Why is adding null to a string legal?

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.

I agree that conceptually strings are just values. However, consider the following code:

int? i = null;
i += 1; // The result of this is that i == null

If the other value type operators used default() the way the string operators are converting null to "", your explanation would make sense.

It's simplest to say that the string operators are a shortcut (special case) for convenience.

I guess the language (or standard library) designers decided this would be a common enough case that they'd do programmers a favour.

(Neat! I always just assumed concating with null would through an exception!)

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).