system.workflow.activities

How to Write a Custom Workflow Activity

Published on January 25, 2007

Normally writing a custom workflow is a common task in Windows Workflow Foundation because there are many situations that we need to apply a development process to workflows. There are several references about writing a custom workflow but I see many newbies who can't understand the process easily. In my opinion the reason is most resources try to talk about all aspects of writing a custom activity at a glance without simple examples. So I want to test my chance and write about this topic in this post and some future posts. I'll begin with a simple custom activity which doesn't have any optional or advanced part of a custom activity.

There are two general types of custom activities: basic and composite. Both basic and composite activities can work with properties, events and ... but composite activities can do something more. Composite activities can contain basic or composite activities. There are some examples of basic and composite activities in built-in workflow activities. Delay or Suspend are two examples of basic activities and While or Parallel are two examples of composite activities.

It's possible to write a custom basic activity or composite activity but process to write a composite activity is a bit longer than basic activity.

Each activity is constructed of some components but for an activity it's not essential to have all these components:

• Definition: Main definition for an activity where you define properties, events and ...
• Executer: This component defines the execution logic for an activity. For example you can use this component to run child activities in a special order.
• Validator: Validator components helps you to validate workflow information before runtime so it can let you to prevent from some exceptions as a result of wrong input.
• Toolbox Item: This component defines default values to be added to a workflow when it's being added to workflow. For example you can use this component to set a default integer value for a property.
• Designer: Designer component is all about visual presentation of a workflow and what you see when you're working with it.

Only first component (Definition) is required for an activity to work but usually other components are implemented for an activity. As you can guess from above description it's not always necessary to have an execution logic, validation logic, default values or a good visual presentation so you can ignore these components based on your needs.

In this post I just want to introduce the core by giving a simple example that only needs first and second components (Definition and Executer). I write a custom workflow activity that gets two string values: a file path a text. On execution it creates a text file and writes text value into it. Before doing anything I create a Class Library project and add appropriate references to it then create a new class file and name it CustomActivity

Alright, I begin with definition component. To design a custom activity I have to derive my definition from a base class. For basic activities I must inherit it from System.Workflow.ComponentModel.Activity and for composite activities I must inherit from System.Workflow.ComponentModel.CompositeActivity base classes. As I want to design a base activity, derive my class from

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