String interning in .Net Framework - What are the benefits and when to use interning

Question

I want to know the process and internals of string interning specific to .Net framework. Would also like to know the benefits of using interning and the scenarios/situations where we should use string interning to improve the performance. Though I have studied interning from the Jeffery Richter's CLR book but I am still confused and would like to know it in more detail.

[Editing] to ask a specific question with a sample code as below:

private void MethodA() {     string s = "String"; // line 1 - interned literal as explained in the answer              //s.intern(); // line 2 - what would happen in line 3 if we uncomment this line, will it make any difference? }  private bool MethodB(string compareThis) {     if (compareThis == "String") // line 3 - will this line use interning (with and without uncommenting line 2 above)?     {         return true;     }     return false; } 

Answer 1

In general, interning is something that just happens, automatically, when you use literal string values. Interning provides the benefit of only having one copy of the literal in memory, no matter how often it's used.

That being said, it's rare that there is a reason to intern your own strings that are generated at runtime, or ever even think about string interning for normal development.

There are potentially some benefits if you're going to be doing a lot of work with comparisons of potentially identical runtime generated strings (as interning can speed up comparisons via ReferenceEquals). However, this is a highly specialized usage, and would require a fair amount of profiling and testing, and wouldn't be an optimization I'd consider unless there was a measured problem in place.

Answer 2

Interning is an internal implementation detail. Unlike boxing, I do not think there is any benefit in knowing more than what you have read in Richter's book.

Micro-optimisation benefits of interning strings manually are minimal hence is generally not recommended.

This probably describes it:

class Program {     const string SomeString = "Some String"; // gets interned      static void Main(string[] args)     {         var s1 = SomeString; // use interned string         var s2 = SomeString; // use interned string         var s = "String";         var s3 = "Some " + s; // no interning           Console.WriteLine(s1 == s2); // uses interning comparison         Console.WriteLine(s1 == s3); // do NOT use interning comparison     } }