MSDN Magazine - December 2008 - (Page 54)

Figure 2 The Execute Method protected override ActivityExecutionStatus Execute( ActivityExecutionContext aec ) { if ( string.IsNullOrEmpty( SSN ) ) { throw new InvalidOperationException( "SSN property value is invalid." ); } ICreditScoring creditSvc = aec.GetService (); Debug.Assert( creditSvc != null ); WorkflowQueuingService queueSvc = aec.GetService (); Debug.Assert( queueSvc != null ); Guid queueName = Guid.NewGuid(); WorkflowQueue queue = queueSvc.CreateWorkflowQueue( queueName, false ); queue.QueueItemAvailable += CreditScoreComputed; creditSvc.ComputeScoreAsync( SSN, queueName ); } return ActivityExecutionStatus.Executing; contained within the activity definition, is registered as a service with the WF runtime: // create an instance of your service type LoggingService logSvc = new LoggingService(); // register it with the WorkflowRuntime instance workflowRuntime.AddService( logSvc ); WorkflowInstance instance = workflowRuntime.CreateWorkflow( typeof( Workflow1 ) ); instance.Start(); During execution, an activity can request access to the services it needs through the ActivityExecutionContext (activities never interact directly with their host runtime) and use the configured services to perform work, while letting the WF runtime and host manage service lifetimes: protected override ActivityExecutionStatus Execute( ActivityExecutionContext aec ) { // ActivityExecutionContext provides access to services LoggingService logSvc = aec.GetService (); Debug.Assert( logSvc != null ); logSvc.LogEvent( EvtType, Message ); return ActivityExecutionStatus.Closed; itly defined via method parameters or return values. Data flows from one activity to another using built-in support for activity data binding. You also have the option of manipulating shared state by interacting with configured services in the logic of your custom WF activities. The basic execution sequence for WF activities is to initialize activity state, execute activity, examine resultant activity state for changes, then repeat for all other activities. So it is precisely the side effects of activity execution (“What is the new value of the PurchaseOrderTotal property?” or “What database value just } The power of the WF service model stems from its simplicity. Any .NET object can be configured as a service. changed?”) that tell you what just happened and what will happen next. You do have the potential for activities to manipulate state in ways undesirable for other downstream activities (however, note that the risk here is no worse than that inherent in any imperative language like C#), but you don’t pay the performance costs associated with data copying. Services, Services, Services As a general purpose programming environment, WF is designed to integrate with external subsystems and code. The most direct way to accomplish this is to author custom workflow activities that make database or Web service calls or call into third-party libraries. In WF terminology, such external systems are known as services. The idea behind WF services is simple: any library or functional element that is required by an activity in your workflow, but is not 54 msdn magazine Typical service examples include logging components, data access layers, and proxies to external subsystems, though the possibilities are limitless. The power of the WF service model stems from its simplicity. Literally any .NET object can be configured as a service. Note that some extra behaviors, such as service lifetime management, are available to services explicitly designed to subclass WorkflowRuntimeService. Also, I usually find it useful to expose services via an interface instead of through a concrete type; this makes it easier to vary the service implementation independent of the activities that depend on it. So the key best practice here is the decoupling of a custom WF activity’s core logic from the constituent pieces needed to accomplish its work. Additionally, you should refactor into a service logic that is stateful and is shared among multiple activities in a workflow (logging, for example); it would be cumbersome to pass a reference to the logging component down the activity graph during execution. This is further complicated for workflows serialized at some point during their execution cycle. I’ll mention one other bit of advice regarding services. Windows WF ships with an infrastructure known as local services for calling out to .NET class instances from activities, as well as handling .NET events inside an activity. You interact with these local services via the CallExternalMethod and HandleExternalEvent activities. They are essentially an extra layer on top of the base WF service infrastructure and provide the same benefits as raw services in a declarative style friendlier to WF host environments such as SharePoint. However, for more general-purpose hosts such as Windows Presentation Foundation (WPF) or ASP.NET, this extra layer can be more cumbersome to use than the equivalent raw service configuration. Keep in mind your anticipated hosting requirements before building solutions with local services. A key principle to employ here is to leverage the power and flexReal-World WF

Table of Contents Feed for the Digital Edition of MSDN Magazine - December 2008

MSDN Magazine - December 2008 Contents Toolbox CLR Inside Out Advanced Basics Cutting Edge Patterns In Practice Team System Real-World WF Visual Studio OBA Tools SOA Data Access Geneva Framework Test Run Foundations Windows With C++ Going Places End Bracket

MSDN Magazine - December 2008

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