.NET Enterprise Services Technologies : Windows Workflow Foundation (WF)

10/13/2010 9:14:10 AM
It is generally possible to describe a business process as a workflow comprised of a discrete series of steps that involves people and software. WF is a Windows platform specifically for workflow technology (Figure 1). At its core, WF enables the execution of steps required to complete a business process. It is used to construct workflow-enabled services and service compositions for which it provides the following:
  • an extensible programming model

  • a workflow engine

  • design tools for Visual Studio

  • a mechanism to invoke services and to publish workflows as services

Figure 1. Windows Workflow Foundation (WF) as a building block of the .NET framework.

WF technology is applicable to document-centric workflows, human workflows, business rules-driven workflows, and other variations. It is used to enable both human and automated workflow steps and to wire up opaque blocks of functionality called activities.

WF Architecture

The major parts of the WF platform (Figure 1) include workflow, activities, the WF base activity library, the WF runtime engine, and WF runtime services. Each part is further explained in Figure 2 and Table 1.

Figure 2. The major moving parts of the WF platform.

Table 1. Descriptions of the major parts of WF.
activitya unit of work or a discrete step in a business process
workflowa sequence of activities
WF runtime enginea workflow instance is created and executed by the WF runtime engine (which also manages state and communication with the host process)
WF designersUI tools used to implement a workflow using shapes (Visual Studio includes a WF designer)
WF runtime servicesservices that provide hosting flexibility and communication
host processan application that hosts the WF runtime engine which executes the workflow (the host process provides support for runtime services, such as persisting the workflow’s state)

WF runtime services are connection points for plug-in resource providers. For example, the default persistence behavior provided by the runtime engine can be changed by providing a runtime service. WF further supports compensating transactions for a given runtime service activity. Within the solution logic, WF can define actual compensators that are invoked when exceptions occur.


A strategic benefit to using WF is that it allows for the use of a common workflow technology to build workflow solutions across other Microsoft products and .NET solution environments.

WF supports both system workflows and human workflows by supporting two built-in workflow types:

  • sequential workflows

  • state machine workflows

Both rely on the same runtime environment and the same set of standard WF activities. A single workflow definition can also be a composite of both types, containing activities that rely on the system and on human action. Let’s briefly explore each workflow type a bit more.

Sequential Workflows

Sequential workflows execute activities in a pre-defined pattern. This workflow type is better suited for system workflows because the execution of activities closely resembles a flowchart with branches, decision logic, loops, and other control structures.

SOA Principles & Patterns

By using WF, rather than embedding workflow routines into the core logic of individual services, each step in the business process is defined explicitly in a graphic designer and executed by the workflow engine. This corresponds directly to Process Centralization together with additional patterns that co-exist to establish an environment as per the Orchestration  compound pattern.

The resulting level of separation can cleanly partition agnostic and non-agnostic logic allowing for the definition of reusable services that are independently maintainable. This is essentially the basis of Functional Decomposition , which is commonly further supplemented with the sequential application of Service Encapsulation , Agnostic Context , Non-Agnostic Context , and Agnostic Capability . These foundational service patterns can be applied to form primitive service modeling and design processes that are initiated with well-defined workflow logic and carried out using WF tools, such as Workflow Designer (explained shortly).

Applying these patterns generally leads to the need to further define the functional contexts of services via Service Layers , such as those established by the application of the Utility Abstraction , Entity Abstraction , and Process Abstraction . Agnostic utility and entity services that result from the application of the former two patterns need to be separated within or outside of WF in order to avoid unnecessary performance overhead and synchronization issues due to deployment-level dependencies on the orchestrated task service logic that encapsulates parent workflow routines.

State Machine Workflows

State machine workflows execute activities as external events occur. This type (based on the well-known Finite State Machine) is better suited for workflows involving human intervention. The subsequent activity to be executed depends on the current state and the event it has received. State machine workflows are useful when the sequence of events is not known in advance or when the number of possibilities makes defining all possible paths impractical.

Workflow Designer

An advantage of creating applications using workflows is the ability to define the workflow graphically, which is why WF includes a designer for Visual Studio. By default, the activities appear in the toolbox, letting a developer drag and drop them onto the tool’s design surface to create a workflow. The workflow designer is a convenient way to interact with the workflow namespace.

Workflows created with the designer are stored in a file based on XAML, a declarative XML language used to define objects, their properties, relationships, and interactions. Upon execution, the runtime engine takes the XAML workflow and creates workflow instances. While there is only one XAML-based workflow file, there can be multiple workflow instances running at any given time.

The designer allows you to model a Sequential Workflow Console Application or a State Machine Workflow Console Application. After selecting the appropriate model you need to add activities by dragging and dropping them from the toolbar. (Note that the console application contains a sub main method that is used to start the workflow.)

Workflow Persistence (with WF)

Workflows can be dehydrated from memory and can later be re-hydrated and re-activated. WF supports dehydration of the instance state by allowing a workflow instance to be serialized to a data store, such as SQL Server. The workflow instance can be restored to the original execution state at any time by de-serializing the data based on events or messages.

The SqlWorkflowPersistenceService class in WF is designed to connect workflows with SQL Server. In order to use this persistence service, we need to create a database in SQL Server with the schema that the persistence service uses. WF comes with SQL scripts to create the database and schema used by the persistence service.

The database and schema scripts are typically placed in the following folder after installing WF:

...\Microsoft.NET\Framework\v3.0\Windows Workflow Foundation\SQL

The scripts provided by WF are SqlPersistenceService_Schema.sql and SqlPersistenceService_Logic.sql. The former defines the structure of the database and the latter defines the stored procedures.

The following example provides code that creates an instance of SqlWorkflowPersistenceService and an instance of the workflow runtime. The workflow runtime is used to generate an instance of Workflow1 by dehydrating it from the persistence database. The runtime starts the instance and the instance ID is stored in the ID property for later use. Finally, the workflow runtime stops and the state is de-serialized back to the database.

Example 1.
void Load()
WorkflowRuntime workflowRuntime = new WorkflowRuntime();
SqlWorkflowPersistenceService sqlPersistenceService =
new SqlWorkflowPersistenceService(this.connectionString);
WorkflowInstance instance =
(WFWorkflow.CalcWorkflow)); = instance.InstanceId;

WF includes a tracking service that allows a developer to save information about a workflow’s execution to the database. For example, the start date, time and end date, time of a workflow and its activities can be saved to the database.

Communicating with the Host Container

The base WF library includes the CallExternalMethod and HandleExternalEvent activities used for communication with the host based on the request-response message exchange pattern.

SOA Principles & Patterns

Persisting workflow logic via a database is a classic application of State Repository  as a result of the application of the Service Statelessness principle to the task or controller service that resides within WF runtime environment.

The WF has a built-in CallExternalMethod activity, which can raise an event to be consumed by the host. The host application needs to implement an event handler for receiving response arguments from the workflow using a standard event-or-delegate pattern. The activity can also be used to send data from the workflow to the host. The HandleExternalEvent is used by the workflow instance to capture an event raised by the host application.

It is critical for the interface outside the runtime execution context to communicate with the code in the host application process. The barrier is bridged using a service designed in the runtime environment called the ExternalDataExchangeService. This service allows you to make calls into a running workflow instance using events, and to call out from a running workflow using method calls. In order to hook up the workflow to the host service, it must contain a class that implements the interface that is intended for communications. This interface will use the ExternalDataExchange attribute to signal the workflow designer and runtime that this interface is intended for communication between the host and workflow.

Once all these code artifacts are defined, the CallExternalMethod and HandleExternalEvent activities can be hooked up to the host service using the WF designer.


A workflow is a sequence of activities executed by the workflow engine. An activity should be modeled as a real-world action required for completing a parent business process. An activity is a class that encapsulates logic and can potentially be reused across different workflows.

WF includes several activities known as the base activity library. Activities from the base activity library are commonly used with sequential workflows (Table 2).

Table 2. Base library activities commonly used with sequential workflows.
IfElseallows conditions to be specified in the workflow and the runtime engine evaluates each condition and acts upon it based on the result (the IfElse activity can contain other IfElse activities and a default IfElse activity if no other condition is met)Looping and Synchronization
Whileaccepts a condition and evaluates it at the beginning of every iteration (if the condition is true, the child activity is run repeatedly until the condition becomes false)
Replicatorexecutes a child activity a given number of times (similar to the foreach statement in C#)
Sequenceis used to execute a group of activities, one at a time, in a predefined order
Parallelexecutes two or more sequences of activities in parallel or in an interleaved manner (all sequence activities must be completed before the workflow moves to the next activity)
Listenis used to idle the workflow process and wait for a wake-up call (the Listen activity is typically used when human interaction is required—it serializes the workflow and goes into a passive mode when it is waiting for human intervention and upon receiving an event, it reactivates the workflow and continues with the processing logic)Human Intervention
EventDrivenis implemented by using the EventDriven activity (a ListenEventDriven activities and child activities that represent human events) activity must contain
HandleExternalEventis invoked when an event specified in an interface is raised (the HandleExternalEvent activity is used by WF to communicate with an external service)
Delayis used to suspend the execution of the workflow for a specified amount of time
Codeallows source code to be injected directly into the workflow (it fires the ExecuteCode event that executes the code, plus this activity can call an external assembly)Execution
CallExternalMethodis used to call a method in a class available to the workflow (the interface and its implementation must be available in the same assembly)
InvokeWorkflowinvokes another workflow to start executing
InvokeWebServiceinvokes a Web service external to the workflow application (creates a Web reference to a Web service and allows operations on the service to be invoked)
WebServiceInputenables a workflow to receive a Web service request
WebServiceOutputpairs with a WebServiceInput activity to respond to a service request (to use this activity, the WebServiceInput activity must be configured first)
TransactionScopeis used to represent System.Transactions in WF (supports all the properties currently supported by System.Transactions)
Terminateis used to terminate the execution of the workflow

State machine workflows provide a way of defining workflows that match an organization’s business process by using states, events, and transitions to model workflow logic. A state represents a snapshot of the business process. The workflow is always in one state and will transition to a new state when it receives an event. Typically, some action will take place in the outside world for the state in the workflow to be transitioned to a new state. On reaching the final state, the workflow is completed.

The base activity library includes several activities designed to enable state machine workflows (Table 3).

Table 3. Base library activities commonly used with state machine workflows.
Staterepresents a state in a state machine workflow (when an event arrives, the workflow will transition from one state activity to a new state activity)
EventDrivenrepresents an event handler in a state machine and is placed inside a state activity
SetStateis used to model transitions in a state machine workflow (includes the TargetStateName property that points to the destination state)
StateInitializationStateFinalization used to perform pre- and post-processing in a state and run when the state machine transitions into the state containing the initialization activity (the StateFinalization activity runs when the state machine transitions out of a state)

A state machine workflow is commonly consumed by one or more UI components that must reflect the current state of the workflow and allow users to only perform legal events. WF includes the StateMachineWorkflowInstance class that provides an API to manage and query a state machine workflow. The class includes properties used to fetch the current state name and find legal transitions for the state. It also includes properties that provide a history of all the states the workflow has been through.

Workflow Runtime Environment

A workflow instance is created and executed by the workflow runtime engine. The runtime engine relies on several runtime services for persisting the workflow’s state, managing transactions, tracking workflow’s execution, and other features. Each instance of the runtime engine can support multiple instances of a workflow concurrently. The workflow instance runs in a host process or in an application domain and can be hosted on ASP.NET Web sites, Windows forms, Windows services, Web services, or SharePoint.

The runtime engine is powered by runtime services that provide an execution environment for transactions, persistence, tracking changes, timer, and threading. Runtime services can be augmented by plugging in custom services that allow changing the behavior of the runtime engine to meet the specific needs of the execution environment.

For most workflow implementations, the default implementation of runtime services satisfies the needs of the execution; however, in some cases the behavior may need to be altered. For example, the workflow may require the host application and the runtime engine to communicate differently, which would require building custom services.

The workflow execution starts by creating an instance of the workflow. It proceeds with carrying out activities until it is required to idle the execution, at which point the instance state is persisted to disk.

WF Programming Model

WF classes are encapsulated in three namespaces, as shown in Figure 3.

Figure 3. Workflow Foundation classes are encapsulated in three namespaces. The System.Workflow.Runtime assembly further contains the WorkflowRuntime class that is used to create an instance of a workflow.

Figure 4 illustrates a simple sequential workflow that contains a Calculator activity. To automate this workflow, the host will need to instantiate it and provide it with parameters including the input values and the operation to perform.

Figure 4. A simple sequential workflow containing one activity that represents calculation logic.

In the following example, we show this workflow hosted in a console application:

Example 2.
using System.Workflow.Runtime.Hosting;
namespace WFWorkflow
class Program
static void Main(string[] args)
workflowRuntime = new WorkflowRuntime())
waitHandle = new AutoResetEvent(false);
+= delegate
(object sender,
WorkflowCompletedEventArgs e)
+= delegate
(object sender,
WorkflowTerminatedEventArgs e)
WorkflowInstance instance =

In the preceding example, the host process initiates the workflow runtime and then starts the workflow method itself. The workflow runtime is started up by instantiating the workflowRuntime class. Passing in the workflow type to the CreateWorkflow method then creates a workflow instance class. The Start method on the workflow instance kicks off the workflow business process.

There are several events raised by the workflow runtime environment. These include WorkflowCompleted and WorkflowTerminated (the latter of which is called when there is an error). The previous example uses anonymous delegates, as WorkflowTerminatedEventArgs provides information on the exception that was generated. When the WorkflowCompleted event is called, we set the waitHandle value.

Passing Parameters into a Workflow Instance

Parameters are passed to a workflow instance using the Dictionary object. The CreatWorkflow operation has an overloaded operation that takes not only the workflow type but also a Dictionary object that accepts parameters. The code fragment in this next example demonstrates this:

Example 3.
Type type = typeof(WFWorkflow.CalcWorkflow);
Dictionary<string, object>
parameters = new Dictionary<string, object>();
parameters.Add("Value1", 11);
parameters.Add("Value2", 19);
parameters.Add("Operation", "+");
WorkflowInstance instance =
workflowRuntime.CreateWorkflow(type, parameters);

Returning Parameters from a Workflow Instance

The WorkflowCompletedEventArgs in the WorkflowCompleted event returns an output parameter, as shown here:

Example 4.
AutoResetEvent waitHandle = new AutoResetEvent(false);
+= delegate
(object sender,
WorkflowCompletedEventArgs e)
int total = (int)e.OutputParameters["Result"];

Output parameters are collections and data is extracted from collection and cast into an integer. The names of the input parameters passed into the workflow instance automatically map to the names of properties defined inside the workflow class. In this case, the Result property maps directly to the result parameter returned. The input properties are used to initialize private variables in the workflow instance:

Example 5.
public partial class CalcWorkflow :
private int value1 = 0;
private int value2 = 0;
private int result = 0;
private String operation;

public int Value1
set{value1 = value;}
public int Value2
set{value2 = value;}
public string Operation
set{operation = value;}
public int Result
get{return result;}
private void Calculator_ExecuteCode
(object sender, EventArgs e)
if (operation == "+")
{result = value1 + value2;}

Workflow-Enabled Services

Only a workflow that uses the WebServiceReceive activity can be published as a Web service. A simple scenario would be to create a workflow project and add WebServiceReceive and WebServiceResponse activities to it. In this case, the workflow is activated by calling a method on the Web service and returning a value when the processing is complete. In Visual Studio, a workflow project can be published as a Web service by right-clicking on a workflow project and selecting “Publish as Web service.” This action will create an ASP.NET project, with ASMX and Web.Config files.

Versioning Orchestrations

Orchestrations in WF can be versioned in two ways:

Execute the XOML file– In this case, the service reads the .XOML file and creates a Workflow instance using the CreateWorkflow method. The XOML file does not support versioning; the file can be manually versioned.

Use assembly level versioning– Each assembly has a version number, and two assemblies that differ by version number are considered by the runtime to be different assemblies. The assembly version can be managed by modifying the assembly.cs file prior to deployment.

Note that because with assembly-level versioning a new version of a workflow is treated as a new assembly version by the runtime, different assembly versions can run concurrently.

WF Extensibility

WF provides various extensibility points that are broad and do not impose semantics on the user. For example, if we are extending the persistence mechanism, WF does not stipulate that we use either the data-store or the serialization techniques.

WF’s extensibility model allows almost every aspect of WF to be extended. Some common extensibility requirements include:

  • creating custom policies

  • creating workflow tracking services

  • adding new activities for persistence, tracking, and communication

  • creating domain-specific activities

WF has a visual designer used in Visual Studio that can also be extended to create domain-specific designers.

Business Rules

A service-oriented solution is typically modeled so as to factor out task services because these services encapsulate business non-agnostic activities specific to the overarching business process. The WF visual designer can be used to glue together various business activities that involve business rules that are exposed in one of two ways:

Conditions can be used with built-in and custom activities to change their execution behavior. There are several built-in rules-based activities that enable conditional logic:

  • IfElse (provides decision logic)

  • While (provides looping behavior)

  • Replicator (analogous to a for-each statement)

Also supported is the conditioned activity group (CAG) that can provide rules-driven behavior over a collection of activities.

PolicyActivity, a rules engine that comes embedded with WF, can be used with a specialized workflow activity class that encapsulates a RuleSet which is stored in a .rules file. At runtime, PolicyActivity retrieves the rules from the .rules file and executes the rules.

SOA Principles & Patterns

The PolicyActivity engine can be used to apply Rules Centralization  so as to centralize business rules logic in the RuleSet, effectively establishing a rules service.

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