C# in Depth

Beyond the basics83derive from a particular type that overloads the == and != operators, those overloads areused. Beware, however—extra overloads that happen to be made available by the typeargument specified by the caller are not used. Listing 3.5 demonstrates this with a simplereference type constraint and a type argument of string.Listing 3.5Comparisons using == and != using reference comparisonsstatic bool AreReferencesEqual (T first, T second)where T : class{B Comparesreturn first==second;references}...string name = "Jon";string intro1 = "My name is "+name;string intro2 = "My name is "+name;Console.WriteLine (intro1==intro2);Console.WriteLine (AreReferencesEqual(intro1, intro2));Even though string overloads == (as demonstrated by C printing True), this overloadis not used by the comparison at B. Basically, when AreReferencesEqual is compiledthe compiler doesn’t know what overloads will be available—it’s as if the parameterspassed in were just of type object.This is not just specific to operators—when the compiler encounters ageneric type, it resolves all the method overloads when compilingthe unbound generic type, rather than reconsidering each possiblemethod call for more specific overloads at execution time. For instance, astatement of Console.WriteLine (default(T)); will always resolve to callConsole.WriteLine(object value)—it doesn’t call Console.WriteLine(string value) when T happens to be string. This is similar to the normalsituation of overloads being chosen at compile time rather than executiontime, but readers familiar with templates in C++ may be surprised nonetheless.Two classes that are extremely useful when it comes to comparing values are Equality-Comparer and Comparer, both in the System.Collections.Generic namespace.They implement IEqualityComparer (useful for comparing and hashing dictionarykeys) and IComparer (useful for sorting) respectively, and the Default propertyreturns an implementation that generally does the right thing for the appropriate type.See the documentation for more details, but consider using these (and similar typessuch as StringComparer) when performing comparisons. We’ll use Equality-Comparer in our next example.Caution!Possiblyunexpectedbehavior!Compares usingstring overloadFULL COMPARISON EXAMPLE: REPRESENTING A PAIR OF VALUESTo finish off our section on implementing generics—and indeed “medium-level” generics—here’sa complete example. It implements a useful generic type—a Pair, which just holds two values together, like a key/value pair, but withno expectations as to the relationship between the two values. As well as providing propertiesto access the values themselves, we’ll override Equals and GetHashCode to allowCLicensed to Rhona Hadida