LINQ to SQL: Interfaces on your Domain objects

There has been some gowning about the lack of interfaces built into the domain objects produced by the Linq to SQL code generator.

Note: a domain object, as I am using the term here, is an object that maps column-to-property with a table. For example, if you have a Customer table with an ID and a Name field, you will have a Customer class with an ID and Name property.

I found that it is trivially easy to create. Every object in Linq to SQL is a partial class. So create a partial class that maps to the generated partial (there is no good way to say that, I swear). Then you can use the built in refactoring "Extract Interface" to create an interface that implements the fields in domain object.

The key point that I did not realize is this: you can assign an interface to class in any of the partial class declarations.

So, in Linq to SQL, keeping with our Customer table paradym, your generated partial class will look like this:

public partial class Customer : INotifyPropertyChanging, INotifyPropertyChanged

Now your other partial class (in an entirely different file) will look like this:

public partial class Customer : ICustomer

Both declarations define interfaces. I didn't know you could do that.

Now the cool part is that the interface can be defined anywhere. If you are trying to create a true separation between layers you will probably want to define it an an assembly other than the assembly that the domain classes are defined in.

Anyway, that is my discovery for today. Have a good weekend.