c＃存储库模式以及lambdas在哪里，如何转换类型

Question

I'm trying to build an Entity framework abstract repository with data transfer objects and separate data providers. We need to be able to switch between oracle, sql server and azure sql depending on the install.

The end repository returns DTO's to the consuming code. Get, Update and delete are working fine. The issue I'm having is with the lambdas for where clauses etc. The generic repository is unaware of the actual Entity object from the data provider so I can't create a where lambda.

PS Using AutoMapper to convert between Entity and DTO

//Repository base
public class Repository<TEntity> : IDisposable, IRepository<TEntity> where TEntity : class, IEntity
{
    public virtual IList<TEntity> GetList(Func<TEntity, bool> where, params Expression<Func<TEntity, object>>[] navigationProperties)
    {
        List<TEntity> list;

        IQueryable<TEntity> dbQuery = Context.Set<TEntity>();

        //Apply eager loading
        foreach (Expression<Func<TEntity, object>> navigationProperty in navigationProperties)
            dbQuery = dbQuery.Include<TEntity, object>(navigationProperty);

        list = dbQuery
            .AsNoTracking()
            .Where(where)
            .ToList<TEntity>();

        return list;
    }
}

//Implementing repository where T is the entity from the EF model
public class UserRepository<T> : IUserRepository, IUnitOfWork
    where T : class, IEntity
{
    public Repository<T> Base { get; private set; }

    public UserRepository(DbContext context)
    {
        Base = new Repository<T>(context);
    }

    public List<UserAccountDTO> GetList(Expression<Func<UserAccountDTO, bool>> where)
        {
            T obj = SupportedRepos.DTOMapper.Map<T>(where.Parameters[0]);
                /* HOW CAN I CONVERT THE FUNC<>? */
                //Base.GetList();
                return null;
        }
    }

public void TestMethod1()
{
    var dbtypeFromConfig = RepoEF.Setup.StorageType.SQLServer;

    using (var repo = RepoEF.Setup.SupportedRepos.Create(dbtypeFromConfig))
    {
        //WORKS FINE, CHANGES AND UPDATES
        var source = repo.Get(3);
        source.LookupId = 111111;
        repo.Add(source);

        //CAN'T CREATE WHERE BECAUSE UserRepository<T> ONLY SEES IEntity
        repo.GetList(x => x.UserAccountId == 3);
    }
}

Is it possible to build a Func<> to pass to the base Repository.

If not any ideas how I change the design to achieve what I need?

Answer 1

Perhaps I could dig into your current implementation of repository pattern and try to fix it, but I believe that, after all, there're many design flaws that will force you to completely refactor your solution.

So I'll give you some hints:

1) A repository is still domain , thus it always work with domain objects

Domain shouldn't work with DTOs. You use DTO pattern when you want to transfer data between logical or physical boundaries. For example, when a domain service need to provide a list of some domain objects to an application service. That list will be exposed as DTOs of the domain object list.

2) Database technology switch is a responsibility of data mapping layer (ie Entity Framework).

Your repositories should remain agnostic to the database technology and coupled with the OR/M framework.

3) If an user repository IS a repository , you should use inheritance instead of composition

Thus, UserRepository should both implement IUserRepository and derive from Repository<User> .

4) A concrete repository shouldn't accept a generic type parameter to provide the domain object to handle

At the end of the day, a concrete repository like IUserRepository handles instances of User , thus, why you want a TEntity generic parameter? Since you've this design flaw, TEntity isn't T ... and this is the reason behind the problem you want to solve...