Example.cs

public interface IRepository<T> where T : class
{
    T Find(int id);
    List<T> All();
    void Delete(int id);
    void Update(T obj);
}

public class ProductRepository : IRepository<Product>
{
    public List<Product> All()
    {
        throw new NotImplementedException();
    }

    public void Delete(int id)
    {
        throw new NotImplementedException();
    }

    public Product Find(int id)
    {
        throw new NotImplementedException();
    }

    public void Update(Product obj)
    {
        throw new NotImplementedException();
    }
}

public class OrderRepository : IRepository<Order>
{
    public List<Order> All()
    {
        string sql = "SELECT * FROM Order";

        // blah blah
    }

    public void Delete(int id)
    {
        throw new NotImplementedException();
    }

    public Order Find(int id)
    {
        throw new NotImplementedException();
    }

    public void Update(Order obj)
    {
        throw new NotImplementedException();
    }
}