C# in Depth

106 CHAPTER 3 Parameterized typing with genericsNotice that the various options for both covariance and contravariance use moregenerics and constraints to express the interface in a more general manner, or to providegeneric “helper” methods. I know that adding a constraint makes it sound lessgeneral, but the generality is added by first making the type or method generic. Whenyou run into a problem like this, adding a level of genericity somewhere with anappropriate constraint should be the first option to consider. Generic methods (ratherthan generic types) are often helpful here, as type inference can make the lack of varianceinvisible to the naked eye. This is particularly true in C# 3, which has strongertype inference capabilities than C# 2.NOTEIs this really the best we can do?—As we’ll see later, Java supports covarianceand contravariance within its generics—so why can’t C#? Well, a lot of itboils down to the implementation—the fact that the Java runtimedoesn’t get involved with generics; it’s basically a compile-time feature.However, the CLR does support limited generic covariance and contravariance,just on interfaces and delegates. C# doesn’t expose this feature(neither does VB.NET), and none of the framework libraries use it. TheC# compiler consumes covariant and contravariant interfaces as if theywere invariant. Adding variance is under consideration for C# 4,although no firm commitments have been made. Eric Lippert has writtena whole series of blog posts about the general problem, and what mighthappen in future versions of C#: http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx.This limitation is a very common cause of questions on C# discussion groups. Theremaining issues are either relatively academic or affect only a moderate subset of thedevelopment community. The next one mostly affects those who do a lot of calculations(usually scientific or financial) in their work.3.6.2 Lack of operator constraints or a “numeric” constraintC# is not without its downside when it comes to heavily mathematical code. The needto explicitly use the Math class for every operation beyond the simplest arithmetic andthe lack of C-style typedefs to allow the data representation used throughout a programto be easily changed have always been raised by the scientific community as barriersto C#’s adoption. Generics weren’t likely to fully solve either of those issues, butthere’s a common problem that stops generics from helping as much as they couldhave. Consider this (illegal) generic method:public T FindMean(IEnumerable data){T sum = default(T);int count = 0;foreach (T datum in data){sum += datum;count++;}Licensed to Rhona Hadida