Introduction
Last week I had a session in Teched Israel 2010 which included approaches for building N-Tier applications on top 
of Entity Framework 4 as the data access technology. During the session, I showed an example of how to use the new Self-Tracking Entities (STE) feature. In this post, I‘m going to show how easy it is to put the STE in a different class library as a jump start for using this T4 Template in N-Tier scenarios.
What is Self-Tracking Entities?
In EF4, a new feature was introduced – the Self-Tracking Entities (STE). The STE are simple entities that track their own changes instead of EF’s ObjectContext. Every entity implements the IObjectWithChangeTracker interface and has a ChangeTracker property that exposes the tracking mechanism. Using STE enables you to use EF in N-Tier scenarios and gain the ability to track changes on the client side. This of course indicates that we will need a dependency on these entities on the client side (and that we will lose interoperability). In order to do that, you will need to separate the entities from the data access layer and to put them in their own class library.
Separating Self-Tracking Entities to Their Own Class Library
You start with two empty class libraries – one for he data access layer and the second for the entities that we are going to generate:
First, create your Entity Framework model in the STEDAL class library. In the EF designer surface, use the Add Code Generation Item in order to pick the Self-Tracking Entities T4 Template from the T4 Template options:
Now the STEDAL will look like:
Pay attention that you have two different tt files. The first one (SchoolModel.Context.tt) holds the context and the second one (SchoolModel.tt) holds the entities. The reason for the separation is obvious – to enable you to move the entities to their own class library.
The next step is to cut and paste the entities to the STEEntities class library. When you do that, pay attention that you will need to add a reference to System.Runtime.Serialization since the STE use WCF data contracts for serialization. Also, you will need to add a reference from the STEDAL to the STEEntities.
Now we need a simple fine tuning in the T4 template itself. Open the T4 template and at the head of the template locate the following line:
string inputFile = @"SchoolModel.edmx";
This line indicates where is the location for the edmx file. You will want to change it to a relative path to the real location of the edmx file for example:
string inputFile = @"../STEDAL/SchoolModel.edmx";
The reason for doing that is that the STE needs the Entity Data Model in order to work properly when we do data access operations.
The last step in the process is to put the context (or the entities if you prefer the opposite) in the same namespace as the entities. This can be achieved by using the SchoolModel.Context.tt file Custom Tool Namespace property. Put the STEEntities there and you are finished:
Now you can build the solution and start using the libraries.
Summary
Let's sum up, when you want to use the new Self-Tracking Entities feature, the first thing to do will be to move the entities to their own class library. The process is very simple and in this post, I showed you how to do exactly that.
