NET runtime, WF needs a host process to load, initialize, and start its runtime before anything interesting can happen. Unlike the traditional server-side usage of ASP. NET, however, WF will be useful in a variety of different hosts. We can host WF in a smart client application, a console application, or a Windows service, for instance.
The class diagram in the screenshot below features the primary classes we use to execute workflows in WF. Creating an instance of the WorkflowRuntime class and calling StartRuntime is all we need to spin up the workflow execution environment. WorkflowRuntime defines methods that allow customization of the execution environment. The class also defines events we can listen for during execution. The CreateWorkflow method returns an object of type WorkflowInstance. The WorkflowInstance class represents an individual workflow.
The Start method on the workflow instance object will begin the execution of a workflow. If an exception occurs, the workflow will invoke the Terminate method which leads to the runtime raising a WorkflowTerminated event.
A typical sequence of calls is shown in the screenshot next. The WorkflowRuntime and WorkflowInstance classes are arguably the most important classes needed at run time, but they are not the only classes available. The WorkflowRuntime class provides only the basic features for executing workflows. Don't worry, these features are available through an extensibility mechanism of WorkflowRuntime — the AddService method.
AddService allows us to make one or more services available to the runtime. These services might be custom services we've written specifically for our domain, like a custom scheduling service, or they might be services already written by Microsoft and included with WF. Let's continue our tour by looking at the services already available.
The DefaultWorkflowSchedulerService creates new threads to execute workflows. Because the threads are separate from the host application, the workflows do not block any application threads and execute asynchronously. The maximum number of simultaneously executing workflows is configurable. Server-side applications typically pull a thread from a pool to service each client request.
It makes sense to loan the thread to the WF runtime, and let the runtime execute the workflow synchronously on the existing request thread instead of using two threads per request, which could reduce scalability. The default transactional service is an instance of the DefaultWorkflowTransactionService class.
Activities inside a running instance of a workflow, and the services operating on the same instance, can all share the same transaction context.
WF relies on the implementation of transactions in. NET's System. Transactions namespace. The Transaction class offers a lightweight, auto-enlisting, and promotable transaction. The transaction can start as a local transaction, and later the runtime can promote the transaction to a heavyweight, distributed transaction if needed. Instead, the runtime can persist the state of the workflow to a durable store and unload the instance from memory.
In 30 days or hopefully, less , the runtime can reload the workflow instance and resume processing. The WF runtime will automatically persist a workflow that is idle or suspended when a persistence service is present. Of course, we'll need a database schema that the persistence service understands. A tracking service will tell the runtime the type of information it wants to know about workflows using a tracking profile.
Once the service establishes a profile, the service can open a tracking channel to receive events and data. Chapter 5 includes more details on tracking profiles and channels. The runtime does not start a tracking service by default, but we can programmatically add a tracking service or configure a tracking service with an application configuration file for the runtime to use.
Maybe you've had one of those product managers who is always at your desk, asking "are you done, yet? The sample isn't meant to demonstrate all the capabilities of the platform, but give a general feel for creating and running a workflow with WF. Supported development tools for the. The extensions are not compatible with the Express editions of Visual Studio We'll choose C as our language and select the Sequential Workflow Console Application template see the screenshot on the next page.
The template gives us a project with references to all the correct WF assemblies, an empty workflow, and a Program. Right-click the workflow and select Delete so we can start a workflow from scratch. If we click to expand the node containing Workflow1. As we mentioned earlier, the partial class will combine with the class generated from the XAML to produce a single type.
Let's modify the class in Workflow1. If we double-click the. We can drag a While activity from the Toolbox and drop the activity between the start and end point of our workflow. The While Activity executes a child task until some condition is met. Our next step is to drag a Code activity from the Toolbox into the center of the While activity. At this point, our designer should resemble the following screenshot:.
Notice both activities display a red exclamation point. The activities are failing their validation checks. We can hover the mouse cursor over the exclamation points and open a smart tag to view the validation error. If we tried to compile the program we'd see these same validation errors as compilation errors. We'll fix these errors now. The Code activity requires us to assign an event handler for the ExecuteCode event.
We can set the event by opening the Properties window F4 and clicking the Code activity to set focus. Double-clicking in the empty space beside the ExecuteCode property will send us into the code-beside file and generate a new event handler. We can place the following code into the event handler. This code will ask the user if a bug is fixed, and then read a key press. The Code activity should now pass validation, so we can turn our attention to the While activity.
A While activity requires a valid Condition property. Several activities in the base activity library work with conditions, including the IfElse, ConditionedActivityGroup , and Replicator activities. These choices represent the two techniques available to express a condition, the first being with code a method that returns a Boolean value , the second being with a rule. Let's select the RuleConditionReference. A rule condition is a named expression that evaluates to true or false, and can live in an external.
A plus sign appears beside the Condition property, and we can click the sign to expand the property editor. When the Condition property expands, the Property window gives us the ability to set a ConditionName and an Expression.
Clicking on the ellipsis … button in the Condition name will launch a Select Condition dialog box. We want the While activity to loop until the bug is fixed. Our rule is! Once we've entered the condition notice the editor provides IntelliSense , we can click OK.
We should select Condition1 and press OK. The While activity should now have a ConditionName and Expression set, and pass validation. Now we need to open the Program. We need to host the WF runtime and ask the runtime to execute our workflow. The item template for a workflow project provides all the boilerplate code we need. Let's review the code:. The code wires up event handlers to the runtime so we know if a workflow terminates because of an exception , or completes successfully. The code instantiates our bug-fixing workflow using the CreateWorkflow method, passing the type of our workflow.
Since the workflow engine executes our workflow asynchronously, we need to block our thread on an AutoResetEvent object and wait for the workflow to complete otherwise, the console mode program would exit before the workflow gets an opportunity to run.
An AutoResetEvent object will block a thread until the object is in a signaled state, which we do with the Set event in the event handlers. Software developers have been implementing workflows to model business processes since the beginning of time. During this time, we've learned that workflows can be long-running and often require input from humans. Net Framework 3. In this article, we took a look at the introductory concepts of Windows Workflows Foundation.
With all the new releases, it is definitely a very exciting time to be a developer. This article is for purely educational purposes and is a compilation of notes, material and my understanding on this subject. Any resemblance to other material is an un-intentional coincidence and should not be misconstrued as malicious, slanderous, or any anything else hereof. View All.
Basic Introduction to Windows Workflow Foundation. Dipal Choksi Updated date Jul 17, Some of the major criteria for workflows are listed below: Visual Representation of process es Can be dynamically modified at run-time Can be long-running There are two major types of workflows - Sequential workflows: used for well-defined, process workflows. State Machine workflows: Organized as state machine diagrams, typically used for workflows with human interaction.
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