Why use Repository Pattern or please explain it to me?

Question

I am learning repository pattern and was reading Repository Pattern with Entity Framework 4.1 and Code First and Generic Repository Pattern - Entity Framework, ASP.NET MVC and Unit Testing Triangle about how they implement the repository pattern with Entity Framework.

Saying

•Hide EF from upper layer
•Make code better testable

Make code better testable I do understand, but why hide EF from upper layer?

Looking at their implementation, it seems just wrap the entity framework with a generic method for query the entity framework. Actually what's the reason for doing this?

I am assuming is for

1. Loose coupling (that's why hide EF from upper layer?)
2. Avoid repeat writting same LINQ statement for same query

Am I understand this correctly?

If I write a DataAccessLayer which is a class have methods

QueryFooObject(int id)
{
..//query foo from entity framework
} 

AddFooObject(Foo obj)
{
.. //add foo to entity framework
}
......
QueryBarObject(int id)
{
..
}

AddBarObject(Bar obj)
{
...
}

Is that also a Repository Pattern?

Explaination for dummy will be great :)

Answer 1

I don't think you should.

The Entity Framework is already an abstraction layer over your database. The context uses the unit of work pattern and each DBSet is a repository. Adding a Repository pattern on top of this distances you from the features of your ORM.

I talked about this in my blog post: http://www.nogginbox.co.uk/blog/do-we-need-the-repository-pattern

The main reason adding your own repository implementation is so that you can use dependency injection and make your code more testable.

EF is not very testable out of the box, but it's quite easy to make a mockable version of the EF data context with an interface that can be injected.

I talked about that here: http://www.nogginbox.co.uk/blog/mocking-entity-framework-data-context

If we don't need the repository pattern to make EF testable then I don't think we need it at all.