Specialized Dictionary

Aug 26, 2008
by David Cumps

Part of my 2008 study notes for exam 70-536 (.NET Framework 2.0).

Overview

Special-case dictionaries
ListDictionary
- Used as a Hashtable for fewer than 10 items.
- Implemented as a simple linked list behind the scenes.
- Same interface as Hashtable, easy to replace.
HybridDictionary
- Used as a Hashtable when the number of items is unknown.
- Implemented as a combination of Hashtable and ListDictionary.
- Switches over to a Hashtable when the collection becomes too big.
- Same interface as Hashtable, easy to replace.
OrderedDictionary
- Used as a Hashtable when you need access by index.
- A plain dictionary mapping, like a cross between Hashtable and ArrayList.

Good practice

When possible, use a generic collection instead.

Examples

ListDictionary: Adding and Removing Elements

ListDictionary a = new ListDictionary();
a.Add("Key2", "Hello");
a.Add("Key1", "World");
a.Add("Key0", "Start");
 
Console.WriteLine(a["Key1"] + " " + a["Key2"]);
Console.WriteLine();
foreach (DictionaryEntry d in a) { Console.WriteLine(d.Value); }
Console.WriteLine();
foreach (object v in a.Values) { Console.WriteLine(v); }
Console.WriteLine();
a.Remove("Key1");
foreach (object k in a.Keys) { Console.WriteLine(a[k]); }

HybridDictionary: Adding and Removing Elements

HybridDictionary a = new HybridDictionary();
a.Add("Key2", "Hello");
a.Add("Key1", "World");
a.Add("Key0", "Start");
 
Console.WriteLine(a["Key1"] + " " + a["Key2"]);
Console.WriteLine();
foreach (DictionaryEntry d in a) { Console.WriteLine(d.Value); }
Console.WriteLine();
foreach (object v in a.Values) { Console.WriteLine(v); }
Console.WriteLine();
a.Remove("Key1");
foreach (object k in a.Keys) { Console.WriteLine(a[k]); }

OrderedDictionary: Adding and Removing Elements

OrderedDictionary a = new OrderedDictionary();
a.Add("Key1", "Hello");
a.Add("Key2", "World");
a.Add("Key0", "Start");
a.Insert(1, "Key10", "Inserted");
 
Console.WriteLine(a["Key1"] + " " + a["Key2"]);
Console.WriteLine();
foreach (DictionaryEntry d in a) { Console.WriteLine(d.Value); }
Console.WriteLine();
foreach (object v in a.Values) { Console.WriteLine(v); }
Console.WriteLine();
a.Remove("Key1");
a.RemoveAt(0);
foreach (object k in a.Keys) { Console.WriteLine(a[k]); }