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

class Product {
   public int ID;
   public int CategoryID;
}
        
public class Test
{
	public static void Main()
	{
	    var products = new List<Product>();
            products.Add(new Product() { ID = 1, CategoryID = 1 });
            products.Add(new Product() { ID = 2, CategoryID = 1 });
            products.Add(new Product() { ID = 3, CategoryID = 7 });
            products.Add(new Product() { ID = 4, CategoryID = 8 });
            products.Add(new Product() { ID = 5, CategoryID = 9 });
            products.Add(new Product() { ID = 6, CategoryID = 10 });

            var catIDs = new[] { 1, 8, 9 };
            var ordered = products
                .OrderByDescending(p => catIDs.Contains(p.CategoryID))
                .ThenBy(p => p.CategoryID);
                
            Console.Write(
                string.Join(Environment.NewLine, 
                ordered.Select(p => 
                    String.Format("ID={0} CatID={1}",p.ID,p.CategoryID)).ToArray()));
	}
}