What is the difference between String.Intern and String.IsInterned?

Question

MSDN states that

String.Intern retrieves the system's reference to the specified String

and

String.IsInterned retrieves a reference to a specified String.

I think that IsInterned should have returned (I know it doesn't) a bool stating whether the specified string is interned or not. Is that correct thinking ? I mean it is atleast not consistent with .net framework naming convention.

I wrote the following code:

 string s = "PK";
 string k = "PK";

 Console.WriteLine("s has hashcode " + s.GetHashCode());
 Console.WriteLine("k has hashcode " + k.GetHashCode());
 Console.WriteLine("PK Interned " + string.Intern("PK"));
 Console.WriteLine("PK IsInterned " + string.IsInterned("PK"));

The output is :

s has hashcode -837830672

k has hashcode -837830672

PK Interned PK

PK IsInterned PK

Why is string.IsInterned("PK") returning "PK"?

Answer 1

String.Intern interns the string if it's not already interned; String.IsInterned doesn't.

IsInterned("PK") is returning "PK" because it's already interned. The reason for it returning the string instead of a bool is so that you can easily get a reference to the interned string itself (which may not be the same reference as you passed in). In other words, it's effectively returning two related pieces of information at once - you can simulate it returning bool easily:

public static bool IsInternedBool(string text)
{
     return string.IsInterned(text) != null;
}

I agree that the naming isn't ideal, although I'm not sure what would have been better: GetInterned perhaps?

Here's an example showing that difference though - I'm not using string literals, to avoid them being interned beforehand:

using System;

class Test
{
    static void Main()
    {
        string first = new string(new[] {'x'});
        string second = new string(new[] {'y'});

        string.Intern(first); // Interns it
        Console.WriteLine(string.IsInterned(first) != null); // Check

        string.IsInterned(second); // Doesn't intern it
        Console.WriteLine(string.IsInterned(second) != null); // Check
    }
}