Try this code of RO C# on iOS, and result is
bool result = ((NSString)null) == ((NSObject)null); // result is false
Then I try code of Objective-C which I think is equivalent, and result is
bool result = ((NSString*)nil) == ((NSObject*)nil); // result is true
Env: Elements 18.104.22.1689
BTW: is there a convenient way to see the generated Objective-C code from RO C#?