Why some identical strings are not interned in .NET?

Question

string s1 = \"test\";
string s5 = s1.Substring(0, 3)+\"t\"; 
string s6 = s1.Substring(0,4)+\"\";   
Console.WriteLine(\"{0} \", object.ReferenceEquals(s1, s5)); //Fa         


        

Answer 1

From msdn documentation of object.ReferenceEquals here:

When comparing strings.If objA and objB are strings, the ReferenceEquals method returns true if the string is interned.It does not perform a test for value equality.In the following example, s1 and s2 are equal because they are two instances of a single interned string.However, s3 and s4 are not equal, because although they are have identical string values, that string is not interned.

using System;

public class Example
{
   public static void Main()
   {
      String s1 = "String1";
      String s2 = "String1";
      Console.WriteLine("s1 = s2: {0}", Object.ReferenceEquals(s1, s2));
      Console.WriteLine("{0} interned: {1}", s1, 
                        String.IsNullOrEmpty(String.IsInterned(s1)) ? "No" : "Yes");

      String suffix = "A";
      String s3 = "String" + suffix;
      String s4 = "String" + suffix;
      Console.WriteLine("s3 = s4: {0}", Object.ReferenceEquals(s3, s4));
      Console.WriteLine("{0} interned: {1}", s3, 
                        String.IsNullOrEmpty(String.IsInterned(s3)) ? "No" : "Yes");
   }
}
// The example displays the following output:
//       s1 = s2: True
//       String1 interned: Yes
//       s3 = s4: False
//       StringA interned: No