MSDN Magazine - January 2009 - (Page 52)

The .NET Framework 3.5 provided two activities— Send and Receive—for sending and receiving messages through WCF. In version 4.0, you’ll find SendMessage and ReceiveMessage activities—meant for sending and receiving one-way messages (similar to Send and Receive in version 3.5)—as well as a higher-level abstraction for request/response operations through the ClientOperation and ServiceOperation activities. A workflow that wants to expose a service operation should use the ServiceOperation activity. Whereas a workflow that wants to consume an external service should use the ClientOperation activity. In addition to these core WCF activities, the .NET Framework 4.0 also provides support for correlation across different one-way operations to ensure that a particular message makes it back to the correct workflow instance. It comes with an activity for defining a new correlation scope (CorrelationScope) and for initializing the correlation values (InitializeCorrelation) before sending an outbound message through WCF. Figure 3 The New FlowChart Activity Designer Figure 4 A Custom CopyFile Activity class CopyFile : Activity { public InArgument Source { get; set; } public InArgument Destination { get; set; } protected override WorkflowElement CreateBody() { return new PowerShellCommand { CommandText = "copy-item", Parameters = { { "path", new InArgument (Source) }, { "destination", new InArgument (Destination) } , { "recurse", new InArgument (false) } }, }; } } one of the most interesting additions because it offers a nice middle ground between the Sequential and StateMachine flow-control models. FlowChart lets you use a step-by-step approach, enhanced with simple decisions and switches, but it also allows you to return to previous activities in the workflow. Flowcharts in general seem more intuitive to many users. Figure 3 shows what the FlowChart designer looks like in the new workflow designer in Visual Studio 2010 (see “The New Workflow Designer” sidebar for more). The .NET Framework 4.0 also introduces some new runtime activities for invoking CLR methods (MethodInvoke), for assigning values to workflow variables (Assign), and for explicitly persisting a running workflow instance (Persist). Finally, the .NET Framework 4.0 comes with a new set of WCFbased activities that simplify the process of exposing workflows as services, or consuming services from within your workflows. 52 msdn magazine Even with these improvements, you may still need to write custom activities sometimes. So to make it easier, Microsoft redesigned the base classes for custom activities. The new base class for custom activities is called WorkflowElement, and there’s another class that derives from it called Activity. The Activity class makes it easy to create new custom activities from existing activities without writing much, if any, code. Figure 4, for example, shows how to define a new custom activity called CopyFile by deriving from Activity and overriding CreateBody. The implementation creates a customized PowerShellCommand instance configured to use the built-in copy-item command. You can also easily build custom activities like this one using the new workflow designer in Visual Studio 2010, through the graphical activity designer, if you’d rather avoid this type of code altogether. When you need to define a custom activity from scratch (something not based on existing activities), you must derive from WorkflowElement and override the Execute method. This approach requires a little more code, and it’s similar to how it works today when deriving from Activity. However, Microsoft has further simplified things around writing these custom activities. One of the things that makes writing a custom activity difficult is managing data flow into and out of the activity. For example, today it is not possible to define a set of typed arguments that get passed into and out of an activity. Normally, developers write custom classes that serialize and deserialize arguments from a workflow queue (for more, see Michael Kennedy’s article, “Web Apps that Support Long-Running Operations,” at msdn.microsoft.com/magazine/ dd296718). Writing this plumbing code isn’t difficult, but it is extra work that distracts from the central goal of modeling application flow. The .NET Framework 4.0 extends the activity programming model with three new data flow concepts that promise to simplify things significantly: arguments, variables, and expressions. .NET 4.0 and “Dublin” WF Activity Programming Model http://msdn.microsoft.com/magazine/dd296718 http://msdn.microsoft.com/magazine/dd296718

Table of Contents Feed for the Digital Edition of MSDN Magazine - January 2009

Toolbox CLR Inside Out Basic Instincts Cutting Edge Test Run First Look� Geneva Framework Silverlight Windows Mobile Service Station Security Briefs Extreme ASP.NET Foundations .NET Matters { End Bracket }

MSDN Magazine - January 2009

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