String interning. How does the compiler know?

Question

I know what string interning is, and why the following code behaves the way it does:

var hello = "Hello";
var he_llo = "He" + "llo";
var b = ReferenceEquals(hello, he_llo); //true

Or

var hello = "Hello";
var h_e_l_l_o = new string(new char[] { 'H', 'e', 'l', 'l', 'o' });
var b = ReferenceEquals(hello, he_llo); //false

...or I thought I did, because a subtle bug has cropped up in some code I'm working on due to this:

var s = "";
var sss = new string(new char[] { });
var b = ReferenceEquals(s, sss); //True!?

How does the compiler know that sss will in fact be an empty string?

Answer 1

If an empty array or null array is passed in a string constructor then it returns an empty string.

It is specified in a comment in the reference code.

 // Creates a new string with the characters copied in from ptr. If
 // ptr is null, a 0-length string (like String.Empty) is returned.

You can also see the same result with null array like:

char[] tempArray = null;
var s = "";
var sss2 = new string(tempArray);
var b = ReferenceEquals(s, sss2); //True!?