using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var myDict = new SizedDictionary<string, string>(5);

        myDict.Add("1","One");

        myDict.Add("2","Two");

        myDict.Add("3","Three");

        myDict.Add("4","Four");

        myDict.Add("5","Five");

        //myDict.Remove("5");

        myDict.Add("6","Six");

        myDict.Add("7","Seven");

        foreach (string s in myDict.Values) {

            Console.WriteLine(s);

        }

    }

}

​

// Sealed class to prevent inheritence becasue, we're hiding the add and delete methods, so unpredictable which will be called if inhenrited

public sealed class SizedDictionary<TKey, TValue> : Dictionary<TKey, TValue> {

    private int maxSize;

    private Queue<TKey> keys;

    public SizedDictionary(int size) {

        maxSize = size;

        keys = new Queue<TKey>();

    }

    new public void Add (TKey key, TValue value) {

        if (key==null) throw new ArgumentNullException();

        base.Add(key, value);

        keys.Enqueue(key);

        if (keys.Count > maxSize) base.Remove(keys.Dequeue());

    }

    new public bool Remove (TKey key) {

        if (key==null) throw new ArgumentNullException();

        if (!keys.Contains(key)) return false;

        var newQueue = new Queue<TKey>();

        while (keys.Count>0) {

            var thisKey = keys.Dequeue();

            if (!thisKey.Equals(key)) newQueue.Enqueue(thisKey);

        }

        keys=newQueue;

        return base.Remove(key);

    }

}