How does appending to a null string work in C#?

Question

I was surprised to see an example of a string being initialised to null and then having something appended to it in a production environment. It just smelt wrong.

I was

Answer 1

it is because

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.

From How to: Concatenate Multiple Strings (C# Programming Guide)

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.

From Addition operator