Вадим Мартынов

RepositoryProvider provider = new RepositoryProvider();
dynamic repo = provider.GetRepository<Entity<int>>();
List<Entity<int>> list = repo.GetAll();
Entity<int> entry = repo.Get(10);

interface Entity { }

class Entity<TId> : Entity
    where TId : struct
{
    public virtual TId Id { get; set; }
}

interface IRepository<T, in TId>
    where T : Entity<TId>
    where TId : struct
{
    T Get(TId id);
    IList<T> GetAll();
}

class Repo<T, TId> : IRepository<T, TId>
    where T : Entity<TId>
    where TId : struct
{
    public T Get(TId id)
    {
        return default(T);
    }

    public IList<T> GetAll()
    {
        return new List<T>();
    }
}

class RepositoryProvider
{
    public dynamic GetRepository<TEntity>()
        where TEntity : Entity
    {
        Type genericParam = typeof(TEntity).GetGenericArguments()[0];
        return Activator.CreateInstance(typeof(Repo<,>).MakeGenericType(typeof(TEntity), genericParam));
    }
}