using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var dictList = new Dictionary<string, List<int>>();

​

        dictList.Add("example", 5);

        dictList.Add("example", 10);

        dictList.Add("example", 15);

        Console.WriteLine(String.Join(", ", dictList["example"])); // 5, 10, 15

        dictList.Remove("example", 5);

        dictList.Remove("example", 10);

        Console.WriteLine(String.Join(", ", dictList["example"])); // 15

        dictList.Remove("example", 15);

        Console.WriteLine(dictList.ContainsKey("example")); // False

    }

}

​

public static class DictListExtensions

{

    public static void Add<TKey, TValue, TCollection>(this Dictionary<TKey, TCollection> dict, TKey key, TValue value)

            where TCollection : ICollection<TValue>, new()

    {

        TCollection list;

        if (!dict.TryGetValue(key, out list))

        {

            list = new TCollection();

            dict.Add(key, list);

        }

​

        list.Add(value);

    }

​

    public static bool Remove<TKey, TValue, TCollection>(this Dictionary<TKey, TCollection> dict, TKey key, TValue value)

        where TCollection : ICollection<TValue>

    {

        TCollection list;

        if (!dict.TryGetValue(key, out list))

        {

            return false;

        }

​

        var ret = list.Remove(value);

        if (list.Count == 0)

        {

            dict.Remove(key);

        }

        return ret;

    }

}