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

If you look at implementations of overloads of == and != in the .net source, they often don't implement != as !(left == right). They implement it fully (like ==) with negated logic. For example, DateTime implements == as

return d1.InternalTicks == d2.InternalTicks;

and != as

return d1.InternalTicks != d2.InternalTicks;

If you (or the compiler if it did it implicitly) were to implement != as

return !(d1==d2);

then you are making an assumption about the internal implementation of == and != in the things your class is referencing. Avoiding that assumption may be the philosophy behind their decision.