IDictionary that keeps each key added to it instead of overwriting

ExpandoDictionary.cs

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

namespace ExpandoDictionary
{
    public sealed class ExpandoDictionary<TKey, TValue> : IDictionary<TKey, TValue>
    {
        private readonly Dictionary<TKey, List<TValue>> content = new Dictionary<TKey, List<TValue>>();

        public int Count { get; }

        public bool IsReadOnly { get; }

        public ICollection<TKey> Keys
        {
            get { return content.Keys; }
        }

        public TValue this[TKey key]
        {
            get { throw new NotSupportedException("Suck it, bitch!"); }
            set { Add(key, value); }
        }

        public ICollection<TValue> Values
        {
            get { return content.Values.SelectMany(i => i).ToList(); }
        }

        public void Add(TKey key, TValue value)
        {
            if (!content.ContainsKey(key))
                content.Add(key, new List<TValue>());

            content[key].Add(value);
        }

        public void Add(KeyValuePair<TKey, TValue> item)
        {
            Add(item.Key, item.Value);
        }

        public void Clear()
        {
            content.Clear();
        }

        public bool Contains(KeyValuePair<TKey, TValue> item)
        {
            return content.ContainsKey(item.Key) && content[item.Key].Contains(item.Value);
        }

        public bool ContainsKey(TKey key)
        {
            return content.ContainsKey(key);
        }

        public void CopyTo(KeyValuePair<TKey, TValue>[] array, int arrayIndex)
        {
            throw new NotSupportedException("Meh");
        }

        public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator()
        {
            return content.Keys.SelectMany(key => content[key].Select(val => new KeyValuePair<TKey, TValue>(key, val))).GetEnumerator();
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }

        public bool Remove(TKey key)
        {
            return content.Remove(key);
        }

        public bool Remove(KeyValuePair<TKey, TValue> item)
        {
            return content[item.Key].Remove(item.Value);
        }

        public bool TryGetValue(TKey key, out TValue value)
        {
            throw new NotSupportedException("Meh");
        }
    }
}