[mullr]

C# has || just like any other language, but objects aren't truthy and nulls aren't falsy. This was a good design decision, since it catches the common = vs. == error if(a=b) while doing the type check rather than silently doing the (probably) wrong thing. As a consequence, the || doesn't work as it does in other places. Hence ??.

[steverb]

?? is the null coalescing operator and it's neither new nor is it hidden.

It is damned handy though.