Reference等于字符串

Question

As indicated here , a string that is created at runtime cannot be interned.

However, the following code:

    class Program {
    static void Main(string[] args)
    {
        string s1 = "Programming Is Fun";
        string s3 = s1.ToString();
        Console.WriteLine(Object.ReferenceEquals(s1, s3));
    }
}

gives (VS 2015):

True

So, is it specified somehow which strings are generated in runtime?

BTW:

The code:

using System;
using System.Text;

class Program {
  static void Main(string[] args)
  {
    string s1 = "hop";
    StringBuilder s2 = new StringBuilder(s1);
    string s3 = s2.ToString();
    Console.WriteLine(Object.ReferenceEquals(s1, s3));
  }
}

gives (VS 2015):

False

in opposite to mono (ver. 4.0.2) which gives

True

see the working example .

Answer 1

String.ToString() returns reference to the same string:

public override String ToString() {
    Contract.Ensures(Contract.Result<String>() != null);
    Contract.EndContractBlock();
    return this;
}

And strings created at runtime can be interned. Quote from the MSDN article you referred to:

the runtime does not guarantee that strings created at runtime are interned

Interning is an expensive operation, usually it's too expensive to do it at runtime. If you want to make sure your string is interned, you can use public static String Intern(String str) method.