Generic Interface Inheritance and Class Implementation for Repository Pattern

Question

I have read a bit about constraints and am trying to implement it in my repository pattern.

I want something like the below but can't quite get it to compile.

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

 //This needs to inherit from IRepository
 //T has to be a model class
 //V has to be a class that implements IEmployeeRepo
 public interface IEmployeeRepo<T, V> where V : EmployeeRepo where T : class : IRepository<T>
 {
    void DoSomethingEmployeeRelated();
 }

 //Dont think this inheritance is correct
 public class EmployeeRepo<Employee, this> : IEmployeeRepo
 {


 }

 //My example model class
 public class Employee
 {
     public string Name {get;set;}
 }

There are far too many errors in your code (I can spot 3 at least) to understand what you want. — leppie
– leppie, Commented Jan 24, 2011 at 11:34
Read the notes above IEmployeeRepo and I also mentioned it will not compile — Jon
– Jon, Commented Jan 24, 2011 at 11:35

RPM1984 · Accepted Answer · 2011-01-24 11:49:24Z

Not sure why you have two type parameters on the Repository - what is the point?

*Here is the classic example of a .NET Repository using Generics: *

*First, the Repository Interface: *

public interface IRepository<T> where T : class
{
   T FindSingle(Expression<Func<T,bool>> predicate);
   IQueryable<T> FindAll(); // optional - matter of preference
   void Add(T entity);
   void Remove(T entity);
}

*Second, the Generic Repository Implementation (EF as the example): *

public abstract class GenericRepository<T> : IRepository<T>
{
   private IObjectSet<T> _ObjectSet; // get this in via DI (for example)

   public T FindSingle(Expression<T,bool>> predicate)
   {
      return _ObjectSet.SingleOrDefault(predicate);
   }

   // you can figure out how to do the other implementation methods
}

*Then, the Specific Repository (you should have one per aggregate root, and also an interface for each specific repository detailing specific methods): *

public EmployeeRepository : GenericRepository<Employee>, IRepository<Employee>
{
   // all regular methods (Find, Add, Remove) inherited - make use of them
   public Employee FindEmployeeByName(string name)
   {
      return FindAll().SingleOrDefault(x => x.Name == name);
      // or you could do: return FindSingle(x => x.Name == name);    
   }
}

Usage:

IRepository<Employee> repository = new EmployeeRepository<Employee>();

Don't go out looking to go too crazy with generics - the only one you need is to constrain the Repository to be used by a entity that is encapsulated behind the Repository.

I simply use where T : class.

Other's use where T : IDomainAggregate or similar, to put constraints on the actual type of entity which is allowed.

Can I ask did you get this documentation from somewhere or did you write it?

TimC · Accepted Answer · 2011-01-24 11:43:34Z

In this situation i usually have a base repo class that implements IRepository<>, and is typed to a base Model class.

public interface IRepository<T> where T : IModel
 {
    void GetAll<T>();
    void GetById<T>(int id);
 }    

 public interface IEmployeeRepo<T> : IRepository<T> where T : IModel
 {
    void DoSomethingEmployeeRelated();
 }

 public class BaseRepo : IRepository<T> where T : IModel
 {

    public void GetAll<T>()
    {

    }

    public void GetById<T>(int id)
    {

    }
 }


 public class EmployeeRepo : BaseRepo<Employee>,  IEmployeeRepo<Employee>
 {
    public void DoSomethingEmployeeRelated()
    {

    }

 }

 //My example model class
 public class Employee : IModel
 {
     public int Id {get;set;}
     public string Name {get;set;}
 }

Rob · Accepted Answer · 2011-01-24 11:36:57Z

1

Try this;

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

 //This needs to inherit from IRepository
 //T has to be a model class
 //V has to be a class that implements IEmployeeRepo
 public interface IEmployeeRepo<T, V> : IRepository<T> where V : EmployeeRepo where T : class
 {
    void DoSomethingEmployeeRelated();
 }

 //Dont think this inheritance is correct
 public class EmployeeRepo : IEmployeeRepo<Employee, EmployeeRepo>
 {
    public void DoSomethingEmployeeRelated()
    {

    }

    public void GetAllData<Employee>()
    {

    }
 }

 //My example model class
 public class Employee
 {
     public string Name {get;set;}
 }

answered Jan 24, 2011 at 11:36

Rob♦

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