Model Extensions

The Web Service Software Factory includes a DSL framework extension that implements a mechanism to add selectable sets of properties to existing model elements without having to modify the DSL designers. Because the mechanism was initially created to add technology-specific properties to service contract elements, it is most often referred to as the Technology Extensions.

Figure 2 illustrates a logical view of that mechanism.

Figure 2
Logical view of the model extension mechanism

For a model element to be extended, it must implement the **IExtensibleObject****interface, defined in the Microsoft.Practices.Modeling.Common project in the Libraries solution folder.

The **IExtensibleObject****interface defines an array of ObjectExtenders, and a property that holds a reference to one of those objects, which is the currently selected extender. ObjectExtender is a container of additional (extension) properties on a model element. The following are the differences between the “regular” properties defined in the model and the extension properties:

The regular properties are always shown in the Properties window when the model element is selected.

Only the properties of the currently selected ObjectExtender are show in the Properties window.

In the Service Factory, all ObjectExtender classes are placed in the projects of the Extenders solution folder, and then they are copied with their assemblies to the Lib folder during the build process, so the Service Factory runtime can find them there at runtime using the AssemblyLoader. A model element does not contain information about what specific extenders can be assigned to it. It is the extenders that know what model element type they apply to. The code that follows is a fragment of the WCFService extender. The ObjectExtender attribute and the generic abstract base class ObjectExtender<Service> define it as the extension to the Service model element defined in the Service Contract model. The following code shows the details of only one extension property called ConcurrencyMode.

[Serializable]
[CLSCompliant(false)]
[ObjectExtender(typeof(Service))]
public class WCFService : ObjectExtender<Service>
{
private ConcurrencyMode concurrencyMode;
private InstanceContextMode instanceContextMode;
#region Constructors
… 
#endregion

#region Properties
[Category(ServiceContractWCFExtensionProvider.ExtensionProviderPropertyCategory),
Description("Specifies whether a service supports single-threaded or multi-threaded modes of operation."),
ReadOnly(false),
Browsable(true)]
[XmlElement("ConcurrencyMode")]
public ConcurrencyMode ConcurrencyMode
{
get { return concurrencyMode; }
set { concurrencyMode = value; }
}
…
#endregion
} 

The ExtensionProvider is a class that provides a list of applicable extender types. The Service Factory has two extension providers, one for the WCF extenders of the Service Model (ServiceContractWCFExtensionProvider) and one for the ASMX extenders of the same model (ServiceContractAsmxExtensionProvider). Both of these extenders are associated with the root model element of the Service Contract model.

The ServiceContractModel implements the ITechnologyProvider interface, which provides the currently selected ExtensionProvider for the model. The logic of creating and associating extender instances with the Service Contract model elements is implemented in the property editor of the ImplementationTechnology property of the ServiceContractModel element. The editor class is named ExtensionProviderEditor and is in the Microsoft.Practices.Modeling.ExtensionProvider project of the Libraries solution folder. The editor is called when the user is selecting the implementation technology of the entire service contract diagram (a necessary step before code can be generated).

When a model is saved, the extenders associated with its model elements are saved together with those elements. When a model is loaded, the extenders are loaded and attached to the extended model element.