Chapter 4, lesson3: Dictionaries.

Dictionaries - opposite end of spectrum to sequental lists. Stores key/value pairs, to allow lookup of values based on key.

Hashtable

• Add() - specify key, specify value.
• Indexer - key in the indexer, and then assign value.
• retrieval - ~ indexer.
• DictionaryEntry - derived from ICollection - a container containing Key and Value. foreach( DictionaryEntry entry in ) .Value;
• Object.GetHashCode() - returns an int that uniquely identifies the object.
• Hashtable - allows only unique hashes of keys, not unique keys.
• Two strings with same text have same hash, even though they are diff. instances.
• Hashtable - tests for equality by testing the hash codes of keys. If equal, it will then call Equals mthd, to see whether 2 objs are indeed equal.
• Consider overriding - GetHashCode, Equals. Thus one can change a class to provide equality.
• ...and if changing the class to provide equality is difficult? e.g. if want to store strings as keys?? - provide equality OUTSIDE the class.
• equality OUTSIDE the class - hashtable can use a class that calculates equality.
• Hashtable - overloaded constructor accepts instance of IEqulaityComparer interface as arg.
• IEqualityComparer - impl. GetHashCode(), Equals(). Mthds allow the Comparer Class to handle equality re. keys, instead of relying on the key class to supply them.
• iterating over a hashtable - returns item sin order of their hsah value.
• If need sorting use SortedList...

SortedList

• SortedLIst - a dictionary class.
• can access stored items in order.
• supports key/value access by index.
• performs sorting as items are added.
• Can supply a Comparer class to constructor to control how sorting is performed.

ListDictionary

• Use like hashtable but for small (<10)>
• If colln will fluctuate bet large & small, use HybridDictionary.
• OrderedDictionary - like a hashtable except it allows access to items by index.