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A dictionary is a collection that enables you to look up information associated with some kind of value. .NET calls this sort of collection a dictionary because it is reminiscent of a traditional printed dictionary: the information is structured to make it easy to find the entry for a particular word if you know what word you re looking for, you can find it very quickly even among tens of thousands of definitions. The information you find when you ve looked up the word depends on the sort of dictionary you bought it might provide a definition of the word, but other kinds exist, such as dictionaries of quotations, or of etymology. Likewise, a .NET dictionary collection is structured to enable quick and easy lookup of entries. The syntax looks very similar to array access, but where you d expect to see a number, the index can be something else, such as a string, as shown in Example 9-1.
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string definition = myDictionary["sea"];
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Just as printed dictionaries vary in what you get when you look up a word, so can .NET dictionaries. The Dictionary type in the System.Collections.Generic namespace is a generic type, letting you choose the type for both the key the value used for the index and the value associated with the index. (Note that there are some restrictions regarding the key type see the sidebar on the next page.) Example 9-1, which models
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a traditional printed dictionary, uses strings for the index, and expects a string as the result, so myDictionary in that example would be defined as shown in Example 9-2.
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Dictionary<string, string> myDictionary = new Dictionary<string, string>();
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Keys, Comparison, and Hashes
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To be able to look up entries quickly, dictionaries impose a couple of requirements on keys. First, a dictionary entry s key must not change in a way that affects comparisons. (This often just means that you should never change a key. However, it s technically possible to build a type for which certain kinds of changes have no impact on comparisons performed by the Equals methods. Such changes are invisible to the dictionary.) Second, it should provide a good hash function. To understand the first requirement that for comparison purposes, keys must not change consider what changing a key would mean in a printed dictionary. Suppose you look up the entry for bug in your dictionary, and then you cross out the word bug and write feature in its place. The usual way of looking up words will now fail for this entry the entry was positioned in exactly the right place for when the key was bug. Anyone looking up feature will not think to look in the location for your amended item. And it s the same with a dictionary collection to enable fast lookup, dictionaries create an internal structure based on the keys items had when they were added to the dictionary. It has no way of knowing when you ve changed a key value. If you really need to do this, you should remove the entry, and then add it back with the new key this gives the dictionary a chance to rebuild its internal lookup data structures. This requirement is most easily met by using an immutable type, such as string, or any of the built-in numeric types. The second requirement that key types should have a good hash function is a bit less obvious, and has to do with how dictionary collections implement fast lookup. The base System.Object class defines a virtual method called GetHashCode, whose job is to return an int whose value loosely represents the value of the object. GetHashCode is required to be consistent with the Equals method (also defined by System.Object) two objects or values that are equal according to Equals are required to return the same hash code. Those are the rules, and dictionaries will not work if you break them. This means that if you override Equals, you are required to override GetHashCode, and vice versa. The rules about hash codes for items that are not equal are more flexible. Ideally, nonequal items should return nonequal hash codes, but clearly that s not always possible: a long can have any of several quintillion distinct values, but a hash code is an int, which has merely a few billion possible values. So inevitably there will be hash collisions nonequal values that happen to have equal hash codes. For example, long returns the same hash code for the values 4,294,967,296 and 1.
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