Dictionary with item limit

Question

I need to provide access to a Key/Value pair store that persists for all users across session.

I could easily create a singleton for this, but for performance reasons I want to limit the size of the dictionary to 10000 items (or any performant number, as the object will persist indefinitely)

Is there a form of dictionary where I can specify a limit to the number of objects stored, and when that limit is exceeded, remove the oldest entry?

Answer 1

There is no such built-in dictionary, but you can build your own. You will need a queue for keys - that will allow you quickly find oldest entry and remove it. Also you will need a simple dictionary for keeping your values - that will allow you quickly search for them:

public class SuperDictionary<TKey, TValue>
{
    private Dictionary<TKey, TValue> dictionary;
    private Queue<TKey> keys;
    private int capacity;

    public SuperDictionary(int capacity)
    {
        this.keys = new Queue<TKey>(capacity);
        this.capacity = capacity;
        this.dictionary = new Dictionary<TKey, TValue>(capacity);
    }

    public void Add(TKey key, TValue value)
    {
        if (dictionary.Count == capacity)
        {
            var oldestKey = keys.Dequeue();
            dictionary.Remove(oldestKey);
        }

        dictionary.Add(key, value);
        keys.Enqueue(key);
    }

    public TValue this[TKey key]
    {
        get { return dictionary[key]; }
    }
}

NOTE: You can implement IDictionary<TKey,TValue> interface, to make this class a 'true' dictionary.