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Effective Repository in C# - Where to put methods?

I'm trying to build a new application using the Repository pattern for the first time and I'm a little confused about using a Repository. Suppose I have the following classes:
public class Ticket
{
}
public class User
{
public List<Ticket>AssignedTickets { get; set; }
}
public class Group
{
public List<User> GroupMembers { get;set; }
public List<Ticket> GroupAssignedTickets { get;set; }
}
I need a methods that can populate these collections by fetching data from the database.
I'm confused as to which associated Repository class I should put those methods in. Should I design my repositories so that everything returning type T goes in the repository for type T as such?
public class TicketRepository
{
public List<Ticket> GetTicketsForGroup(Group g) { }
public List<Ticket> GetTicketsForUser(User u) { }
}
public class UserRepository
{
public List<User> GetMembersForGroup(Group g) { }
}
The obvious drawback I see here is that I need to start instantiating a lot of repositories. What if my User also has assigned Widgets, Fidgets, and Lidgets? When I populate a User, I need to instantiate a WidgetRepository, a FidgetRepository, and a LidgetRepository all to populate a single user.
Alternatively, do I construct my repository so that everything requesting based on type T is lumped into the repository for type T as listed below?
public class GroupRepository
{
public List<Ticket> GetTickets(Group g) { }
public List<User> GetMembers(Group g) { }
}
public class UserRepository
{
public List<Ticket> GetTickets(User u) { }
}
The advantage I see here is that if I now need my user to have a collection of Widgets, Fidgets, and Lidgets, I just add the necessary methods to the UserRepository pattern and don't need to instantiate a bunch of different repository classes every time I want to create a user, but now I've scattered the concerns for a user across several different repositories.
I'm really not sure which way is right, if any. Any suggestions?

The repository pattern can help you to:
Put things that change for the same reason together
As well as
Separate things that change for different reasons
On the whole, I would expect a "User Repository" to be a repository for obtaining users. Ideally, it would be the only repository that you can use to obtain users, because if you change stuff, like user tables or the user domain model, you would only need to change the user repository. If you have methods on many repositories for obtaining a user, they would all need to change.
Limiting the impact of change is good, because change is inevitable.
As for instantiating many repositories, using a dependency injection tool such as Ninject or Unity to supply the repositories, or using a repository factory, can reduce new-ing up lots of repositories.
Finally, you can take a look at the concept of Domain Driven Design to find out more about the key purpose behind domain models and repositories (and also about aggregate roots, which are relevant to what you are doing).

Fascinating question with no right answer. This might be a better fit for programmers.stackexchange.com rather than stackoverflow.com. Here are my thoughts:
Don't worry about creating too many repositories. They are basically stateless objects so it isn't like you will use too much memory. And it shouldn't be a significant burden to the programmer, even in your example.
The real benefit of repositories is for mocking the repository for unit testing. Consider splitting them up based on what is simplest for the unit tests, to make the dependency injection simple and clear. I've seen cases where every query is a repository (they call those "queries" instead of repositories). And other cases where there is one repository for everything.

As it turns out, the first option was the more practical option in this case. There were a few reasons for this:
1) When making changes to a type and its associated repository (assume Ticket), it was far easier to modify the Ticket and TicketRepository in one place than to chase down every method in every repository that used a Ticket.
2) When I attempted to use interfaces to dictate the type of queues each repository could pull, I ran into issues where a single repository couldn't implement an generic interface using type T multiple times with the only differentiation in interface method implementation being the parameter type.
3) I access data from SharePoint and a database in my implementation, and created two abstract classes to provide data tools to the concrete repositories for either Sharepoint or SQL Server. Assume that in the example above Users come from Sharepoint while Tickets come from a database. Using my model I would not be able to use these abstract classes, as the group would have to inherit from both my Sharepoint abstract class and my SQL abstract class. C# does not support multiple inheritance of abstract classes. However, if I'm grouping all Ticket-related behaviours into a TicketRepository and all User-related behaviours into a UserRepository, each repository only needs access to one type of underlying data source (SQL or Sharepoint, respectively).

Relationship between EF-Generated classes and model?

I'm using ASP .NET MVC (C#) and EntityFramework (database first) for a project.
Let's say I'm on a "Movie detail" page which shows the detail of one movie of my database. I can click on each movie and edit each one.
Therefore, I have a Movie class, and a Database.Movie class generated with EF.
My index action looks like :
public ActionResult MovieDetail(int id)
{
Movie movie = Movie.GetInstance(id);
return View("MovieDetail", movie);
}
GetInstance method is supposed to return an instance of Movie class which looks like this for the moment :
public static Movie GetInstance(int dbId)
{
using (var db = new MoviesEntities())
{
Database.Movie dbObject = db.Movies.SingleOrDefault(r => r.Id == dbId);
if (dbObject != null)
{
Movie m = new Movie(dbObject.Id, dbObject.Name, dbObject.Description);
return m;
}
return null;
}
}
It works fine but is this a good way to implement it? Is there an other cleaner way to get my instance of Movie class ?
Thanks

is this a good way to implement it?
That's a very subjective question. It's valid, and there's nothing technically wrong with this implementation. For my small-size home projects, I've used similar things.
But for business applications, it's better to keep your entities unrelated to your MVC application. This means that your data context + EF + generated entities should be kept in a separate project (let's call it the 'Data' project), and the actual data is passed in the form of a DTO.
So if your entity resembles this:
public class Person {
public int Id { get; set; }
public string Name { get; set; }
}
You'd expect there to be an equivalent DTO class that is able to pass that data:
public class PersonDTO {
public int Id { get; set; }
public string Name { get; set; }
}
This means that your 'Data' project only replies with DTO classes, not entities.
public static MovieDTO GetInstance(int dbId)
{
...
}
It makes the most sense that your DTOs are also in a separate project. The reason for all this abstraction is that when you have to change your datacontext (e.g. the application will start using a different data source), you only need to make sure that the new data project also communicates with the same DTOs. How it works internally, and which entities it uses, is only relevant inside the project. From the outside (e.g. from your MVC application), it doesn't matter how you get the data, only that you pass it in a form that your MVC projects already understand (the DTO classes).
All your MVC controller logic will not have to change, because the DTO objects haven't changed. This could save you hours. If you link the entity to your Controller AND View, you'll have to rewrite both if you suddenly decide to change the entity.
If you're worried about the amount of code you'll have to write for converting entities to DTOs and vice versa, you can look into tools like Automapper.
The main question: Is this needed?
That, again, is a very subjective question. It's relative to the scope of the project, but also the expected lifetime of the application. If it's supposed to be used for a long time, it might be worth it to keep everything interchangeable. If this is a small scale, short lifetime project, the added time to implement this might not be worth it.
I can't give you a definitive answer on this. Evaluate how well you want the application to adapt to changes, but also how likely it is that the applicaiton will change in the future.
Disclaimer: This is how we do it at the company where I work. This is not the only solution to this type of problem, but it's the one I'm familiar with. Personally, I don't like making abstractions unless there's a functional reason for it.

A few things:
The naming you're using is a little awkward and confusing. Generally, you don't ever want to have two classes in your project named the same, even if they're in different namespaces. There's nothing technically wrong with it, but it creates confusion. Which Movie do I need here? And if I'm dealing with a Movie instance, is it Movie or Database.Movie? If you stick to names like Movie and MovieDTO or Movie and MovieViewModel, the class names clearly indicate the purpose (lack of suffix indicates a database-backed entity).
Especially if you're coming from another MVC framework like Rails or Django, ASP.NET's particular flavor of MVC can be a little disorienting. Most other MVC frameworks have a true Model, a single class that functions as the container for all the business logic and also acts as a repository (which could be considered business logic, in a sense). ASP.NET MVC doesn't work that way. Your entities (classes that represent database tables) are and should be dumb. They're just a place for Entity Framework to stuff data it pulls from the database. Your Model (the M in MVC) is really more a combination of your view models and your service/DAL layer. Your Movie class (not to be confused with Database.Movie... see why that naming bit is important) on the other hand is trying to do triple duty, acting as the entity, view model and repository. That's simply too much. Your classes should do one thing and do it well.
Again, if you have a class that's going to act as a service or repository, it should be an actual service or repository, with everything those patterns imply. Even then, you should not instantiate your context in a method. The easiest correct way to handle it is to simply have your context be a class instance variable. Something like:
public class MovieRepository
{
private readonly MovieEntities context;
public MovieRepository()
{
this.context = new MovieEntities();
}
}
Even better, though is to use inversion of control and pass in the context:
public class MovieRepository
{
private readonly MovieEntities context;
public MovieRepository(MovieEntities context)
{
this.context = context;
}
}
Then, you can employ a dependency injection framework, like Ninject or Unity to satisfy the dependency for you (preferably with a request-scoped object) whenever you need an instance of MovieRepository. That's a bit high-level if you're just starting out, though, so it's understandable if you hold off on going the whole mile for now. However, you should still design your classes with this in mind. The point of inversion of control is to abstract dependencies (things like the context for a class that needs to pull entities from the database), so that you can switch out these dependencies if the need should arise (say perhaps if there comes a time when you're going to retrieve the entities from an Web API instead of through Entity Framework, or even if you just decide to switch to a different ORM, such as NHibernate). In your code's current iteration, you would have to touch every method (and make changes to your class in general, violating open-closed).

entity-model never should act as view-model. Offering data to the views is an essential role of the view-model. view-model can easily be recognized because it doesn’t have any other role or responsibility other than holding data and business rules that act solely upon that data. It thus has all the advantages of any other pure model such as unit-testability.
A good explanation of this can be found in Dino Esposito’s The Three Models of ASP.NET MVC Apps.

You can use AutoMapper
What is AutoMapper?
AutoMapper is a simple little library built to solve a deceptively complex problem - getting rid of code that mapped one object to another. This type of code is rather dreary and boring to write, so why not invent a tool to do it for us?
How do I get started?
Check out the getting started guide.
Where can I get it?
First, install NuGet. Then, install AutoMapper from the package manager console:
PM> Install-Package AutoMapper

ASP.NET MVC Design Issues

Whats the difference between Model and ViewModel? I should use both of them or I better skip one of them? who grabs the data from the database?
I wonder whats the best/right way to take my data from the database.
One option is to have a DAL (Data Access Layer) and instantiate it in every controller,
fill with it the viewmodels like:
var viewmodel = Dal.GetArticles();
Another option is to let the model itself grab the information from the Database:
var model = new ArticlesModel();
model.GetArticles();
public void GetArticles()
{
var context = new DbContext();
_articles = context.Articles
}
Another similar option is to have a static DAL so you can access it inside every model,
so each model will have a method to grab the data using the static DAL class (Which contain a DbContext class inside to access the Database)
public void GetArticles()
{
_articles = DAL.GetArticles();
}
So the general question is if the model itself needs to grab the data from the database or the controller itself can have access to the DAL.

While someone is writing a more useful answer, I will quickly address your points.
Model is the data you want to display.
More often than not, you will want to use object relational mapping so most of your business object classes correspond to database tables and you don't have to manually construct queries.
There are plenty of ORM solutions available, including Entity Framework, NHibernate and (now dying) LINQ to SQL.
There is also an awesome micro-ORM called Dapper which you may like if bigger frameworks feel unneccessarily bloated for your solution.
Make sure you learn about the differences between them.
DAL is more idiomatic in .NET than classes that “know” how to load themselves.
(Although in practice your solution will very likely be a mixture of both approaches—the key is, as usual, to keep the balance.)
My advice is to try keeping your models plain old CLR objects as long as your ORM allows it and as long as this doesn't add extra level of complexity to the calling code.
These objects, whenever possible (and sensible—there are exceptions for any rule!), should not be tied to a particular database or ORM implementation.
Migrating your code to another ORM, if needed, will be just a matter of rewriting data access layer.
You should understand, however, that this is not the main reason to separate DAL.
It is highly unlikely you'll want to change an ORM in the middle of the project, unless your initial choice was really unfit for the purpose or you suddenly gained a traction of 100,000 of users and your ORM can't handle it. Optimizing for this in the beginning is downright stupid because it distracts you from creating a great product capable of attracting even a fraction of hits you're optimizing for. (Disclaimer: I've walked this path before.)
Rather, the benefit of DAL is that you database access becomes always explicit and constrained to certain places where you want it to happen. For example, a view that received a model object to display will not be tempted to load something from the database, because in fact it is the job of controller to do so.
It's also generally good to separate things like business logic, presentation logic and database logic. Too often it results in better, less bug-ridden code. Also: you are likely to find it difficult to unit-test any code that relies on objects being loaded from the database. On the other hand, creating a “fake” in-memory data access layer is trivial with LINQ.
Please keep in mind that again, there are exceptions to this rule, like lazy properties generated by many ORMs that will load the associated objects on the go—even if called within a view. So what matters is you should make an informed decision when to allow data access and why. Syntaxic sugar might be useful but if your team has no idea about performance implications of loading 20,000 objects from ORM, it will become a problem.
Before using any ORM, learn how it works under the hood.
Choosing between Active Record-style objects and a DAL is mostly a matter of taste, common idioms in .NET, team habits and the possibility that DAL might eventually have to get replaced.
Finally, ViewModels are a different kind of beast.
Try to think of them like this:
You shouldn't have any logic in views that is more sophisticated than an if-then-else.
However, there often is some sophisticated logic in showing things.
Think pagination, sorting, combining different models in one view, understanding UI state.
These are the kinds of thing a view model could handle.
In simple cases, it just combines several different models into one “view-model”:
class AddOrderViewModel {
// So the user knows what is already ordered
public IEnumerable<Order> PreviousOrders { get; set; }
// Object being added (keeping a reference in case of validation errors)
public Order CurrentOrder { get; set; }
}
Models are just data, controllers combine the data and introduce some logic to describe data to be shown in view models, and views just render view models.
View model also serves as a kind of documentation. They answer two questions:
What data can I use in a view?
What data should I prepare in controller?
Instead of passing objects into ViewData and remembering their names and types, use generic views and put stuff in ViewModel's properties, which are statically typed and available with IntelliSense.
Also, you'll likely find it useful to create ViewModel hierarchies (but don't take it to extremes!). For example, if your site-wide navigation changes from breadcrumbs to something else, it's cool to just replace a property on base view model, a partial view to display it and the logic to construct it in the base controller. Keep it sensible.

A model represents the structure you like your data in and is not concerned about the view which may consume it. A model's intend is purely that of representing the structure.
A model may contain properties irrelevant to the view consuming it.
A view-model is designed with the view in mind. A view-model is intended for a 1-to-1 relationship to a view. A view-model only contains the basic fields and properties the view it is intended for requires.
In general you would have your controller contact a repository (In your example your DAL) obtaining the data and then populating either a model or view-model with the results, sending it down to the view.

Model (Domain Model): is the heart of the application, representing the biggest and most important business asset because it captures all the complex business entities, their relationships and their functionality.
ViewModel: Sitting atop the Model is the ViewModel:The two primary goals of the ViewModel are
1. to make the Model easily consumable by the View and
2. to separate and encapsulate the Model from the View.
Eg.
Model:
public class Product
{
...............
}
public class Category
{
...........
}
ViewModel:
public class ProductViewModel
{
public ProductViewModel(List<Product> products, List<Category> categories)
{
this.Products = products;
this.Categories = categories;
}
public List<Product> Products { get; set; }
public List<Category> Categories { get; set; }
}

Confusion over IRepository

I'm a bit confused about when using the "IRepository pattern", when actually to load the data.
Currently I have something like this:
public class MainViewModel : ViewModelBase
{
// EF4 generated ObjectContext
private ScorBotEntities context = new ScorBotEntities();
// Custom IUserRepository class
private IUserRepository userRepository;
public MainViewModel()
{
this.userRepository = new UserRepository(context.Users);
}
public ObservableCollection<User> Users
{
get
{
return new ObservableCollection<User>(userRepository.GetAll());
}
}
}
ScorBotEntities are autogenerated using EF4 (I had a look at POCOs, to much work for this sized project).
You can find the definition of the UserRepository here: http://code.google.com/p/i4prj4-g2/source/browse/ScorBotRobotics/ScorBotRobotics/Repositories/UserRepository.cs
But basically, what I'm wondering about is, why do it even make sense to use a repository here, instead of just writing it like this:
public class MainViewModel : ViewModelBase
{
private ScorBotEntities context = new ScorBotEntities();
public MainViewModel()
{
}
public ObservableCollection<User> Users
{
get
{
return new ObservableCollection<User>(context.Users);
}
}
}
It makes sense to abstract functionality away such as with the UsernameAndPassword method. But in that case, perhaps using some Query Objects would be more ideal?

I am a bit baffled that your context has made its way down to your ViewModel. I believe your GUI layer should never see the context. Context must be opened/kept/closed by the IRepository. Let the data layer (IRepository) return an array/list of Users.

There are a couple of different points here. First, your view models should have no knowledge of the repository - keep your view models as simple as possible.
Second, the IRepository is your public API - so you should have dependencies to this (depend on abstractions rather than concrete implementation between layers).
There are a couple of different (perfectly acceptable ways) to implement the IRepository. One is to have the repository encapsulate the context directly. Another is to use the "unit of work" pattern and have your unitOfWork encapsulate the context and pass the unitOfWork object to each repository. Either way, since you're using EF4, testability is much easier than it used to be. For example, EF4 introduced IObjectSet so that it is easy to provide test doubles and mocks to test your repository.
I highly recommend checking out this whitepaper on Testability and Entity Framework 4.

Separation of concerns
What if you want to change the storage of 'Users', from say SQL to a flat file?
Then context would not be needed, and you'd have to change every use of it, instead of just your IRepository implementation.
Also, ideally you would have your IRepository injected. So you're MainViewModel doesn't care how it gets it's Users.

MVVM where to put Data Access Layer?

I am investigating WPF's MVVM design pattern. But am unsure where to put the Data Acess code?
In some examples I have looked at, data access is performed directly in the ViewModel. It seems odd to put something like linq to sql in the ViewModel? Other examples have a seperate project for Data Access, this seems more like it?
Is this there a general approach? I feel like I am missing something here!
Thanks

Here's how I've been organizing my MVVM w/ LINQ projects:
Model - I think of the Model as the state of the system. It provides an interface to the data, and it keeps track of system status. The Model does not know about the ViewModel or View--it just provides a public interface to its data and various events to let the consumers (usually ViewModels) know when the state has changed.
ViewModel - The ViewModel is in charge of organizing or structuring all the data needed by the View, keeping track of the status of the view (such as the currently selected row of a data grid), and responding to actions on the view (such as button pushes). It knows what the view needs, but it doesn't actually know about the view.
View - The View is the actual look and feel of the UI. It contains all the built-in and custom controls, how they arranged, and how they are styled. It knows about the ViewModel, but only for the purpose of binding to its properties.
Gateway - This is the part that directly addresses your question. The Gateway (which is basically my way of saying "DataAccessLayer") is its own separate layer. It contains all the code (including LINQ queries) to CRUD or select, insert, update, and delete data from/to your data source (database, XML file, etc.). It also provides a public interface to the Model, allowing the Model to focus on maintaining system state without having to concern itself with the details (i.e., the queries) needed to update the data source.
DataAccess Classes - In C#, these are very simple classes that model your elemental data objects. When you select something using a LINQ query, you will usually create an IEnumerable<T> or List<T> where T is one of your data objects. An example of a data object would be:
public class Person
{
public string Name { get; set; }
public int Age { get; set; }
}
The big advantage of a design like this is that it really separates your concerns. Everything has a specialized job, and it's (usually) pretty easy to know what kind of thing goes where.
The disadvantage is that it may be overkill for small projects. You end up creating a lot of infrastructure for public interfaces that basically pass a single wish through several layers. So, you might end up with a scenario like this: [user clicks Submit, ViewModel tells Model to AddNewPerson, Model tells Gateway to InsertPerson] instead of a scenario like this [user clicks Submit, ViewModel adds new record to the database directly].
Hope that helps.

I would add another layer, essentially what you want is a data factory. You want to create a set of classes that will CRUD to the database for you and return clean POCO objects to the ViewModel.
A good example to look at would the Nerd Dinner book. It covers MVC not MVVM but the patterns are very similar and the way they access data in that solution would be good starting point.
Hope this helps.

Data access should not be in the view model, as this is supposed to be a view specific (possibly simplified) representation of the domain model.
Use a mapper of some sort to map your view model (the VM in MVVM) to your model (the first M). New objects in your model can be created using the factory pattern. Once created, you can store them in a database using the repository pattern. The repositories would then represent your data access layer. In your repository you could use an O/R mapper like NHibernate or Entity Framework.
EDIT:
I see that GraemeF suggests putting the data access code in the model. This is a NOT a good approach, as this would force you to update your domain model if you were to move from e.g. SQL Server to Oracle or XML files. The domain objects should not have to worry about how they are persisted. The repository pattern isolates the domain from its persistence.

MVVM stands for Model, View, and ViewModel. The piece you are missing is the Model, which is where your data access code lives.
The ViewModel takes the Model and presents it to the View for display, so typically you would have something like this:
class PersonModel : IPerson
{
// data access stuff goes in here
public string Name { get; set; }
}
class PersonViewModel
{
IPerson _person;
public PersonViewModel(IPerson person)
{
_person = person;
}
public Name
{
get { return _person.Name; }
set { _person.Name = value; }
}
}
The PersonView would then bind to the properties of the PersonViewModel rather than directly to the model itself. In many cases you might already have a data access layer that knows nothing about MVVM (and nor should it) but you can still build ViewModels to present it to the view.

Your ViewModel should be a thin layer that just services the view. My rule of thumb: if it has to do with the presentation of the UI, then it belongs in the ViewModel, otherwise it should be in the Model.

The WPF Application Framework (WAF) contains a sample application that shows how the Model-View-ViewModel (MVVM) pattern might be used in combination with the Entity Framework.