C# in Depth

Generic collection classes in .NET 2.099calls. To make those operations thread-safe, the collection needs to be locked for theduration of the operation. (It requires cooperation from other code using the samecollection, of course.) In short, the Synchronized method gave the appearance ofsafety without the reality. It’s better not to give the wrong impression in the firstplace—developers just have to be careful when working with collections accessed inmultiple threads. SynchronizedCollection performs broadly the same role as asynchronized ArrayList. I would argue that it’s still not a good idea to use this, for thereasons outlined in this paragraph—the safety provided is largely illusory. Ironically,this would be a great collection to support a ForEach method, where it could automaticallyhold the lock for the duration of the iteration over the collection—but there’sno such method.That completes our coverage of List. The next collection under the microscopeis Dictionary, which we’ve already seen so much of.3.5.2 DictionaryThere is less to say about Dictionary (just called Dictionary forthe rest of this section, for simplicity) than there was about List, although it’sanother heavily used type. As stated earlier, it’s the generic replacement for Hashtableand the related classes, such as StringDictionary. There aren’t many features presentin Dictionary that aren’t in Hashtable, although this is partly because the ability tospecify a comparison in the form of an IEqualityComparer was added to Hashtable in.NET 2.0. This allows for things like case-insensitive comparisons of strings withoutusing a separate type of dictionary. IEqualityComparer and its generic equivalent,IEqualityComparer, have both Equals and GetHashCode. Prior to .NET 2.0 thesewere split into IComparer (which had to give an ordering, not just test for equality) andIHashCodeProvider. This separation was awkward, hence the move to IEquality-Comparer for 2.0. Dictionary exposes its IEqualityComparer in the publicComparer property.The most important difference between Dictionary and Hashtable (beyond thenormal benefits of generics) is their behavior when asked to fetch the value associatedwith a key that they don’t know about. When presented with a key that isn’t in themap, the indexer of Hashtable will just return null. By contrast, Dictionary willthrow a KeyNotFoundException. Both of them support the ContainsKey method totell beforehand whether a given key is present. Dictionary also providesTryGetValue, which retrieves the value if a suitable entry is present, storing it in theoutput parameter and returning true. If the key is not present, TryGetValue will setthe output parameter to the default value of TValue and return false. This avoidshaving to search for the key twice, while still allowing the caller to distinguish betweenthe situation where a key isn’t present at all, and the one where it’s present but its associatedvalue is the default value of TValue. Making the indexer throw an exception isof more debatable merit, but it does make it very clear when a lookup has failedinstead of masking the failure by returning a potentially valid value.Licensed to Rhona Hadida