Make type-casting inside a workflow easier with TypeCastActivity

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Introduction

While doing some research on Windows Workflow Foundation, I came across a single lack of functionality that I really missed in my projects. That was object down-casting functionality in workflow binding architecture.

Consider an example: I have a custom activity (call it LoadDataActivity) to load some data objects using API. Loaded object reference is stored in LoadDataActivity.Data property, which is of type Object, as I assume I don't know objects of what type can be loaded.

I also have a custom activity ShowMessageActivity to display some information text for me. It has ShowMessageActivity.Message property of type String to store message information.

Consider I am designing a sequential workflow to load a Person object and display his FullName property in the message box. For that purpose I am using LoadDataActivity to load data and ShowMessageActivity to display FullName information. I also would like to bind ShowMessageActivity.Message property to LoadDataActivity.Data.FullName property, but this is the place where I get stuck.

Workflow property binding dialog does not allow me to expand LoadDataActivity.Data property as it is of type Object and it does not contain FullName property and looks like I have no means to type-cast this property (without writing additional code!) in order to complete my binding operation.

The purpose of this article is to present custom workflow activities which could be used to type-cast any workflow field, property or activity property to the chosen type. The key value of using such activities is the ability to down-cast objects in order to access them in workflow property binding dialog.

Alternative solutions and limitations

1. No support for down-casting in WF binding architecture in WWF V1; http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=381688&SiteID=1
2. It is possible to add fields to workflow code-behind and store type-casted values there. Those values could be accessed from within workflow property binding dialog. The main drawback of such method is the requirement to make code changes, which are not always acceptable and even not always possible (i.e. in XAML-only workflows);
3. Implementing separate activities that would have properties of different types, i.e. instead of having a single LoadDataActivity, have multiple activities: LoadPersonActivity, LoadEmployeeActivity, etc. The main drawback - large number of activities to implement.

Proposed solution - TypeCastActivity and TypeCastActivitySequence

I am proposing an activity TypeCastActivity with two parameters - first one serving as an input parameter of type Object, and other one of type 'T', storing type-casted input parameter value. 'T' is any type selected by the user. Such activity is implemented as a generic type, so it cannot be added into toolbox and used the same way as regular activities at design time. It also has to be compiled into a separate class library (not Activity Library) in order to avoid validation errors during compile process.

public class TypeCastActivity <T > : 
    Activity, ITypeCastActivity
{
…
}

In order to use TypeCastActivity in workflow projects, TypeCastActivitySequence activity was implemented. It has a special activity designer associated with it which allows adding TypeCastActivity instances as child activities of the TypeCastActivitySequence.

Using TypeCastActivity and TypeCastActivitySequence activities

In order to perform type-casting inside the workflow, drop TypeCastActivitySequence activity on the workflow design surface. Right click on it and choose menu item "Add Type Cast Activity…"

After clicking activity designer menu item, type browser dialog pops up. In order for TypeCastActivity to be created, it is necessary to specify a Type, which you want to perform type-casting to.

In the example above I am choosing a "Person" type. As a result of such action, I get new TypeCastActivity generated inside the TypeCastActivitySequence and the generated activity contains Parameter property of type "Person".

TypeCastActivity.Source property could be bound to any object, i.e. workflow input parameter, storing Person instance. A type-casted reference to such object would then automatically be stored inside TypeCastActivity.Parameter property, which can be used in the rest of the workflow for property binding, Person object property modifications, etc.

Points of interest

Implemented activities TypeCastActivity and TypeCastActivitySequence can also be reviewed as examples on following topics:

• Implementing activity of generic type;
• Using custom activity designer to add menu item to designer context menu;
• Custom activity property serialization using modified WorkfowMarkupSerializer;
• Using standard type browser dialog;
• Validating names for dynamically added activities;
• Instantiating generic type objects using Reflection;