We want a derivation type
constraint but in the other direction, constraining the S to be somewhere up the
inheritance tree of T instead of down.
Given that this isn??™t possible, what can we do? There are fewer options this time
than before. First, you could create a generic class with the following declaration:
ComparisonHelper
: IComparer
where TDerived : TBase
You??™d then create a constructor that takes (and stores) an IComparer as a
parameter. The implementation of IComparer would just return the result
of calling the Compare method of the IComparer. You could then sort the
List by creating a new ComparisonHelper that uses the
area comparison.
The second option is to make the area comparison class generic, with a derivation
constraint, so it can compare any two values of the same type, as long as that type
implements IShape. Of course, you can only do this when you??™re able to change the
comparison class??”but it??™s a nice solution when it??™s available.
13 You didn??™t really expect to get through the whole book without seeing a shape-related example, did you?
106 CHAPTER 3 Parameterized typing with generics
Notice that the various options for both covariance and contravariance use more
generics and constraints to express the interface in a more general manner, or to provide
generic ???helper??? methods.
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