using System;

using System.Linq;

using System.Collections;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var products = new List<Product>

        {

            new Product(1, "Fruit", "Apple"),

            new Product(2, "Fruit", "Banana"),

            new Product(3, "Fruit", "Orange"),

            new Product(4, "Vegetables", "Carrot"),

            new Product(4, "Vegetables", "Pea"),

            new Product(3, "Soft Drink", "Orange Juice"),

            new Product(2, "Soft Drink", "Lemonade"),

            new Product(1, "Alcoholic", "Bitter"),

            new Product(1, "Alcoholic", "Lager"),

            new Product(2, "Alcoholic", "Vodka")

        };

        IEnumerable<GroupResult> groups = products.GroupByMany(x => (x.CategoryId > 2) ? 0 : 1, y => (y.CategoryId < 4) ? 1 : 0);

        var primeGroup = groups.GetPrimeGroup();

        Console.WriteLine("Result Group: " + primeGroup.Key);

        foreach (var item in primeGroup.Items)

        {

            Console.WriteLine(item);

        }

    }

}

​

public class Product

{

    public string Name { get; set; }

    public int CategoryId { get; set; }

    public string Subcategory { get; set; }

    public Product(int categoryId, string subcategory, string name)

    {

        CategoryId = categoryId;

        Subcategory = subcategory;

        Name = name;

    }

    public override string ToString()

    {

        return String.Concat(CategoryId, " => ", Name, " -> ", Subcategory);

    }

}

​

public static class IEnumerableExtensions

{

    public static IEnumerable<GroupResult> GroupByMany<TElement>(

        this IEnumerable<TElement> elements,

        params Func<TElement, int>[] groupSelectors)

    {

​

        if (groupSelectors.Length > 0)

        {

            var selector = groupSelectors.First();

​

            //reduce the list recursively until zero

            var nextSelectors = groupSelectors.Skip(1).ToArray();

            return 

                elements.GroupBy(selector).Select(

                g => new GroupResult

                {

                    Key = g.Key,

                    Count = g.Count(),

                    Items = g,

                    SubGroups = g.GroupByMany(nextSelectors)

                });

        }

​

        return null;

    }

    public static GroupResult GetPrimeGroup(this IEnumerable<GroupResult> groups)

    {

        foreach (var group in groups.OrderBy(x => x.Key))

        {

            // Do we even have any elements?

            if (group.Count > 0)

            {

                if (group.SubGroups != null && group.SubGroups.Count() > 0)

                {

                    return group.SubGroups.GetPrimeGroup();

                }

                else

                {

                    return group;

                }

            }

        }

​

        return null;

    }

}

​

public class GroupResult

{

    public int Key

    {

        get;

        set;

    }

​

    public int Count

    {

        get;

        set;

    }

​

    public IEnumerable Items

    {

        get;

        set;

    }

​

    public IEnumerable<GroupResult> SubGroups

    {

        get;

        set;

    }

​

    public override string ToString()

    {

        return string.Format("{0} ({1})", Key, Count);

    }

}