Comparisons

Comparisons work out-of-box only for primitive types. For others they need to be defined.

Comparisons do not work well with inheritance. It’s not recommneded to implement comparisons on unsealed classes.

IComparable

This interface defines CompareTo method that returns int:

• -1 (less then)
• 0 (equal)
• +1 (greater then)

It’s also recommended to implement non-generic IComparable for legacy code.

This interface is used by arrays to sort its items - if item does not implement, exception will be thrown.

Operators

There are 4 operators that can be implemented: <, <=, >, >=. If these get implemented, it’s recommended to also implement == (and !=).

IComparer

Interface to be implemented by classes that can compare T. When implementing it there’s a convention:

• it two objects are NULL return “0”
• if first object is NULL, return “-1”
• if second object is NULL, return “+1”

There is actually a recomendation to inherit from Comparer<T> base class instead of implementing the interface from zero (because it also implements non-generic IComparer, so our inherited class doesn’t have to do that explicitly).

Comparers are good candidates for singletons.

A good comparer should be deterministic and the result should not depend on the input order. For example, if a class has 2 properties and our comparer sorts them by one of these properties, it might happen that two instances will be considered equal when their second property has different values. So, our comparer should actually also look at the value of second property when first property values are equal.

Writing comparers for non-sealed classes is problematic, mostly due to the problem outlined in previous paragraph. The result of sorting will depend from order of input elements since the comparer does not know anything about the inheriting instances (unles it actually does). There is no real solution for this issue.

Comparer<T>.Default - every type can use the default comparer, which uses the T’s IComparer<T> implementation (or IComparer if generic one is missing). What if both are missing? An exception?

StringComparer is a static class that has 6 various comparers for strings (implements IComparer and IEqualityComparer).