The Windows Workflow
Foundation (WF) ships with a Policy Activity that allows you to execute a set
of rules against your workflow. This activity contains a design time rules
editor that allows you to create a set of rules. At run time, the Policy
Activity runs these rules using the WF Rules engine.
features, the rules engine allows you to prioritize rules and to set a chaining
policy to govern rules evaluation. The
rules engine uses a set of Code DOM expressions to represent the rules. These
rules can be run against any managed object, not just a workflow. Hence, the
mechanisms of the rules engine have nothing to do with workflow. You can
actually instantiate and use this rules engine without having to embed it
inside of a workflow. You can use this rules engine to build rules-driven .NET
I gave a talk at
the last Las Vegas VSLive! that demonstrates how to do this. The first sample
in the talk uses a workflow to demonstrate the power of the rules engine. The
second and third samples use a very simple example to demonstrate how to use
the engine outside of a workflow.
Two problems have to
be solved. You have to create a set of
Code DOM expressions for the rules. You have to host the engine and supply it
the rules and the object to run the rules against.
While the details
are in the slides and the examples, here is the gist of the solution.
To use the rules
engine at runtime, you pull the workflow rules out of some storage mechanism.
The first sample uses a file. A WorkflowMarkupSerializer instance deserializes
the stored rules to an instance of the RuleSet class. A RuleValidation instance validates the rules
against the type of the business object against which you will run the rules
against. The Execute method on the RuleExecution class is used to invoke the
rules engine and run the rules.
How do you create
the rules? Ideally you would use some domain language, or domain based
application, that would generate the rules as Code DOM expressions. If you were
masochistic enough, you could create those expressions by hand.
As an alternative,
the second sample hosts the Workflow rules editor dialog (RuleSetDialog class)
to let you create the rules. Unfortunately, like the workflow
designer, this is a programmer's tool, not a business analyst's tool. A WorkflowMarkupSerializer
instance is used to serialize the rules to the appropriate storage.
I would be
interested in hearing about how people use this engine to build rules driven