C# 4 and .NET 4

248 ❘ ChaPTer 10 cOllectiOns
diCTionaries
Dictionaries represent a sophisticated data structure that allows you to access an element based on a
key. Dictionaries are also known as hash tables or maps. The main feature of dictionaries is fast lookup
based on keys. You can also add and remove items freely, a bit like a List, but without the performance
overhead of having to shift subsequent items in memory.
Figure 10-5 shows a simplified representation of a dictionary. Here employee-ids such as B4711 are the
keys added to the dictionary. The key is transformed into a hash. With the hash a number is created to
associate an index with the values. The index then contains a link to the value. The figure is simplified
because it is possible that a single index entry can be associated with multiple values, and the index can
be stored as a tree.
Keys
Index
Values
B4711
0
1
B12836
Tony Stewart
B12836
2
. . .
B4711
Jimmie Johnson
N34434
31
32
. . .
N34434
Matt Kenseth
60
61
figure 10-5
The .NET Framework offers several dictionary classes. The main class you can use is Dictionary
.
Key Type
A type that is used as a key in the dictionary must override the method GetHashCode() of the Object class.
Whenever a dictionary class needs to work out where an item should be located, it calls the GetHashCode()
method. The int that is returned by GetHashCode() is used by the dictionary to calculate an index of
where to place the element. We don’t go into this part of the algorithm. What you should know is that it
involves prime numbers, so the capacity of a dictionary is a prime number.
The implementation of GetHashCode() must satisfy these requirements:
➤
➤
➤
➤
➤
The same object should always return the same value.
Different objects can return the same value.
It should execute as quickly as possible; it must be inexpensive to compute.
It must not throw exceptions.
It should use at least one instance field.
www.it-ebooks.info