Tuesday, July 7, 2009

Windows Workflow Foundation 4.0 (Part 3 Activities)

On of the most visible areas of change in WF 4.0 is the out of the box activities. In this article we’ll take a look at the out of the box activities that are included in beta 1. The in the following articles we’ll begin to use these activities as well as custom activities to write a functional workflow that I’ll use to demonstrate some of the other features of WF 4.0.


  The out of the box activities are divided into five categories.

  • Procedural – These are the core workflow activities including activities such as the ForEach activity, While activity and a number of others described below.
  • Flowchart – As the name implies these are the core flowchart activities.
  • Messaging – These are the WCF workflow activities that include Send and Receive activities as well as several others.
  • Power Shell – These are a new set of activities for working with Windows Power Shell from WF.
  • Interop – There is only one activity in this category.



Collection Management


Adds an Item to a Collection.


Removes an Item from a Collection.


Clears a Collection, removing all items stored in it.


Verifies if an Item exists in a Collection. If the item exists, its Result argument will yield True.

Control of Flow


The If activity selects a child activity for execution based on the value of a Boolean expression.

If the Boolean expression Condition yields True (and “Then” activity is configured), the “Then” activity is scheduled. If the expressions yields False (and “Else” activity is set), the “Else” expression is scheduled.


Executes its Body until the Condition evaluates to True. The Body will be executed at least once.

ForEach / ForEach<T>

ForEach activity contains a list of Values and a Body. At runtime, the list is iterated and the body is executed for each value in the list.


The Pick Activity provides event-based control flow modeling in WF. The only valid children for a Pick activity are PickBranches.

At the beginning of a Pick execution, all the Trigger activities from all its Branches are scheduled. When the first Trigger completes its corresponding Action activity is scheduled, and all other Trigger activities are canceled.


PickBranch represents a branch in a Pick. It consists of a Trigger and Action. PickBranch can only be added to a Pick activity.


The Sequence activity allows for the execution of one or more Activities in order.


Switch activity is similar to switch statement in C#. It contains an Expression and a set of Cases (each case has a key and an activity). After the expression is evaluated, the Switch activity looks for a Case with a key that matches the result of the expression and if found, it schedules the activity associated with that Case.


The While activity executes it's Body while a Boolean Condition is True.

Parallel Execution


Parallel activity allows parallel execution of its children. It operates by scheduling each WorkflowElement in its Branches collection at the beginning of its execution.  It completes when all of its Branches complete or when its CompletionCondition property evaluates to true.

ParallelForEach /  ParallelForEach<T>

The ParallelForEach activity enumerates the elements of a collection(Values) and executes an Activity for each element of the collection, in a similar way than the ForEach activity does. The main difference is that the embedded statement is executed in a parallel fashion.

Just like the Parallel Activity, ParallelForEach has a CompletionCondition, so that the ParallelForEach activity could complete early if the evaluation of the CompletionCondition returns true. The CompletionCondition is evaluated after each iteration is completed.

Error Handling


TryCatch activity is similar to the try..catch construct in C#: all activities in the Try block are executed and if an exception occurs, it will schedule the Catch block that best matches that exception (if no matching catch is found, the workflow is aborted). All Catch blocks are contained in a collection called Catches.

TryCatch activity also has a Finally block that is executed after the Try (and any eventual Catch).

A note on unhandled exceptions:

TryCatch provides exception handling at the workflow level. When an unhandled exception is thrown, the workflow is aborted and therefore the Finally block won’t be executed. This behavior is consistent with C#.


Represents one catch block to be used in a TryCatch activity. If an exception is thrown within a Try Element, the TryCatch will attempt to find a matching Catch element based on the type of the thrown exception.

Catch<T> can only be used inside a TryCatch activity


Throw activity throws an exception within a workflow. Throw activity has an Exception property that contains the exception that will be thrown at execution time.



The Assign activity assigns the value of its Value argument to its To argument.

The types of both arguments must be compatible. This compatibility is verified at runtime.


Delay Activity, as its name suggests, will block the current workflow execution path for a Duration specified by user. After the duration expires, the workflow continues execution as expected. The duration of the delay is set using a TimeSpan.

InvokeMethod / InvokeMethod<T>

InvokeMethod is the activity that allows you to call an existing CLR instance or static method. To invoke a method all you need to do is provide the owner of the method (TargetType for static methods, TargetObject for instance methods), the MethodName, and its Parameters.

InvokeMethod supports the following method invocation scenarios:

· Public instance and static methods

· Parameter passing by value and by reference

· Support for parameter arrays

· Support for generic parameters

· Asynchronous method invocation


Writes text to the configured output console.

Writing beyond the System.Console

WriteLine has a TextWriter argument can be configured to write to different outputs. For example, we can configure the TextWriter property to send the text to an ASP.NET page. If the TextWriter is not set, it will be set by default to the System Console.

Advanced (Cancellation, Compensation, Transactions, and Persistence)


The CancellationScope activity consists of two main parts, the Body and the CancelHandler. The body is the code path that normally executes. If the activity gets canceled, then the cancel handler is called.


CompensableActivity is used to define a potentially long running activity with accompanying Compensation and Confirmation logic.

Compensation allows the user to specify corrective action to be taken on an activity based upon activity which occurs after the successful completion of the Body of the activity.


Compensate is used to explicitly invoke the compensation handler of a CompensableActivity.


Confirm is used to explicitly invoke the confirmation handler of a CompensableActivity.


Persists the workflow instance. Persistence will be done using the configuration of the WorkflowInstance that is being executed (this activity doesn’t have any arguments).


The TransactionScopeActivity provides the mechanism for initializing a new transaction, making the transaction handle ambient (a workflow execution property) and calling complete on the transaction once the Body of the TransactionScope activity has completed.

TransactionScopeActivity supports “Requires” semantics. If there is already an ambient transaction it is used, else a new one is created.

Nested transaction scopes:

TransactionScopeActivity can be nested in another TransactionScopeActivity. A TransactionScopeActivity nested in another TransactionScopeActivity will use the existing transaction.





This is the root for a Flowchart. Since Flowchart is an activity like any other, it can be composed inside any container activity.

For example, we can add a Flowchart inside of a Sequence or a Flowchart inside another Flowchart.

The green ball in the image at the right represents the start node of the Flowchart.


FlowDecision models conditional forks within a Flowchart. It can be seen as the equivalent of the procedural If activity in the Flowchart world.

This activity contains a Boolean expression Condition. If the expression evaluates to “True”, the true path is executed (otherwise, the false path is scheduled).


FlowSwitch activity selects a next node depending on the value of an expression. FlowSwitch can be seen as the equivalent of the procedural Switch activity in the Flowchart world.





Models one way receive of a message. It can receive data of the following types: Message, DataContract types, XmlSerializable types, and MessageContracts.


This activity template represents a correlated Receive activity and SendReply activity.

By using this template you can wait for an incoming message and then send a reply to the sender.

Since the Receive and the SendReply are inside a sequence, you can add any activity between them.


Models one way send of a message. It can receive data of the following types: Message, DataContract types, XmlSerializable types, and MessageContracts.

This activity can be used in two ways:

1. Client: this activity can be used to send a request to a service. This is equivalent to a WCF client calling a service operation. No contract inference is performed on the client side.

2. On the server side the Send activity can be used to send a reply to a previous Receive. In this case the Send and Receive activities must have the same OperationName, Action and CorrelationHandle.


This activity template represents a correlated Send activity and ReceiveReply activity.

By using this template you can send a message and then wait for a reply from the destination.

Since the Send and the ReceiveReply activities are inside a sequence, you can add any activity between them.





Invokes a PowerShell cmdlet that does not have a return value. InvokePowerShell can be used to invoke simple cmdlets and scripts. We can also pass parameters and input objects to the cmdlet. After execution, the activity provides a set of errors (if any occurred).


An activity that invokes and retrieves the resultant output from a PowerShell cmdlet. This flavor of the activity has all the same arguments than the non-generic version plus an InitializationAction.

The InitializationAction is used to map the results of the execution of the cmdlet to variables in our workflows.





The Interop activity is a WF4 activity that wraps a WF3 activity (a non-abstract CLR type that derives from System.Workflow.ComponentModel.Activity) thus allowing the WF3 activity to be used in WF4 workflows.  Note that the WF3 activity can be a single leaf activity, or an entire compiled workflow (tree of activities).

The Interop activity bridges the WF4 and WF3 activity execution models, facilitates data flow across the interop boundary, and enables persistence and tracking of WF3 activity instances within WF4 workflow instances.

The Interop activity allows WF developers to move to the WF4 model in an incremental fashion:

· Quickly experiment with WF4 using existing WF3 activity artifacts

· Wrap WF3 activities the developer isn’t ready to redesign on the WF4 model

· Wrap WF3 activities for which the developer doesn’t own or possess the source code (e.g. they purchased the activity from a third party)

Overall this is quite an impressive list of activities when you consider the WF 3.5 came with a total of 30 out of the box activities. Missing are the event driven activities, code activity and web service activities to name a few.


  The descriptions in the beta documentation on MSDN are pretty weak. I was able to find  more complete descriptions on the .Net endpoint blog, which I’ve used because of the detail in descriptions.

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