Why is adding null to a string legal?

Question

The MSDN article on String Basics shows this:

string str = \"hello\";
string nullStr = null;
string emptyStr = \"\";

string tempStr = str + nullStr; // temp         


        

Answer 1

Conceptually, strings are normally thought of as values as opposed to references to objects which have identity. One of the main reasons that they aren't structs with value semantics is because of the overhead that comes with copying-on-assignment. If strings were values they couldn't be nullable and so a null is just treated by the "+" operator as if it were an empty string (i.e., as if default(string) == "" just as default(int) == 0).