Why string interned but has different references?

Question

string s1 = "abc";
string s2 = "ab";
string s3 = s2 + "c";

Console.WriteLine(string.IsInterned(s3));           // abc
Console.WriteLine(String.ReferenceEquals(s1, s3));  // False

I just cannot understand why s3 interned, but ReferenceEquals was False.

Dose they have two copies in intern pool?

Thanks in advance.

Answer 1

There are basically three different situations possible when string.IsInterned is invoked. To illustrate, here is a test method:

static void MyInternCheck(string str)
{
  var test = string.IsInterned(str);

  if ((object)test == (object)str)
    Console.WriteLine("Yes, your string instance is in the intern pool");
  else if (test == str)
    Console.WriteLine("An instance with the same value exists in the intern pool, but you have a different instance with that value");
  else if (test == null)
    Console.WriteLine("No instance with that value exists in the intern pool");
  else
    throw new Exception("Unexpected intern pool answer");
}

You can "hit" all three situations with this code:

static void Main()
{
  string x = "0";
  MyInternCheck(x);
  string y = (0).ToString(CultureInfo.InvariantCulture);
  MyInternCheck(y);
  string z = (1).ToString(CultureInfo.InvariantCulture);
  MyInternCheck(z);
}

Output:

Yes, your string instance is in the intern pool
An instance with the same value exists in the intern pool, but you have a different instance with that value
No instance with that value exists in the intern pool

Since the literal "0" is mentioned in the program text, a string instance with value "0" will exist in the intern pool. The variable x is a reference to that instance.

The variable y has the same value as x, but that is not computed until run-time (the C# compiler makes no guesses as to what int.ToString(IFormatProvider) could be returning). As a consequence, y is another instance than is x.

The variable z has a value that is not found in the intern pool.