using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var lookup = new Lookup<int, string>();

        lookup.Add(1,"hello");

        lookup.Add(1,"there");

        lookup.Add(2,"how");

        lookup.Add(2,"are");

        lookup.Add(3,"you");

        lookup.Add(3,"doing");

        foreach(var group in lookup)

        {

            Console.Write(group.Key);

            Console.Write(". ");

            group.Value.ForEach(g => Console.Write(g + " "));

            Console.Write("\n");

        }

    }

}

​

public class Grouping<TKey, TElement> : List<TElement>, IGrouping<TKey, TElement>

{

    public Grouping(TKey key, TElement element){

        Key = key;

        this.Add(element);

    }

    public TKey Key

    {

        get;

        set;

    }

}

​

public class Lookup<TKey, TElement> : Dictionary<TKey,Grouping<TKey, TElement>>

{

    public void Add(TKey key, TElement element)

    {

        if(this.ContainsKey(key))

        {

            this[key].Add(element);

        }

        else

        {

            this.Add(key, new Grouping<TKey, TElement>(key,element));

        }

    }

}