Prerequisites

To follow along with the examples throughout this article series, you need the following installed on your system:

• .NET Framework 3.0 - Because Windows Workflow Foundation is built on top of this, you need the .NET Framework 3.0 installed for runtime functionality. Windows Vista comes with the .NET 3.0 Framework out of the box as long as you select this option at the time of installing Windows Vista. If you are running an operating system other than Windows Vista, you need to install the .NET Framework 3.0.
• Visual Studio 2005  (any edition) - Required for workflow development.
• Windows Workflow Foundation extensions for Visual Studio 2005  - Available for download from Microsoft.

Activities in Windows Workflow

As mentioned previously, Workflows are made up of a set of activities. An activity can be defined as the smallest piece of action that the flow employs. An activity can be anything from a straightforward action to a composite action that consists of an assembly of several activities. They can be aggregated to a workflow by using the workflow visual designer supplied with Visual Studio 2005. Activities can also be sequential, as the whole workflow itself, where the order of the steps is predetermined at design time, or it can be event-driven, where the order of the actions is determined at run time in response to external events, as in the case of state machine workflow.

For the purposes of demonstrating activities, let us create a new Visual Studio project by selecting File -> New from the menu. In the New Project dialog box, select Workflow under the Visual C# node in the Project types pane and select Sequential Workflow Console Application as the template in the right pane. Name the project WorkflowActivities as shown below:

Figure 1: Creating the WorkflowActivities project

After the project is created, select View -> Solution Explorer from the menu so that you can examine the files within the project. The Workflow1.cs file is the workflow itself, and the Program.cs file contains the .NET code that starts the Console application. To start with, add a new workflow named IfElseWorkflow to the project and modify the below line of code in the Program.cs file to invoke that workflow.

Every time you add a new workflow to the project, just modify the above line of code in the Program.cs file to create the instance of the new workflow for invocation purposes.

IfElse Activity

Using the IfElse activity, you can simulate an if-then-else conditional expression. As you would imagine, the IfElse activity requires you to provide the logic that evaluates to a Boolean value. Windows Workflow enables you to provide this logic using two different ways.

One method of providing this logic is to set the Condition property of an IfElseBranch activity to an instance of the DeclarativeRuleCondition class. This class acts as a pointer to a rule that exists in the workflow’s rules set.

Another way of providing this logic is to set the Condition property to an instance of the CodeCondition class. This class exposes an event, also called Condition that is fired when the IfElseBranch activity’s logic needs to be evaluated. The event handler accepts an argument of type ConditionalEventArgs (that is of type Boolean) through which you can specify the results of the conditional expression you build into the event handler. You accomplish this by setting the Result property of the ConditionalEventArgs object to either true or false. Depending on the Result value, the IfElse activity directs workflow execution to one of two branches. Visually, in the Microsoft Visual Studio workflow visual designer, true executes the path shown on the left and false executes the path to the right. Note that both of these branches are containers for other activities meaning that you can insert child workflow activities that encapsulate the logic of the workflow depending on the result of the condition evaluation. The below example leverages this approach to provide the logic to the IfElse activity.

For the purposes of this example, let us pass a parameter to the IfElseWorkflow and use its value to drive the IfElse condition. To this end, add a new private variable named _approvalRequired to the workflow and corresponding getter and setter methods.

From within the workflow, let us examine the value contained in the ApprovalRequired parameter in the IfElse activity and execute appropriate logic inside the If and Else condition.

Now drag and drop an IfElse activity from the tool box onto the workflow designer between the Start and End steps. After the activity is added, you will see a red icon indicating that the condition property has not been set. Click the down arrow right next to the icon to set the condition property. That will bring up the properties dialog box through which you can set the Condition property to either CodeCondition or DeclarativeRuleCondition. For the purposes of this example, select the CodeCondition from the list box and set its Condition subproperty value to IfElseCondition_ExecuteCode and click Enter to bring up the code window.

Drag and drop a new Code activity right below the left IfElseBranchActivity and set its ExecuteCode property to IfCondition_ExecuteCode. Now modify the IfCondition_ExecuteCode procedure to look as follows:

Now add another Code activity right below the right IfElseBranchActivity and set its ExecuteCode property to ElseCondition_ExecuteCode. Now modify the ElseCondition_ExecuteCode procedure to look as follows:

Now modify the workflow invocation code in the Program.cs file to assign a value to the ApprovalRequired parameter in the workflow.

Now you are ready to execute the workflow through the console application by pressing F5. You should see an output that is somewhat similar to the following.

Figure 2: Output for the IfElse activity example

Since you passed in false to the ApprovalRequired parameter, the above picture shows that being reflected inside the workflow.

While Activity

The While activity is very similar to the while loop construct in programming languages such as C#. Like the IfElseBranch activity, which uses its Condition property to determine whether the branch executes, the While activity also has a Condition property. You can set the Condition property to either Rule based Condition or Code Condition, similar to the IfElse activity.

Note that the While activity can contain only one child activity. You can overcome this limitation by using the Sequence activity as the root child activity of While activity and add child activities to the Sequence activity.

For the purposes of this example, add a new workflow named WhileWorkflow to the WorkflowActivities project. For the purposes of this example, let us consider a simple scenario wherein you want to execute the logic contained inside the While activity specific number of times that is decided at run time. To this end, let us start by declaring a public property inside the workflow that specifies the number of times the logic inside the while loop needs to be executed.

In addition to the _count variable, let us also add another private variable named _noOfTimesExecuted that keeps track of the number of times the While logic is executed.

Now that you have added the required private variables, drag and drop a While activity to the workflow designer and set its Condition property to Code Condition. Set the value of the Condition sub property to WhileActivity_CodeCondition and click Enter to bring up the code window. Modify its event handler to look as follows:

Now drag and drop the Code activity as the child of the While activity and modify its ExecuteCode event handler to look as follows:

Now that you have created the workflow, the next step is to modify the Program.cs file to invoke the WhileWorkflow. If you pass in a value of 5 to the Count parameter, you will see an output that is somewhat similar to the following.

Figure 3: Output for the While activity example

Parallel Activity

As the name suggests, the Parallel activity enables you to have multiple activities executing independent of one another. Once you configure the Parallel activity with multiple branches, all of those branches execute in a semi-parallel fashion at run time. Note that each branch requires a Sequence activity as its first child. Although this activity is named Parallel, the branches don’t execute in a true parallel nature. This is due to the fact that workflows are executed on a single thread. Therefore, multiple branches cannot run at the same time. Because of this limitation, the Windows Workflow simulates parallel execution. For example, the Parallel activity runs the first activity of the first branch, and after that activity’s execution has completed, the first activity of the second branch is run. This process continues until all activities in all branches have been executed. The following picture shows the result of placing a Parallel activity onto the workflow designer.

Figure 4: A Parallel activity in the Workflow Designer

Although you see only two branches as children of the Parallel activity, you can add additional branches through the "Add Branch" hyperlink that can be found at the bottom of the properties dialog box.

Sequence Activity

Sequence activity is a utility activity that is fundamental to the creation of sophisticated workflows. As the name suggests, it is a container activity that allows you to host multiple child activities and subsequently have them execute in a defined order. The Sequence activity is mainly used in conjunction with other activities that allow only one child. For example, you can use the While activity with a Sequence activity to allow multiple children to execute in a loop iteration. In addition, the branches of the Parallel activity require that their sole child be a Sequence activity.

Replicator Activity

This activity creates any number of instances of its single child activity. You can combine this activity with the Sequence activity or any other composite activity to allow multiple children. The Replicator activity has a collection property called CurrentChildData that dictates how many times the child activity is replicated. You can populate this collection in an event handler known as Initialized. The below table provides a brief overview of the events exposed by the Replicator activity.

In addition to the rich set of events, the Replicator activity has a property called ExecutionType, which you can set to one of two values in the ExecutionType enumeration.

If you set this property to ExecutionType.Sequential, an instance of the child activity is created and executed before any other instances are created and subsequently executed.

If you set the ExecutionType property to ExecutionType.Parallel, all child activity instances are created first and then executed in a parallel fashion.

In some situations, you might want the execution of child activities to be canceled if a certain condition evaluates to true. In those cases, you can set the UntilCondition property to the desired code or rule condition.

Similar to a Foreach statement, the Replicator activity creates and executes a given number of instances of the specified child activity. You can specify only one child activity, but using composite or custom activities is allowed. You can't control the number of iterations through a declarative property. Instead, you write a handler for the Initialized event and populate the CurrentChildData collection with initialization data for each of the desired instances:

The above code snippet adds two instances of the child activity to the replicator, each initialized with a given string. Note that if you leave the CurrentChildData collection empty, the Replicator won't run any child activity and is limited to firing top-level events such as Initialized and Completed. Now drag and drop a Code activity inside the Replicator activity and modify its ExecuteCode event handler to look as follows:

By default, child instances run in sequence. However through the ExecutionType property, you can opt for a parallel execution. If no global condition is set through the UntilCondition property, the replicator ends when all child activities have completed execution. If the UntilCondition property is set, the activity terminates when the UntilCondition is true.

Conclusion

Activities are the reusable building blocks for composing workflows. In this installment, you have understood the steps involved in leveraging out of the box activities to create your workflows. Specifically you learned how to embed flow logic in your workflow by looking at the activities that are specifically meant for representing conditional flow logic. To this end, you explored out of the box activities such as IfElse, While, Replicator, Parallel and Sequence activities. The next installment in this series will get into the details of creating custom activities through Windows Workflow.

About Thiru Thangarathinam

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