Programming Visual Basic 2005

Dictionaries

A dictionary is a collection that associates a key with a value. A language dictionary, such as Webster's, associates a word (the key) with its definition (the value).

To see the benefit of dictionaries , start by imagining that you want to keep a list of the state capitals. One approach might be to put them in an array:

Dim stateCapitals(50) As String

The stateCapitals array will hold 50 state capitals. Each capital is accessed as an offset into the array. For example, to access the capital for Arkansas, you need to know that Arkansas is the fourth state in alphabetical order:

Dim capitalOfArkansas As String = stateCapitals(4)

It is inconvenient, however, to access state capitals using array notation. After all, if I need the capital for Massachusetts, there is no easy way for me to determine that Massachusetts is the 21st state alphabetically.

It would be far more convenient to store the capital with the state name. A dictionary allows you to store a value (in this case, the capital) with a key (in this case, the name of the state).

A .NET Framework generic dictionary can associate any kind of key (string, integer, object, etc.) with any kind of value (string, integer, object, etc.). Typically, of course, the key is fairly short, the value fairly complex.

The most important attributes of a good dictionary are that it is easy to add values and that it is quick to retrieve values and some of the most important properties methods of the Dictionary class are shown in Table 17-5 ...