Using Method of Inherited Interface with Generic Repository Pattern, UnitOfWork in ASP.NET MVC 5

Question

My Problem is from here,

I have written an Interface for country repository and I am following generic repository with UnitOfWork, I am also using ninject for DI

public interface ICountryRepository : IRepository<Country>
{
    List<Country> GetAll();
}

Implemented country repository interface,

public class CountryRepository : BaseRepository<Country>, ICountryRepository
{
    public CountryRepository(DbContextcontext) : base(context)
    {
    }

    public List<Country> GetAll(){
    // Not implemented
    }
}

There is a additional method in ICountryRepository interface and I have implemented it too. But when I need that method to use using UnitOfWork, I can't use that method. That is giving System.NullReferenceException

I have tried,

ICountryRepository repository = UnitOfWork.Repository<Country>() as ICountryRepository;
return repository.GetAll();

The downcasting suggest the method but without casting the method is not accessible.

Additional Codes are given,

Entities

Base Entity

    public class BaseEntity
    {
        public int Id { get; set; }
    }

Product Entity

public class Country : BaseEntity
{
    public string Name { get; set; }
}

Repositories

Interface

public interface IRepository<T>
{
    void Add(T entity);
}

Base Repository

public class BaseRepository<TEntity> : IRepository<TEntity> where TEntity : BaseEntity
{
    protected IDbContext _context;
    private readonly IDbSet<TEntity> _dbEntitySet;
    private bool _disposed;

    public BaseRepository(IDbContext context)
    {
        _context = context;
        _dbEntitySet = _context.Set<TEntity>();
    }

    public void Add(TEntity entity)
    {
        _context.SetAsAdded(entity);
    }

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }

    public virtual void Dispose(bool disposing)
    {
        if (!_disposed && disposing)
        {
            _context.Dispose();
        }
        _disposed = true;
    }
}

Unit Of Work

Interface

public interface IUnitOfWork : IDisposable
{
    IRepository<TEntity> Repository<TEntity>() where TEntity : BaseEntity;

    void BeginTransaction();

    int Commit();

    Task<int> CommitAsync();

    void Rollback();

    void Dispose(bool disposing);

}

Unit of work implemented

public class UnitOfWork : IUnitOfWork
{
    private readonly IDbContext _context;
    private bool _disposed;
    private Hashtable _repositories;

    public UnitOfWork(IDbContext context)
    {
        _context = context;
    }

    public IRepository<TEntity> Repository<TEntity>() where TEntity : BaseEntity
    {
        if (_repositories == null)
        {
            _repositories = new Hashtable();
        }

        var type = typeof(TEntity).Name;

        if (_repositories.ContainsKey(type))
        {
            return (IRepository<TEntity>)_repositories[type];
        }

        var repositoryType = typeof(BaseRepository<>);

        _repositories.Add(type, Activator.CreateInstance(repositoryType.MakeGenericType(typeof(TEntity)), _context));

        return (IRepository<TEntity>)_repositories[type];
    }

   /* Other Implementation
    *
    *
    *
    */
}

It's probably in this line: `_context.Set()`. Your're passing `BaseProduct` which is not an entity and then EF cannot create a `DBSet`. `UnitOfWork.Repository`should do fine, but `UnitOfWork.Repository` will not. — smoksnes, Dec 05 '16 at 12:44
I have edited that, Unfortunately that is not the issue. I am still facing the issue @smoksnes — Abdur Rahim, Dec 05 '16 at 12:46
Nothing seems to implement `IBaseProductRepository` so your cast will always be null. Also, any particular reason for using `Activator`? There's nothing magic going on. You should be able to use `new BaseRepository(_context)`. — smoksnes, Dec 05 '16 at 12:54
@smoksnes, I've changed that mistake. And I am using Ninject for Dependency Injection that is doing the magic. But The problem is not still solved. — Abdur Rahim, Dec 05 '16 at 13:07
Possible duplicate of [What is a NullReferenceException, and how do I fix it?](http://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Ant P, Dec 05 '16 at 13:10
You are resolving an instance of type `BaseRepository`, not an instance of type `CountryRepository`. — Maarten, Dec 05 '16 at 13:11
Now you're casting to `ICountryRepository`, but you're creating a `BaseRepository`. So the cast will still result in null. — smoksnes, Dec 05 '16 at 13:31
`ICountryRepository` is NOT the same thing as `IRepository`. You cannot cast the latter to the former. — DavidG, Dec 05 '16 at 13:52

score 1 · Accepted Answer · answered Dec 05 '16 at 14:13

As said in the comment, you are resolving an instance of type BaseRepository<Country>, not your derived CountryRepository class, which is the one you want to resolve in case of a Country.

One crude hard coded solution is to replace your generic-repository-type-to-resolve with a custom-type-to-resolve. Something like this. You will need to manually add entries to the dictionary.

private static readonly Dictionary<Type, Type> s_RepositorySuperTypes = new Dictionary<Type, Type> {
    { typeof(BaseRepository<Country>), typeof(CountryRepository) }
};

public IRepository<TEntity> Repository<TEntity>() where TEntity : BaseEntity {
    if (_repositories == null) {
        _repositories = new Hashtable();
    }

    var type = typeof(TEntity).Name;
    if (_repositories.ContainsKey(type)) {
        return (IRepository<TEntity>)_repositories[type];
    }

    var closedRepositoryType = typeof(BaseRepository<>).MakeGenericType(typeof(TEntity));
    if (s_RepositorySuperTypes.ContainsKey(closedRepositoryType)) {
        closedRepositoryType = s_RepositorySuperTypes[closedRepositoryType];
    }
    _repositories.Add(type, Activator.CreateInstance(closedRepositoryType, _context));

    return (IRepository<TEntity>)_repositories[type];
}

score 0 · Answer 2 · answered Dec 12 '16 at 05:35

You should make your custom repository methods extension methods.

So if your extra method was called GetCountriesByContinent something like

public static class CountryRepository
{
    public static IEnumberable<Country> GetCountriesByContinent(this IRepository<Country> repo, string continent)
    {
        return repo.Where(c => c.Continent == continent);
    }
}

You should also look into covariance vs contravariance which may help you understand why you have this issue in the first place.

Using Method of Inherited Interface with Generic Repository Pattern, UnitOfWork in ASP.NET MVC 5

Entities

Repositories

Unit Of Work

2 Answers2