C# borrows the behavior of its + operator from Java. If either operand to + is a string, the + operator will call String.Concat, which accepts type Object and concatenates the results of calling ToString on every non-null object that's passed to it. The fact that null references are simply ignored is only a small part of the way in which the operands to String.Concat aren't regarded as "strings" per se; a much more noticeable aspect of this behavior is that types which aren't strings have their ToString method called, whether or not they would otherwise be implicitly convertible to string.
