Why must we define both == and != in C#?

Question

The C# compiler requires that whenever a custom type defines operator ==, it must also define != (see here).

Why?

I\'m curious to k

Answer 1

Other than that C# defers to C++ in many areas, the best explanation I can think of is that in some cases you might want to take a slightly different approach to proving "not equality" than to proving "equality".

Obviously with string comparison, for example, you can just test for equality and return out of the loop when you see nonmatching characters. However, it might not be so clean with more complicated problems. The bloom filter comes to mind; it's very easy to quickly tell if the element is not in the set, but difficult to tell if the element is in the set. While the same return technique could apply, the code might not be as pretty.