Wednesday, April 16, 2008

WCF: Client Domain Objects, Decorator Pattern

The client domain objects (aka business entity - BE) must be related to the data contracts provided by the web-services. Still, they will need to be enhanced with e.g. dirty-tracking, validation, state, property conversions (xs:date), business rules, and other aspects needed to produce a fully working client application. This article describes how to achieve this when sharing domain objects across layers is not an option (i.e. interop with JEE services), and the same applies to AOP.

One option for implementing the BE objects, described in this article, is to apply the Decorator pattern to the WSDL scema-based DTO objects to enhance them into full-fledged domain objects. Read ‘
Design of Client Domain Objects’ for an intro this option and the related Mapper option.

If you do not like this Decorator option due to the weak [XML attribute] link to the data contracts (see end of article), you can always use a derived decorator class instead of a partial class. Make all your BE enhancements in the derived class using a combination of DTO overrides and new domain logic. If you prefer explicit, debuggable code, you're better off using the Mapper option.

Service Proxy

Use the SVCUTIL.EXE /REFERENCE switch when generating the proxy code and messages to reuse your BE objects as DTOs and to avoid getting new, duplicate DTO objects generated (might not compile due to name clashes). Alternatively, just remove the DTO code from the generated proxy. Make sure that the proxy code reference the .NET namespace and interface assembly that contains your BE objects.

Business Entities

You can use the data contract DTO objects generated by SVCTIL.EXE /DCONLY to get started. This will give you data-binding enabled (/EDB) anemic entity objects with all the properties in place, annotated with XML serialization attributes. Just ensure that the output files from SVCUTIL.EXE do not overwrite your code. The DTO objects become your generated BE objects - ready for decoration.

The BE objects must be manually maintained whenever you make changes to the generated BE code, e.g. to add EntLib3 validation attributes on the properties. Keep your additions to the generated BE in a partial class as far as possible, e.g. add new properties and methods in a partial class extending the decorated class. If all your decorator code can be isolated into a partial class, then the generated BE code can be re-generated without losing your extended BE code.

Our programming model for decorator-based client domain objects is based on using two partial classes; one T.schema.cs class file for all code based originally on the data contracts, and one T.cs class file for all added aspects. T is the type name of the BE.

Note that BE constructors will not be called during deserialization. Create a method to be used as a surrogate constructor to be run after deserialization and mark it with [OnDeserialized]. This is a good place for resetting e.g. the dirty flag:

void DeserializationCtor(StreamingContext context)
_isDirty = false;

Your BE class must still have a constructor to initialize the object for normal use. The constructor must initialize all properties that must have a defined initial value. In addition, the c'tor must initialize all child objects that are exposed as (de-normalized) public properties, to avoid null reference exceptions when using the BE (make it simple to use the BE). This also applies to child collections; create the collection to make it ready for use.

public NominationType()
//initialize all normalized valueobjects with members
this.NominatedValueField = new UnitValueType();
this.NominatedValueField.Unit = String.Empty;

Also initialize all fields that are required by the XSD schema, failure to initialize these fields will give serialization errors:
[DataMemberAttribute(IsRequired=true, . . .) ]

Don't break the DRY principle by adding the value object initialization code multiple places.

Dirty Tracking

If your BE implements INotifyPropertyChanged to enable data-binding, then the event is a good place to add dirty-tracking: Simple Dirty Tracking in Domain Objects

Converting XML Data Types to/from Strongly Typed Properties

Note that xs:date becomes string in .NET as there is no Date only type, just DateTime. Do not let this kind of weak type design leak into your BE objects. Contain the weak typing inside your BE object by converting to/from strongly typed properties.

private string _demandDateData; //XML xs:date datatype

public DateTime DemandDate
return DateTime.ParseExact(this._demandDateData, WSDateFormat, null);
string xsDate = value.ToString(WSDateFormat);
if (this._demandDateData.Equals(xsDate) != true)
this._demandDateData = xsDate;

This keeps your BE object within the "Simple vs Easy" principle.

De-normalization of BE Value Objects

The data contracts provided by the web-services will most likely contain some DDD value objects for numeric types annotated with some related attributes (e.g. money with currency) and other similar cases. These value objects will become child objects of the BE object, which is not very data-binding friendly as most controls (grids, etc) needs the data to be non-nested properties of the bound object. In addition, one control can only have one binding source. Thus, unlike master-details type binding that can be resolved using multiple binding sources, nested value objects must be de-normalized and projected as normal properties directly on the BE objects.

Start by making the value object private and give the property a new name (e.g. suffix with Data) while specifying the original name in the serialization attribute to keep the link to the XML. Keep the RaisePropertyChanged parameter equal to the property name.

[DataMemberAttribute(Name = "NominatedValue", IsRequired = true, EmitDefaultValue = false, Order = 2)]
private UnitValueType NominatedValueData
get { return this.NominatedValueField; }
set { . . . }

Then create the de-normalized public properties (in a partial class) as required by the BE and data-binding needs. The new public properties should be wrappers exposing an element of the original private member value object, using a friendly name and the correct data type. Make sure that the new-old value comparison is correct. Keep the RaisePropertyChanged parameter equal to the new property name.

public Decimal NominatedValue
get { return this.NominatedValueField.Value; }
if (this.NominatedValueField.Value.Equals(value) != true)
this.NominatedValueField.Value = value;

public string NominatedValueUnit
get { return this.NominatedValueField.Unit; }
set { . . . this.RaisePropertyChanged("NominatedValueUnit"); }

As can be seen from the example, the new public properties are named by concatenating the value object name and the exposed property name; except for when the property name is just repeating the last part of the value object (i.e. NominatedValue.Value).

Remember to initialize all the value object children in the BE c'tor; both create the child objects and set their initial values.

Keeping the BE Synchronized with the Data-Contract Schema

It is imperative that the XML serialization attributes in code are correct according to the data contract XSD when using the decorator approach on proxy DTO objects to implement BE objects. There are three typical changes that can break the mapping between the BE code and the payload XML:

  • Changes to the code by renaming properties without updating the XML serialization attributes accordingly
  • Changes to the data contract XSD schema without adding properties with correct XML serialization attributes
  • Changes to the data contract XSD schema without updating the XML serialization attributes accordingly
The first issue can happen when e.g. re-exposing a property by renaming and hiding the original property (such as for de-normalizing a value object). This will require updating the XML serialization attribute to keep the mapping of the code to XSD correct:

[DataMemberAttribute(Name = "NominatedValue", . . .) ]
private UnitValueType NominatedValueData

The next two issues are caused by adding new items to the data contract or changing some existing items. The latter is not recommended and is definitively not a recommended practice for published contracts:

  • It is a proven practice that published contracts are not changed, except for non-breaking changes in minor versions.
  • For new, non-breaking versions of the contract, items must only be added at the end of the existing contract.
When a new minor version of a published data contract must be incorporated into your BE class, then start by adding the properties to hold the new data items, then add the applicable XML serialization attributes with the correct sequence order value:

[DataMemberAttribute(. . . , Order=<order number in XML xs:sequence element for item>) ]

For new major versions of data contracts, you must consider creating a new version of the BE class in a new .NET namespace. This allows you to do controlled versioning of your domain objects in synchronization with the versioning policy of the service contracts.

However, during the development of the web-services, the contracts are bound to get breaking changes as the data contracts evolve (which is OK as they are still not published). For this reason, wait as long as you can to make changes to the generated BE code, stick to working on the partial class used to decorate the BE. This will allow you to re-generate the schema-bound parts of your BE to keep up with the changes in the data contracts.

A side note: deriving your BE objects from the DTO objects is an alternative to partial classes that strictly enforces the separation of decorator code from generated code, and allows the generated code to be auto-generated over and over again - like for the Mapper option. Alas, there are drawbacks to inheriting the DTOs also, as none of the generated proxy artifacts are geared towards this. Look into using an AOP framework to do dynamic subclassing if chosing to derive your BE objects.

For data contracts under development, any combination of adding, changing, removing and reordering of data contract items can be expected to occur. The XML serialization attributes must then be updated accordingly.

1 comment:

Anonymous said...

I read your first post about Client Domain
Objects and was hoping for some detailed explanation of the patterns. Thanks for you post, find them very informative

Regards Rob