New Dictionary: HashList – or How to build an ordered Hashtable

Sometimes the world is quite awful. You need a fast key access to a collection because of the performance of your application, and you need the garantuee that you can read the items in the same sequence like you put it in the list.

This scenario happend to me a few times during my career as a developer. In past I hold the elements twice in a hashtable and in a collection in order to have both – fast key access and the correct ordering. The problem was that this method causes a lot of overhead in managing the consistency of your data.

DotNet 2.0 offers a new collection called “KeyedCollection” which is one big step into the right direction. The problem is that the KeyedCollection ist abstract and therefore must be inherited, furthermore it does not implement the IDictionary interface.

That’s why I wrote a wrapper that is based on the new KeyedCollection and does implement the IDictionary interface. I called it HashList.

Look at this piece of code:

IDictionary contacts = new Dictionary();

contacts.Add("a", contact_a);
contacts.Add("b", contact_b);
contacts.Add("c", contact_c);
Dump(contacts);

contacts.Remove("a");
contacts.Remove("b");
contacts.Remove("c");

contacts.Add("a", contact_a);
contacts.Add("b", contact_b);
contacts.Add("c", contact_c);
Dump(contacts);

a.) contact a
b.) contact b
c.) contact c

The second list is NOT in the right order anymore.
c.) contact c
b.) contact b
a.) contact a

Now the same piece of code using the new HashList (you can download it with the link at the end of the blog).

IDictionary contacts = new HashList();

contacts.Add("a", contact_a);
contacts.Add("b", contact_b);
contacts.Add("c", contact_c);
Dump(contacts);

contacts.Remove("a");
contacts.Remove("b");
contacts.Remove("c");

contacts.Add("a", contact_a);
contacts.Add("b", contact_b);
contacts.Add("c", contact_c);
Dump(contacts);

a.) contact a
b.) contact b
c.) contact c

Now everything is ordered, like we had put them in the list.
a.) contact a
b.) contact b
c.) contact c

That’s my solution for using the KeyedCollection Microsoft offers. Hope you enjoy the hybrid between a Dictionary and a Collection.

The example can be downloaded using the following link:
New Dictionary: HashList – or How to build an ordered Hashtable

PS: Before I get another comment like the first one . It’s not the intention of this example to sort the items by their Key. The intention is to keep the items in the same order like they have been put in the Hashtable.