String interning?

Question

The second ReferenceEquals call returns false. Why isn\'t the string in s4 interned? (I don\'t care about the advantages of StringBuilder over string concatenation.)

Answer 1

First of all, everything written so far about immutable strings is correct. But there are some important things which are not written. The code

string s1 = "tom";
string s2 = "tom";
Console.Write(object.ReferenceEquals(s2, s1)); //true

display really "True", but only because of some small compiler optimization or like here because CLR ignore C# compiler attributes (see "CLR via C#" book) and place only one string "tom" in the heap.

Second you can fix the situation with following lines:

s3 = String.Intern(s3);
s4 = String.Intern(s4);
Console.Write (object.ReferenceEquals (s3, s4)); //true

Function String.Intern calculates a hash code of the string and search for the same hash in the internal hash table. Because it find this, it returns back the reference to already existing String object. If the string doesn't exist in the internal hash table, a copy of the string is made and the hash computed. The garbage collector doesn't free memory for the string, because it is referenced by the hash table.