Is Unit Of Work and Repository Patterns very useful for big projects?
I'm starting a new web project using ASP.NET Webforms + EF4. I'm trying to apply a repository pattern with a unit of work pattern following this tutorial : http://www.dotnetage.com/publishing/home/2011/07/05/6883/the-repository-pattern-with-ef-code-first-dependeny-injection-in-asp-net-mvc3.html
I think I got the idea but my question is that, when I create a new object in the model, do I also have to define that object in IDALContext of the Unit Of Work? Isn't that a handbreak for rapid development? Also if you work with multiple developers and if you don't want other developers to see your DAL, how can you manage this? Because in this pattern as I understand, when you create a new object in the model you also have to define it in the IDALContext for this tutorial. Sorry I'm so confused by this.
Answer
Now, the first question should be, why do I need a repository or unit of work pattern at all? Couldn't I just use the EF context from the controller, having the full power of directly writing the query I need and returning the data?
Answer: You could, but the real intent behind is testability and thus higher quality, more maintainable code. If you separate your data access and concentrate it on one place, you can mock it out during testing. This allows you to unit test the logic defined within your controller without effectively writing to a data store.
Before starting with the Unit of Work, just use take a look at the Repository pattern. This basically abstracts the data access for a given entity. So you define methods like Filter(), All(), Update(..), Insert(..), Delete(...) and finally, Save(). Actually most of these could be quite easily abstracted to a BaseRepository<TEntity> class such that in the end you'd just have to create a new Repository in rare cases with special behavior. Otherwise it would be something like BaseRepository<Person> personRepo = new BaseRepository<Person>() or BaseRepository<Address> addressRepo = new BaseRepository<Address>() etc.
Why is the Unit of Work needed?
A unit of work represents all operations done during a certain cycle, in a web environment normally per Http request. This means when a new request enters, you instantiate a new Unit of Work, you add new stuff, update or delete it and then you "commit" the changes by invoking the .save()or .commit()..whatever. Actually if you take a closer look at the Entity Framework DbContext (or ObjectContext), they are already representing some kind of Unit of Work.
However if you want to further abstract it, because you'd not necessarily like to have your EF context in your controller classes (remember: testability), then you create a UoW to group your Repositories and also to ensure they all share the same EF context instance. You might achieve the latter also through a DI container (Dependency Injection container).
To your questions: Is it useful in big projects?:
Definitely, especially in big projects. It's all about keeping responsibilities separated (data access, business logic, domain logic) and thus making things testable.