (the and ) with the flow control (the ). You can develop discrete, independent activities containing your reusable business logic and then knit them together using the flow control activities.

Windows Workflow Foundation (WF) provides the ability to optionally register services with the workflow runtime engine. This ability provides an extensible way to modify the behavior of the workflow runtime. WF defines a set of core workflow services that handle thread scheduling, workflow persistence, transactions, and workflow tracking. The designers of WF could have embedded the implementation of these services in the runtime engine itself, but they wisely chose to externalize them, placing their implementations in pluggable services. This places you in control. You decide which services to use (some are optional) and which implementation to use for each service.

E suspend execution of a workflow until an event is received. The event is commonly raised externally by a local service, but it can also be raised internally from within a workflow. These types of activities are vital to state machine workflows, since they are typically used to trigger state transitions. But they are also a valuable tool for sequential workflows.

An important capability of workflows is that they can be (saved and reloaded at a later time). Workflow persistence is especially important when developing applications that coordinate human interactions, since those interactions could take a long period of time. But persistence is also applicable to other types of applications. Without persistence, the lifetime of your workflows is limited. When the host application is shut down, any workflow instances simply cease to exist.

The focus of this chapter is state machine workflows. are different from sequential workflows in a number of ways. Most importantly, they don’t define a hardwired sequence of steps within the workflow. Instead, they define a set of application states with possible transitions between states. Each state can handle multiple external events that trigger execution of child activities including a possible transition to another state.

Integrity and consistency are important qualities to have when performing work in any application. You generally don’t want to perform work in an inconsistent manner, or in a way that might leave the integrity of the data in doubt. These are important qualities for workflow applications as well as traditional applications.