using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        List<Item> items = new List<Item>();

        items.Add(new Item() { ItemId = 1, ItemName = "Test1", ListId = 1 });

        items.Add(new Item() { ItemId = 2, ItemName = "Test2", ListId = 1 });

        items.Add(new Item() { ItemId = 3, ItemName = "Test3", ListId = 1 });

        items.Add(new Item() { ItemId = 4, ItemName = "List", ListId = 2 });

        items.Add(new Item() { ItemId = 5, ItemName = "List2", ListId = 2 });

        items.Add(new Item() { ItemId = 6, ItemName = "Testing", ListId = 3 });

        items.Add(new Item() { ItemId = 7, ItemName = "Testing2", ListId = 3 });

        items.Add(new Item() { ItemId = 8, ItemName = "Testing3", ListId = 3 });

​

        var groupByResult = items.GroupBy(i => i.ListId);

        List<GroupedItems> betterGroups = new List<GroupedItems>();

        foreach (var groupedItem in groupByResult)

        {

            betterGroups.Add(new GroupedItems() { ListId = groupedItem.Key, Items = groupedItem.ToList() });

        }

    }

    public class Item

    {

        public int ItemId { get; set; }

        public string ItemName { get; set; }

        public int ListId { get; set; }

    }

    public class GroupedItems

    {

        public int ListId {get; set;}

        public List<Item> Items {get; set;}

    }

}