Make Hashtable immutable

Question

How do I make a derived class from Hashtable, objects of which can be added, but cannot be removed or replaced?

What do I have to override and particularly how do I override the [] operator?

Answer 1

At a minimum, you should override Clear, Remove, the property Values and the indexer Item. You can override the indexer Item using the syntax:

public override this[object key] {
    get { // get implementation }
    set { // set implementation }
}

You need to override Clear so that a user can't clear the hashtable. You need to override Remove so that a user can't remove entires from the hashtable. You need to override Values because a user could use the ICollection that is returned to modify the values in the hashtable. You need to override Item for a similar reason.

Answer 2

Instead of deriving from the Dictionary (which you should use rather than a HashTable), you should probably encapsulate it in this case.

The Dictionary has a lot of methods that allow changing the collection, it's easier to make it a private member, and then just implement the methods to add and access items.

Something like:

public class StickyDictionary<Key, Value> : IEnumerable<KeyValuePair<Key, Value>>{

   private Dictionary<Key, Value> _colleciton;

   public StickyDictionary() {
      _collection = new Dictionary<Key, Value>();
   }

   public void Add(Key key, Value value) {
      _collection.Add(key, value);
   }

   public Value this[Key key] {
      get {
         return _collection[key];
      }
   }

   public IEnumerable<KeyValuePair<Key, Value>> GetEnumerator() {
      return _collection.GetEnumerator();
   }

}