C# in Depth

Simple generics for everyday use67I do not like green eggs and ham.";Dictionary frequencies = CountWords(text);foreach (KeyValuePair entry in frequencies){string word = entry.Key;int frequency = entry.Value;Console.WriteLine ("{0}: {1}", word, frequency);}Prints eachkey/value pairfrom mapThe CountWords method B first creates an empty map from string to int. This willeffectively count how often each word is used within the given text. We then use a regularexpression C to split the text into words. It’s crude—we end up with two empty strings(one at each end of the text), and I haven’t worried about the fact that “do” and “Do”are counted separately. These issues are easily fixable, but I wanted to keep the code assimple as possible for this example. For each word, we check whether or not it’s alreadyin the map. If it is, we increment the existing count; otherwise, we give the word an initialcount of 1 D. Notice how the incrementing code doesn’t need to do a cast to int inorder to perform the addition: the value we retrieve is known to be an int at compiletime. The step incrementing the count is actually performing a get on the indexer forthe map, then incrementing, then performing a set on the indexer. Some developersmay find it easier to keep this explicit, using frequencies[word] = frequencies[word]+1; instead.The final part of the listing is fairly familiar: enumerating through a Hashtablegives a similar (nongeneric) DictionaryEntry with Key and Value properties foreach entry E. However, in C# 1 we would have needed to cast both the word and thefrequency as the key and value would have been returned as just object. That alsomeans that the frequency would have been boxed. Admittedly we don’t really have toput the word and the frequency into variables—we could just have had a single call toConsole.WriteLine and passed entry.Key and entry.Value as arguments. I’ve reallyjust got the variables here to ram home the point that no casting is necessary.There are some differences between Hashtable and Dictionarybeyond what you might expect. We’re not looking at them right now, but we’ll coverthem when we look at all of the .NET 2.0 collections in section 3.4. For the moment, ifyou experiment beyond any of the code listed here (and please do—there’s nothinglike actually coding to get the hang of a concept) and if it doesn’t do what you expect,just be aware that it might not be due to a lack of understanding of generics. Checkthe documentation before panicking!Now that we’ve seen an example, let’s look at what it means to talk aboutDictionary in the first place. What are TKey and TValue, and why dothey have angle brackets round them?3.2.2 Generic types and type parametersThere are two forms of generics: generic types (including classes, interfaces, delegates,and structures—there are no generic enums) and generic methods. Both are essentiallya way of expressing an API (whether it’s for a single generic method or a wholeELicensed to Rhona Hadida