具有通用接口的多级通用存储库

Question

I have problems with implementing of repository logic. In a real project I use Ninject, so I use interfaces for some classes. For now I made a very simple demo of the problem:

// ------- Base User -------------- //

public interface IBaseUser
{
    int Id { get; set; }
}
public class BaseUser : IBaseUser
{
    public int Id { get; set; }
}

// -------- Individual User --------//

public interface IIndividualUser : IBaseUser
{
    string Gender { get; set; }
}
public class IndividualUser: BaseUser, IIndividualUser
{
    public string Gender { get; set; }
}

// -------- Legal User --------//

public interface ILegalUser : IBaseUser
{
    string VatNumber { get; set; }
}
public class LegalUser : BaseUser, ILegalUser
{
    public string VatNumber { get; set; }
}

// -------- Base User Repo --------//

public interface IBaseUserRepo<T> where T: BaseUser
{
    T GetById(int id);
}
public class BaseUserRepo<T> where T : BaseUser, IBaseUserRepo<T>
{
    public T GetById(int id)
    {
        // logic...
    }
}

// -------- Individual User Repo --------//

public interface IIndividualUserRepo : IBaseUserRepo<IndividualUser>
{
    IndividualUser GetByGender(string gender);
}
public class IndividualUserRepo : BaseUserRepo<IndividualUser>, IIndividualUserRepo
{
    public IndividualUser GetByGender(string gender)
    {
        // logic...
    }
}

// -------- Legal User Repo --------//

public interface ILegalUserRepo : IBaseUserRepo<LegalUser>
{
    LegalUser GetByVatNumber(string vatNumber);
}
public class LegalUserRepo : BaseUserRepo<LegalUser>, ILegalUserRepo
{
    public LegalUser GetByVatNumber(string vatNumber)
    {
        // logic...
    }
}

In this 2 lines of code:

public class IndividualUserRepo : BaseUserRepo<IndividualUser>, IIndividualUserRepo

public class LegalUserRepo : BaseUserRepo<LegalUser>, ILegalUserRepo

the compiler fires this errors:

The type 'genericInterfaceTest.IndividualUser' cannot be used as type parameter 'T' in the generic type or method 'genericInterfaceTest.BaseUserRepo'. There is no implicit reference conversion from 'genericInterfaceTest.IndividualUser' to 'genericInterfaceTest.IBaseUserRepo'. C:\\Work\\Tests\\genericInterfaceTest\\genericInterfaceTest\\Program.cs 77 18 genericInterfaceTest

and

The type 'genericInterfaceTest.LegalUser' cannot be used as type parameter 'T' in the generic type or method 'genericInterfaceTest.BaseUserRepo'. There is no implicit reference conversion from 'genericInterfaceTest.LegalUser' to 'genericInterfaceTest.IBaseUserRepo'. C:\\Work\\Tests\\genericInterfaceTest\\genericInterfaceTest\\Program.cs 91 18 genericInterfaceTest

That happens only then I'm implementing the interface IBaseUserRepo in this line of code:

public class BaseUserRepo<T> where T : BaseUser, IBaseUserRepo<T>

Can somebody explain me please where I'm wrong.

Answer 1

I guess you intented to implement IBaseUserRepo<T> interface in BaseUserRepo<T> class, but what you did instead you constrained T should be of basetype BaseUser and also T should implement IBaseUserRepo<T>

Change this

public class BaseUserRepo<T> where T : BaseUser, IBaseUserRepo<T>

To this

public class BaseUserRepo<T> : IBaseUserRepo<T> where T : BaseUser

Now you've implemented IBaseUserRepo<T> interface in BaseUserRepo<T> class and constrained T generic parameter to BaseUser .

Ordering matters :)