Question

Why by default only literal strings are saved in the intern pool?

Example from MSDN:

String s1 = "MyTest";
String s2 = new StringBuilder().Append("My").Append("Test").ToString(); 
String s3 = String.Intern(s2); 
Console.WriteLine("s1 == '{0}'", s1);
Console.WriteLine("s2 == '{0}'", s2);
Console.WriteLine("s3 == '{0}'", s3);
Console.WriteLine("Is s2 the same reference as s1?: {0}", (Object)s2==(Object)s1); 
Console.WriteLine("Is s3 the same reference as s1?: {0}", (Object)s3==(Object)s1);

/*
This example produces the following results:
s1 == 'MyTest'
s2 == 'MyTest'
s3 == 'MyTest'
Is s2 the same reference as s1?: False
Is s3 the same reference as s1?: True
*/

Answer 1

The short answer: interning literal strings is cheap at runtime and saves memory. Interning non-literal strings is expensive at runtime and therefore saves a tiny amount of memory in exchange for making the common cases much slower.

The cost of the interning-strings-at-runtime "optimization" does not pay for the benefit, and is therefore not actually an optimization. The cost of interning literal strings is cheap and therefore does pay for the benefit.

I answer your question in more detail here:

https://ericlippert.com/2009/09/28/string-interning-and-string-empty