What's the Comparer class for?

Question

What purpose does the Comparer class serve if the type that you specify already implements IComparable?

If I specify Comparer.Defaul

Answer 1

If a type implements IComparable, it is almost certainly better to use that than IComparable. With value types, the performance of IComparable is often much better than that of the non-generic IComparable. With inheritable reference types, IComparable can offer better semantics than IComparable by allowing rankings based upon derived-type fields.

For an example of the latter benefit, suppose one has an abstract base class ScheduleEvent, with a property EventTime, that implements IComparable by sorting EventTime. Derived types include ScheduledPopupMessageEvent with a message string, a ScheduledGongEvent with a GongVolume parameter. Multiple ScheduleEvents with the same EventTime must report zero for IComparable.CompareTo, because there is no safe and consistent way to rank ScheduleEvents of different types, and because two ScheduleEvents which both report themselves as unranked relative to a third must, for consistency, report themselves as unranked relative to each other. On the other hand, there would be no problem with having ScheduledGongEvent implementing IComparable take GongVolume into account as well as EventTime, or with ScheduledPopupMessageEvent doing likewise with its Message parameter.

It is useful, then, to have things like sorting routines use IComparable if it exists, but be able to fall back to IComparable if IComparable does not exist. Checking for whether a class implements IComparable and selecting an appropriate implementation if it does, however, is a little expensive. Fortunately, once a type is determined to have an IComparable implementation, it can be relied upon to always have one; likewise, if a type is found not to have such an implementation, it never will. Further, if a generic class has any static fields, every combination of type parameters will yield a different class with different fields. Thus, the first time Comparer.Default is run with a particular type parameter T, it will store the Comparer routine it returns into a static field. If Comparer is run again with that same type, it will return the same comparer routine. Although it might seem odd to have a static Comparer class with just one method, creating a separate Comparer class for every type T provides a place to store the created compare routine.