Implementando SOLID - OCP

ProductFilter.cs

using System.Collections.Generic;
using System.Linq;

namespace OCP.Filter.Model
{
  class ProductFilter
  {
    public IEnumerable<Product> ByCriteria(IList<Product> products, ICriteria criteria)
    {
      foreach (var product in products)
      {
        if (criteria.Match(product)) yield return product;
      }
    }
  }

  interface ICriteria
  {
    bool Match(Product product);
  }

  class MultiCriteria : ICriteria
  {
    private readonly IEnumerable<ICriteria> _criterias;

    public MultiCriteria(IEnumerable<ICriteria> criterias)
    {
      _criterias = criterias;
    }

    public bool Match(Product product)
    {
      return _criterias.All(c=>c.Match(product));
    }
  }

  class SizeCriteria : ICriteria
  {
    private readonly ProductSize _productSize;

    public SizeCriteria(ProductSize productSize)
    {
      _productSize = productSize;
    }

    public bool Match(Product product)
    {
      return product.Size == _productSize;
    }
  }

  class ColorCriteria : ICriteria
  {
    private readonly ProductColor _productColor;

    public ColorCriteria(ProductColor productColor)
    {
      _productColor = productColor;
    }

    public bool Match(Product product)
    {
      return product.Color == _productColor;
    }
  }
}