6.4 Logical Pairs

It is quite common to overload the equals operator (==) to test whether two objects are equal (however equality might be defined for your object). C# insists that if you overload the equals operator, you must also overload the not-equals operator (!=). Similarly, the less-than (<) and greater-than (>) operators must be paired, as must the less-than or equals (<=) and greater-than or equals (>=) operators.

C and C++ programmers take note: Many C++ libraries require the implementation of only the < operator or the = operator, and other operations are derived from these. C# requires the implementation of both halves of these paired operators.