Search code examples
linqinterfacedata-access-layer

Why and when to use an Interface with Linq

Assuming you have three layers (Business, Data and UI). My data layer would have a linq to sql file with all the tables added.

I've seen some examples where an Interface is created in the business layer and then implemented in another class (type is of IQueryable/IEnumerable), yet other classes are using normal Linq syntax to get/save/delete/update data.

Why and when would i use an Interface which has an IQueryable/IEnumerable type?

Solution

  • Two of the most common situations, which you may want to do this are:

    1. you want to protect yourself from changes to that part of your system.
    2. you want to be able to write good unit tests.

    For example, you have a business layer that talks directly to LINQ to SQL. In the future you mat have a requirement to use nHibernate or Entity Framework instead. Making this change would impact on your business layer, which is probably not good.

    Instead, if you have programmed to an interface (say IDataRepository), you should be able to swap in and out concrete implementations like LINQtoSQLRepository or HibernateRepository without having to change your business layer - it only cares that it can call, say Add(), Update(), Get(), Delete() etc - but doesn't care how these operations are actually done.

    Programming to interfaces is also very useful for unit testing. You don't want to be running tests against a database server for a variety of reasons such as speed and reliability. So, you can pass in a test double, fake or mock implementation to test your data layer. E.g. You have some test data that implements your IDataRepository, which allows tou to then test add(), delete() etc from your business layer without having a DB connection.

    These points are generally good practice in all aspects of your application. I suggest reading up on The Repository Pattern, SOLID principles and maybe even Test Driven Development. This is a large and sometimes complex area and its difficult to give a detailed answer of exactly what to do and when as it needs to suit your scenario.

    I hope this helps you get started.