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

From MSDN:

In string concatenation operations, the C# compiler treats a null string the same as an empty string, but it does not convert the value of the original null string.

More information on the + binary operator:

The binary + operator performs string concatenation when one or both operands are of type string.

If an operand of string concatenation is null, an empty string is substituted. Otherwise, any non-string argument is converted to its string representation by invoking the virtual ToString method inherited from type object.

If ToString returns null, an empty string is substituted.