MSDN Magazine - January 2009 - (Page 53)

You use arguments to define the way data flows in and out of an activity. Each argument has a specified binding direction such as input, output, or input/output. The following example shows how to author a simple Audit activity with a single input argument called “AuditMessage”: public class Audit: WorkflowElement { public InArgument AuditMessage{ get; set; } protected override void Execute(ActivityExecutionContext context) { WriteToEventLog(AuditMessage.Get(context)); } } Variables provide a way for declaring named storage for data. You can define variables in code as well as in XAML-based workflows. Variables can also be defined at different scopes within a workflow (within nested workflow elements), and they are part of the workflow program definition at design time, while the values are stored in the workflow instance at run time. The following example shows how to write a XAML workflow that defines a variable named message, assigns it a value (using the new Assign activity), and then passes the variable’s value into the custom Audit activity defined previously: Another thing to note is that there’s nothing special about the root activity. Anything that derives from WorkflowElement can be used at the root or anywhere within the workflow. This lets you arbitrarily compose different workflow styles within one another (for example, a state machine within a flowchart within a sequence). Most of these programming model changes help make declarative workflows first-class citizens because you no longer need imperative codebehind files for defining data-flow properties—it can all be done declaratively in XAML now. However, since these are pretty fundamental changes, they required some significant changes to the WF runtime that ultimately break compatibility with your .NET Framework 3.x activities (see the sidebar on migrating workflows for more information). Despite that, though, Microsoft believes that simplifying the WF programming model will greatly benefit WF developers over time. More New WCF Features in the .NET Framework 4.0 It’s easy to see how you might implement a WCF service with a workflow, but to truly produce a declarative workflow service, you will also need a way to define service contracts and to configure endpoint definitions using the declarative XAML-only model. This is precisely what WCF in the .NET Framework 4.0 brings to the table. Suppose you have this WCF service contract definition: [ServiceContract] public interface ICalculator { [OperationContract] int Add(int Op1, int Op2); [OperationContract] int Subtract(int Op1, int Op2); }; An expression is a construct that takes one or more input arguments, performs some operation or behavior on those input arguments, and then returns a value. Expressions are defined by deriving a class from ValueExpression, which itself derives from WorkflowElement, so expressions can be used anywhere an activity can be used. Expressions can also be used as a binding to another activity’s argument. This example defines a simple Format expression: public class Format : ValueExpression { public InArgument FormatString { get; set; } public InArgument Arg { get; set; } protected override void Execute(ActivityExecutionContext context) { context.SetValue(Result, String.Format(FormatString.Get(context), Arg.Get(context))); } In the .NET Framework 4.0, you can define the same contract declaratively using the following XAML definition: } The New Workflow Designer A new workflow designer added to Visual Studio 2010 provides a compelling graphical user experience for many of the key WCF and WF features discussed in this article. It provides features such as improved workflow navigation (drilling into composite activities) with bread-crumb trails for going back in scope, in-place activity editing (reducing the need for the Properties window), zoom capabilities, and overview navigation. In addition, the new designer provides an improved model for customization and re-hosting. Visual Studio 2010 will also offer a suite of new or improved project templates that make it easy to get started quickly with flowcharts and XAML-only workflows, and it will provide full support for XAML-based debugging when working with declarative workflows and services. January 2009 53 You can incorporate this expression into your XAML workflows like any other activity. For example, the following workflow illustrates how to pass the result of the Format expression into the Audit activity to write the result to the event log (this assumes that Audit’s content maps to the message argument): msdnmagazine.com http://www.msdnmagazine.com

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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