Sunday, January 15, 2006

VB8: Friend setters, but no friend assemblies

The new version of Visual Basic (VB8, a.k.a VB.NET2) released with VS2005, has got several new language features, such as generics and friend property setters. The latter is a feature I was looking forwards to employ in my current project. The need for easily restricting which code is allowed to modify the content of business entity objects (i.e. setting property values) seemed to be a perfect match for friend setters (internal in C#).

The scenario is that we have the typical set of components, objects, and layers in our application: business entities (BE), business logic (BL), data access logic (DAL), and also a client application (PL) that consumes the services provided by the business layer. The PL code should not be able to modify some of the entity object properties, such as the .Id (primary key) of the entity. The main task of the DAL is to transform the entities to and from the database, thus DAL must be able to set all of the properties in a BE object. Just what friend setters are for:

Private _id As Integer
Public Property Id() As Integer
Return _id
End Get
Friend Set(ByVal value As Integer)
_id = value
End Set
End Property

One object model design task remains, and the plan was to use another new VB8 feature promoted on several MSDN blogs through the Whidbey beta period: friend assemblies. First, me explain why friend assemblies are needed as part of the application's design.

As this is a distributed solution, we want to deploy a minimum set of components to the clients, both for operational and for security reasons. Thus, PL has only access to BE objects and a set of service proxies, but the BL and DAL objects are not deployed to the client. In fact, PL, BL, BE, and DAL are four different assemblies (VB projects). As the DAL and the BE components are separate assemblies, and some of the property setters cannot be public if the PL (and other future components and service consumers) should not be able to change those properties, the need for the InternalsVisibleTo mechanism arised.

To my disappointment, VB8 does not support friend assemblies (well, actually the VB8 compiler does not). I guess the "beta information - subject to change without notice" disclaimer has been applied to this feature.

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