Conditional Activities in Windows Workflow Foundation

Conditional Activities

Conditional Execution is one of the most basic requirements in any workflow solution. Windows Workflow foundation provides some out-of-the-box Activities for supporting Conditional Execution.

1. IfElseActivity Activity
2. WhileActivity Activity
3. ConditionedActivityGroup Activity 

IfElseActivity Activity: This Activity represents the logical If-Then-Else decision point. The first branch of the IfElseActivity that results to a "true" condition is executed.

WhileActivity Activity: This Activity represents the familiar While loop. Child activities of the While Activity are executed until the condition specified with the While Activity evaluates to "true".

ConditionedActivityGroup Activity: This activity is a combination of the parallel execution feature of the ParellelActivity Activity with a looping behavior.

Looping feature: The ConditionedActivityGroup activity specifies an "Until" condition in the root of the activity. The child activities of the ConditionedActivityGroup activity are iteratively executed until the "Until" condition of the ConditionedActivityGroup activity returns a "true" value.

Parallel feature: Child Activities included within the ConditionedActivityGroup activity are repeatedly executed until certain criteria are met. Each child activity may specify an optional "When" condition to control the repeat execution of the child activity.

ConditionedActivityGroup activity operation

1. If the "Until" condition of a ConditionedActivityGroup activity evaluates to "false", it's Child Activities are evaluated.
2. If the Child Activity does not have a "When" condition, it is executed only in the first iteration of the ConditionedActivityGroup activity. This activity is not re-executed in subsequent iterations.
3. If the Child activity's "When" condition results in a "true" value, it is scheduled for execution
4. All the child activities of the ConditionedActivityGroup activity are executed (as described in step 3) and the "Until" condition is re-evaluated.
5. The ConditionedActivityGroup activity is re-iterated until:
   1. Either the "Until" condition returns false. In this case, all executing child activities are cancelled.
   2. Or there are no more Child Activities with the "When" condition returning a "true" value.
   3. A Halt keyword is specified in the sequence of execution. 

The parallel feature of execution for the child activities does not guarantee the order of execution.

How are Conditions specified in the Conditional Activities?

Conditions can be specified for the Conditional Activities either through code or declaratively via Rules.

Code Conditions

Code condition for a conditional activity is specified as a code-behind handler method that returns a Boolean value in the Result property.

A sample handler method is listed below

private void CheckMinimumBalance(object sender, ConditionalEventArgs args)

{

          if (this.CurrentBalance - this.CreditAmount > 0)

{

          e.Result = true;

}

          else

                   e.Result = false;

}

Rule Conditions

Conditions can be specified declaratively through RuleConditionReference. The condition in this scenario is saved in a .Rules xml formatted file in the workflow project. The .rules file can include multiple RuleConditions and the RuleConditionReference references to one of the RuleConditions by it's Name.

RuleConditions can be specified Rule Condition Editor dialog box and get saved in the .rules file.

Example of Specifying a Rule Condition within the Rule Condition Editor Dialog Box:

this.CurrentBalance - this.CreditAmount > 0

Advantages of using Rule Conditions:

• Rule conditions are included in the declarative model and can be dynamically updated at run-time
• The .rules file provides code separation and enables external analysis /tools to be built on top of the .rules file.  

Valid Expressions

The following usages are valid when specifying conditions using either Code or Rule Conditions. Expressions must result in a Boolean value. Types that are available in the .Net framework's System.CodeDom can be used to specify expressions.

• Access the class Properties/Invoke Methods
• Access other classes from the Base Class Library
• Specify Regular Expressions 

Dynamically changing expressions at runtime

The value of the Condition property on the Conditional Activities cannot be changed at runtime. However, you can dynamically change the definition of the rule condition in the .rules file using workflow changes.

Conclusion

In this article we had an introductory look at Conditional Activities and Condition types in the Windows Workflow Foundation.

Disclaimer

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.