Strange string literal comparison

Going deeper in C#, I have encountered a little (strange) problem with object reference equality. Let says I have two strings:

String a = "Hello world!";
String b = "Bonjour le monde";
bool equals = ReferenceEquals(a, b);  // ******************* (1)
b = "Hello world!";
equals = ReferenceEquals(a, b);       // ******************* (2)

(1) Is false and that is expected. ReferenceEquals Documentation says

ReferenceEquals compares instances

but then:

  • Why does (2) returns true?
  • Strings a and b are not the same object are they? If yes then how did they become the same given that I never explicitly did a=b

Answers


This is because of string interning.

The common language runtime conserves string storage by maintaining a table, called the intern pool, that contains a single reference to each unique literal string declared or created programmatically in your program. Consequently, an instance of a literal string with a particular value only exists once in the system.

For example, if you assign the same literal string to several variables, the runtime retrieves the same reference to the literal string from the intern pool and assigns it to each variable.


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