Two different "strings" are the same object instance?

Question

The code is pretty self explanatory. I expected when I made a1 and b1 that I was creating two different string instances that contain the same text. So I figure a1 == b1 would be true but object.ReferenceEquals(a1,b1) would be false, but it isn't. Why?

//make two seemingly different string instances
string a1 = "test";
string b1 = "test";         
Console.WriteLine(object.ReferenceEquals(a1, b1)); // prints True. why?

//explicitly "recreating" b2
string a2 = "test";
string b2 = "tes";
b2 += "t";    
Console.WriteLine(object.ReferenceEquals(a2, b2)); // prints False

//explicitly using new string constructor
string a3 = new string("test".ToCharArray());
string b3 = new string("test".ToCharArray());    
Console.WriteLine(object.ReferenceEquals(a3, b3)); // prints False

Answer 1

Literal string objects are coalesced into single instances by the compiler. This is actually required by the specification:

Each string literal does not necessarily result in a new string instance. When two or more string literals that are equivalent according to the string equality operator (Section 7.9.7) appear in the same assembly, these string literals refer to the same string instance.