Develop Reference

Use reflection to forcefully cast object into interface it sort of implements

I have the following setup:
public interface IRepository<T>
{
void Save(T entity);
}
public interface IEmailRepository : IRepository<Email> {} // completely empty!
public class MockRepository<T> : IRepository<T> {}
What I wish to do is the following:
void SomeBusinessMethod(IEmailRepository repo) { ... }
MockRepository<Email> mockRepository = CreateMockRepository<Email>();
IEmailRepository mockedEmailRepository = (IEmailRepository) mockRepository; // Invalid cast!
BusinessMethod(mockedEmailRepository);
I am aware that this isn't possible because MockRepository<T> does not inherit from IEmailRepository, even through the class and interface have the exact same signatures.
This is done in a production project and my goal is to inject a mock repository into business classes. I would prefer the possibility of using reflection to forcefully do this conversion anyways to not update the business logic code.
I found that you can use TypeBuilder and ModuleBuilder to create new types and methods with IL, but I just want to assign an interface to an instance that already sort of implements it. Is there a way to achieve this?
There are several other questions slightly related to this problem, but none have been answered with a solution that allows this setup.

Repository pattern connection string

I am building a generic Repository for a WinForms kinda small app like this:
public interface IRepository<T> where T : class
{
IEnumerable<T> GetAll();
IEnumerable<T> Find(Expression<Func<T, bool>> query);
T GetByID(int id);
void Add(T item);
void Update(T item);
void Delete(T item);
}
I know that many recommend against using the repository pattern, but in this case saves a lot of code, since all the POCO's will be very similar and use all the methods declared in the interface.
This will be implemented using ServiceStack.OrmLite.
I don't want to use ConfigurationManager to get the connection string. What would be a good way to achieve this? I thought about using a BaseRepository class to initialize a static connection string, but I am not sure how to implement this.
Any suggestions?

A BaseRepository class woudl be an abstract class that implements the methods in IRepostiory<T> abstractly:
public class BaseRepository<T> : IRespository<T> where T : class
{
protected readonly string ConnectionString = "your connection string here"; /*could event be static?*/
public abstract IEnumerable<T> GetAll();
/*....*/
}
I've not used OrmLite, but it looks like you could probably implement most of the methods in the BaseRepository class. Any decent DI container could then use this, rather than having to have a separate repository class for each entity.
However, as the Repository pattern is typically used with the Unit of Work pattern, you might want to consider how this all communicates.
One final note: I'd warn against GetAll as IEnumerable - apart from maybe lookup tables will only ever have a small number of rows. Once you start to have any volume of data, you'll need to retrieve it in chunks - loading hundreds of thousands (or more?) records in one go will be pretty slow.

Generic Repository Pattern have repetition of code so what will be benefit?

I am understanding repository pattern in c#. I am confused when i study about generic repository pattern. There is lot of repetition in it. i have some question about this pattern.
I am using Entity Framework code first approach and i have two model classes
Student
Teachers
How many generic interface i will use for example if i have one
generic interface
public interface IRepository<TEntity>
{
IQueryable<TEntity> FindAll(Expression<Func<TEntity, bool>> where = null);
TEntity FindOne(Expression<Func<TEntity, bool>> where = null);
}
So this interface can be use in both model classes.
if Student class have more methods where i can define these methods? for example
public class StudentRepo<TEntity> : IRepository<TEntity> where TEntity : class
{
public virtual IQueryable<TEntity> FindAll(Expression<Func<TEntity, bool>> where = null)
{
return null != where ? Context.Set<TEntity>().Where(where) : Context.Set<TEntity>();
}
public virtual TEntity FindOne(Expression<Func<TEntity, bool>> where = null)
{
return FindAll(where).FirstOrDefault();
}
public void update()
{
}
public int FindId()
{
}
}
So i have added two new methods update() and FindId() in StudentRepo where can I define these methods?
If I want to add these two methods in IRepository then I have to call these methods for Teacher class. what will be benefit of it?
Is this better approach if i create separate interface for both classes? like IStudent and ITeacher so i can define those methods which i want to use and unnecessary methods will not be in use.
Kindly guide me i am so confused.

You can have one implementation for the IRepository that you have, something like:
public class GenericRepository<TEntity> : IRepository<TEntity> where TEntity : class
{
public virtual IEnumerable<TEntity> FindAll(Expression<Func<TEntity, bool>> where = null)
{
// implementation ...
}
public virtual TEntity FindOne(Expression<Func<TEntity, bool>> where = null)
{
// implementation
}
public void Update(TEntity entity)
{
// update your entity ...
}
// etc...
}
Then have your own repository inheriting from it:
public class StudentRepository : GenericRepository<Student>
{
// here you get all the goodies + you can add your own stuff
}
And:
public class TeacherRepository : GenericRepository<Teacher>
{
// here you get the same goodies, you don't need to re-implement them
}
This way you don't have to re-implement all the methods defined in the generic repository, you can however add your own more complex methods.

Generic repositories are worthless. They just do the same thing as entity framework and most of the implementations out there expose IQueryable<T>.
So why is that so bad?
The repository pattern is used to create an abstraction between the data source and your code. That abstraction is created to reduce complexity and to reduce coupling between those tiers.
A generic repository might seem to be a good choice at first, but as every entity (root aggregate) have their own unique features you will always have to write custom queries to fetch them.
To solve that, most generic implementations expose IQueryable<T>. That is a bad thing since there is no 100% complete Linq to Sql provider (a set of classes which translates the LINQ statement to a SQL statement). Every provider has to resort to custom commands to support eager/lazy loading, support for IN sql clause etc.
You always have to be aware of those customizations every time you use the repository through IQueryable<T>.
Hence you still have to know about how Entity Framework works. You could therefore use EF directly instead of using a generic repository.
If you truly want to use the repository pattern, please design your code first with all your classes. And THEN create the database. That is, fit the DB after your code and not vice versa. And make sure that your repository is 100% complete abstraction (google for instance persistance ignorance)

I think that the Generic Repository idea is trying to generalize too much, seperate interfaces are better in my opinion as they provide a more meaningful contract, this blog explains it very good, and offers to use the generic repository "behind the scenes".

Cast a concrete class instance to an interface that uses generics

Here is my concrete Repository that derives from an abstract base class
public class DepartmentRepository : RepositoryBase<Department>, IDepartmentRepository
public abstract class RepositoryBase<T> : IRepository<T> where T : class, IPersistantBusinessObject
Here is my interfaces
public interface IDepartmentRepository : IRepository<Department>
public interface IRepository<T> where T : IPersistantBusinessObject
public interface IPersistantBusinessObject { ... }
And here is my entities
public class Department : BusinessObjectBase
public abstract class BusinessObjectBase : IPersistantBusinessObject
How do I cast my DepartmentRepository to the generic IRepository?
DepartmentRepository rep = new DepartmentRepository();
(IRepository<IPersistentBusinessObject>)rep; // this doesn't work
Ideas? In case you're wondering, this is an asp.net MVC3 application using the repository pattern. Did I over architect the interfaces? How am I suppose to use a generic repository now?
Thanks.
EDIT:
Here is my IRepository interface. As asked about why I need to cast to IRepository, it is actually only so I can use the "SaveChanges" method, because I want to intercept any exceptions from the database and handled in a common way.
public interface IRepository<T> where T: IPersistantBusinessObject
{
IQueryable<T> Retrieve();
T Retrieve(int id);
IQueryable<T> RetrieveActiveAndInActive();
T RetrieveActiveAndInActive(int id);
void Add(T domainObject);
void SaveChanges();
void Delete(T domainObject);
void Delete(int id)
}

What are you trying to do here? I can't think of a situation in which you would want to cast to IRepository<IPersistentBusinessObject>. What code is dependent on what abstraction?
I would usually expect other code to depend on the abstraction of IDepartmentRepository, and can't really see the use case for IRepository<T> or even IPersistentBusinessObject at all. Again, if you can find a use case for code that depends on that abstraction, then they are valuable; otherwise, YAGNI.
(By the way, this is one reason why test-driven development is valuable. You can figure out which abstractions you need by writing unit tests, and seeing what classes need to be mocked to isolate testing of certain functionality, instead of just vaguely abstracting everything into interfaces because you were told they were better than concrete types. Start with concrete types, and when your tests force you to extract an interface, that's the right time!)
Really though, this is a question of covariance of generic types. It should work if you say
public interface IRepository<out T> where T : IPersistantBusinessObject

You need to cast it to IRepository<Department>.
DepartmentRepository derives from RepositoryBase<Department> which implements IRepository<Department>.
You might want to have a look at co and contravariance...

As Daniel implies, you can't cast it to IRepository, because you've defined the IRepository interface to only exist in conjunction with another type T. So "IRepository" as an interface does not exist.
If you want to be able to use an interface "IRepository" then it needs to be a standalone interface without the type dependence.
Perhaps you need two interfaces, one which implements the basic functionality you want in IRepository, and one which implements the generic methods for type T.
e.g.
<!-- language: lang-c# -->
public interface IRepository
{
SaveChanges()
...
}
public interface IRepositoryFor<T> where T : IPersistantBusinessObject
{
T GetFirst(); // or whatever
...
}

What is a IRepository and what is it used for?

What is a IRepository? Why is it used, brief and simple examples won't hurt.

MVC promotes separation of concerns, but that doesn't stop at the M V C level.
Data Access is a concern in itself. It should be done in the M bit of MVC, ie the model. How you structure your model is up to you, but people usually follow tried and tested patterns (why reinvent the wheel?). The Repository Pattern is the current standard. Don't expect a simple formula, however, because the variations are as many as there are developers, almost.
IRepository is just an interface that you create (it is not part of MVC or ASP.NET or .NET). It allows you to "decouple" your repositories from real implementations. Decoupling is good because it means your code...:
Your code is much more reusable. This is just plain good.
Your code can use Inversion of Control (or Dependency Injection). This is good to keep your concerns well separated. It is especially good because this allows Unit Testing...
Your code can be Unit Tested. This is especially good in large projects with complex algorithms. It is good everywhere because it increases your understanding of the technologies you are working with and the domains you are trying to model in software.
Your code becomes built around best practices, following a common pattern. This is good because it makes maintenance much easier.
So, having sold you decoupling, the answer to your question is that IRepository is an interface that you create and that you make your Repositories inherit from. It gives you a reliable class hierarchy to work with.
I generally use a generic IRepository:
IRepository
Where TEntity is, well, an entity. The code I use is:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Wingspan.Web.Mvc
{
public interface IRepository<TEntity> where TEntity : class
{
List<TEntity> FetchAll();
IQueryable<TEntity> Query {get;}
void Add(TEntity entity);
void Delete(TEntity entity);
void Save();
}
}
A concrete implementation of this interface would be:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Linq;
using Wingspan.Web.Mvc;
namespace ES.eLearning.Domain
{
public class SqlRepository<T> : IRepository<T> where T : class
{
DataContext db;
public SqlRepository(DataContext db)
{
this.db = db;
}
#region IRepository<T> Members
public IQueryable<T> Query
{
get { return db.GetTable<T>(); }
}
public List<T> FetchAll()
{
return Query.ToList();
}
public void Add(T entity)
{
db.GetTable<T>().InsertOnSubmit(entity);
}
public void Delete(T entity)
{
db.GetTable<T>().DeleteOnSubmit(entity);
}
public void Save()
{
db.SubmitChanges();
}
#endregion
}
}
This allows me to write:
SqlRepository<UserCourse> UserCoursesRepository = new SqlRepository<UserCourse>(db);
Where db is a DataContext instance injected into, say, a Service.
With UserCoursesRepository I can now write methods in my Service class like:
public void DeleteUserCourse(int courseId)
{
var uc = (UserCoursesRepository.Query.Where(x => x.IdUser == UserId && x.IdCourse == courseId)).Single();
UserCoursesRepository.Delete(uc);
UserCoursesRepository.Save();
}
And now in my controllers, I can just write:
MyService.DeleteUserCourse(5);
MyService.Save();
With this pattern the development of your app becomes more of an assembly line that leads up to a VERY simple controller. Every piece of the assembly line can be tested independently of everything else, so bugs are nipped in the bud.
If this is a long, unwieldy answer it is because the real answer is:
Buy Steven Sanderson's book Pro ASP.NET MVC 2 Framework and learn to think in MVC.

An IRepository is an interface you specify when you want to implement the Repository Pattern. As #Brian Ball stated, it's not part of .NET it is an interface that you create.
Developers using the Repository Pattern widely recommend the use of an interface for the implementation. For example, in the application I am developing right now, I have 5 repositories. 4 specific and 1 generic. Each one inherits from an IRepository which ensures I will not have issues down the road with differences in implementations.
As far as code examples, I'll try:
interface IRepository<T> where T : class {
IQueryable<T> Select();
}
Implemented as a generic repository:
public class Repository<T> : IRepository<T> where T : class {
public IQueryable<T> Select() {
return this.ObjectContext.CreateObjectSet<T>();
}
}
Implemented as a specialized repository:
public class EmployeeRepository : IRepository<Employee> {
public IQueryable<Employee> Select() {
return this.ObjectContext.Employees;
}
}
Both the Repository<T> and EmployeeRepository implement IRepository, however they go about performing the querying slightly differently. The generic repository has to create an object set of T before it can try to do anything.
Keep in mind that Repository<T> is supposed to be locked to the interface, where as EmployeeRepository can implement more specialized methods to accomplish more complex logic.
I hope this helps you a little.

IRepository is not a defined type in the .Net framework. Usually when you see an interface named that, the program uses the Repository Pattern ( https://web.archive.org/web/20110503184234/http://blogs.hibernatingrhinos.com/nhibernate/archive/2008/10/08/the-repository-pattern.aspx ). Generally when people use this pattern, they will create an interface that all the repositories adhere to. There are many benefits to doing this. Some of the benefits are code de-copupling, and unit testing.
It is also common for this to be done so it can be taken advantage of with IoC ( http://en.wikipedia.org/wiki/Inversion_of_control ).

A repository is an abstraction which represents any underlying and arbitrary data store as if it were an in memory collection of objects.
This definition is morphed into a more practical form due to common practices and system limitations as a collection of objects in memory which represent some underlying and arbitrary data store, possibly a disconnected one. Under the hood the repository may be linked to a database, a flat file, an in-memory collection of objects, or whatever else you may imagine. The user of a repository doesn't care.
So an IRepository is the interface contract which defines how Api code wishes client code to interact with the repository. This often includes add, update, delete, and get contracts as for example, this very common example of a repository contract:
public interface IRepository<TEntity> where TEntity : class
{
List<TEntity> GetAll();
void Add(TEntity entity);
void Delete(TEntity entity);
void Save();
}
But I prefer to use a different interface for a few reasons.
First, you typically wont be using a repository by itself, you will probably be using it with a unit of work pattern, so the repository shouldn't have a Save() method. It might have an Update(T entity) method - but why? The object which you receive from the repository will automatically be updatable/updated just like any other object you would receive from any kind of collection of objects because you have retrieved references to the objects themselves. (For example: if your TEntity is a Person object, and you get person "Chuck", and you change his last name from "Bartowski" to "Carmichael", the repository has presumably already updated said entity. If this seems tenuous in your mind, there is nothing wrong with implementing an Update(T entity) method.)
Second, most repositories should be able to handle disconnected environments. If your solution does not have this requirement, you can still create an interface that handles disconnected scenarios and simply leave it unimplemented. Now you are ready for the future.
At last, our contract makes more sense to the true nature of a repository - a collection of objects in memory which represent some arbitrary data store, possibly a disconnected one.
public interface IRepository<TEntity> where TEntity : class
{
List<TEntity> GetAll();
List<TEntity> Get(Func<TEntity, bool> where);
void Insert(TEntity entity);
void Insert(IEnumerable<TEntity> entities);
void Remove(TEntity entity);
void Remove(IEnumerable<TEntity> entities);
void SyncDisconnected(TEntity entity, bool forDeletion = false);
void SyncDisconnected(IEnumerable<TEntity> entities, bool forDeletion = false);
}
If you define a base class for all of your entities, let's call it DomainObject, and you give it an Id field, then you can do the following:
public interface IRepository<TEntity> where TEntity : DomainObject
{
TEntity GetById(object Id);
List<TEntity> GetAll();
List<TEntity> Get(Func<TEntity, bool> where);
void Insert(TEntity entity);
void Insert(IEnumerable<TEntity> entities);
void Remove(TEntity entity);
void Remove(IEnumerable<TEntity> entities);
void SyncDisconnected(TEntity entity, bool forDeletion = false);
void SyncDisconnected(IEnumerable<TEntity> entities, bool forDeletion = false);
}
If you don't like the optional parameter forDeletion, you can add a method which allow syncing deleted objects as well:
void SyncDisconnectedForDeletion(TEntity entity);
The reason you need to do this is because in most cases, syncing disconnected objects for deletion is incompatible with syncing disconnected objects for addition or modification. (Try it. You will see for yourself the requirements for deletion against a store vary wildly from that of addition or modification). Hence, the interface should define a contract so the implementation can discern between the two.
You can implement this interface against ANY repository of ANY underlying data store, connected or disconnected, including other abstractions to underlying data stores such as Entity Framework.