C# in Depth

82 CHAPTER 3 Parameterized typing with generics}...Console.WriteLine(CompareToDefault("x"));Console.WriteLine(CompareToDefault(10));Console.WriteLine(CompareToDefault(0));Console.WriteLine(CompareToDefault(-10));Console.WriteLine(CompareToDefault(DateTime.MinValue));Listing 3.4 shows a generic method being used with three different types: string,int, and DateTime. The CompareToDefault method dictates that it can only beused with types implementing the IComparable interface, which allows us to callCompareTo(T) on the value passed in. The other value we use for the comparison is thedefault value for the type. As string is a reference type, the default value is null—andthe documentation for CompareTo states that for reference types, everything should begreater than null so the first result is 1. The next three lines show comparisons with thedefault value of int, demonstrating that the default value is 0. The output of the last lineis 0, showing that DateTime.MinValue is the default value for DateTime.Of course, the method in listing 3.4 will fail if you pass it null as the argument—the line calling CompareTo will throw NullReferenceException in the normal way.Don’t worry about it for the moment—there’s an alternative using IComparer, aswe’ll see soon.DIRECT COMPARISONSAlthough listing 3.4 showed how a comparison is possible, we don’t always want to constrainour types to implement IComparable or its sister interface, IEquatable,which provides a strongly typed Equals(T) method to complement the Equals(object)method that all types have. Without the extra information these interfaces give us accessto, there is little we can do in terms of comparisons, other than calling Equals(object),which will result in boxing the value we want to compare with when it’s a value type.(In fact, there are a couple of types to help us in some situations—we’ll come to themin a minute.)When a type parameter is unconstrained (in other words, no constraints are appliedto it), you can use == and != operators but only to compare a value of that type withnull. You can’t compare two values of T with each other. In the case where the typeargument provided for T is a value type (other than a nullable type), a comparison withnull will always decide they are unequal (so the comparison can be removed by the JITcompiler). When the type argument is a reference type, the normal reference comparisonwill be used. When the type argument is a nullable type, the comparison will do theobvious thing, treating an instance without a value as null. (Don’t worry if this last bitdoesn’t make sense yet—it will when you’ve read the next chapter. Some features aretoo intertwined to allow me to describe either of them completely without referring tothe other, unfortunately.)When a type parameter is constrained to be a value type, == and != can’t be used withit at all. When it’s constrained to be a reference type, the kind of comparison performeddepends on exactly what the type parameter is constrained to be. If it’s just a referencetype, simple reference comparisons are performed. If it’s further constrained toLicensed to Rhona Hadida