Open closed principle for different filter conditions

Question

The class ProductService, below, gets products based on different filters like by date, country, etc., from a database. ProductsService doesn't follow OCP, as adding new filter like getting products by Price requires changing the ProductsService code. How it can be fixed? Any suggestions/comments would be really helpful.

public class ProductsService : IProductsService
{
    public FilteredProducts GetProductsByDate(DateTime startDate, DateTime EndDate) 
    {   
        //.....

    }
    public FilteredProducts GetProductsByCountry(string country)
    {
        //.....

    }

    public FilteredProducts GetProductsByCity(string city) 
    {
        //.....

    }

}

public class FilteredProducts
{
    public IEnumerable<Product> Products{set;get;}
    public int uniqueProducts { set; get; }
}

public class Product
{
    public int ID{set;get;}
    public string Name{set;get;}
    public decimal Cost{set;get;}
}

Answer 1

Best way is to represent each operation as separate class.

    public interface IProductFilter
    {
        FilteredProducts GetProducts(); 
    }

    public class GetProductsByDate : IProductFilter
    {
        private DateTime _startDate;
        private DateTime _endDate;

        public GetProductsByDate(DateTime startDate, DateTime EndDate)
        {
            _startDate = startDate;
            _endDate = EndDate;
        }

        public FilteredProducts GetProducts()
        {
            // filter
        }
    }

You can then pass this implementation into your service, where it gets executed.

    public class ProductsService : IProductsService
    {
        public FilteredProducts FilterProducts(IProductFilter filter)
        {
            // execute the filter
            // return the products
        }
    }

You could even go as far as turning it into generic Command (for example here) and executing all your database logic through it, ditching the "service" anti-pattern.