C# | How to retrieve a reference to a specified String

String.IsInterned(String) Method method is used to retrieve a reference to a specified String. This method looks up the specified string in the intern pool.
If the specified string has already been interned, a reference to that instance is returned. Otherwise, null is returned. Here the intern pool is a table that contains a single instance of each unique literal string constant declared in a program, as well as any unique instance of String you add programmatically by calling the Intern method.

Note: The main difference between String.Intern(String) Method and String.IsInterned(String) Method is that Intern method returns a reference if the string doesn’t exist by adding a reference of the specified string to the intern pool. But the IsInterned method returns null if the string doesn’t exist.

Syntax:

public static string IsInterned (string strA);

Here, strA is a string which is used to search for in the intern pool.

Return Value: The return type of this method is System.String. This method will return a reference to strA if it is present in the CLR(Common Language Runtime) intern pool. Otherwise, return null.

Exception: This method will give ArgumentNullException of the str is null.

Below given are some examples to understand the implementation in a better way:

Example 1:

// C# program to check if the given  
// string is in intern pool or not 
using System; 
  
class GFG { 
      
    // main method 
    public static void Main() 
    { 
        // strings 
        string str1 = "Monday"; 
        string str2 = str1 + "-day"; 
        string[] pool = {"Tuesday", "Wednesday", 
                         "Thursday", str2}; 
                           
        // checking in intern pool 
        foreach(var item in pool) 
        { 
  
            // retrive reference of strings 
            bool value = String.IsInterned(item) != null; 
  
            if (value) 
  
                // if the string in intern pool then print 
                Console.WriteLine("{0}: Yes", item); 
                  
            else
              
                // if string in not intern pool then print 
                Console.WriteLine("{0}: No", item); 
        } 
    } 
} 

Output:

Tuesday: Yes
Wednesday: Yes
Thursday: Yes
Monday-day: No

Example 2:

// C# program to check if the given  
// string is in intern pool or not 
using System; 
  
class GFG { 
      
    // main method 
    public static void Main() 
    { 
        // strings 
        string str1 = "Geeks"; 
        string str2 = str1 + "-day"; 
          
        // this will give error as  
        // null is present in pool 
        string[] pool = {"GFG", "DSA", 
                         "C#", null}; 
                           
        // checking in intern pool 
        foreach(var item in pool) 
        { 
  
            // retrive reference of strings 
            bool value = String.IsInterned(item) != null; 
  
            if (value) 
  
                // if the string in intern pool then print 
                Console.WriteLine("{0}: Yes", item); 
                  
            else
              
                // if string in not intern pool then print 
                Console.WriteLine("{0}: No", item); 
        } 
    } 
} 

Runtime Error:

Unhandled Exception:
System.ArgumentNullException: Value cannot be null.
Parameter name: str

Reference:

https://docs.microsoft.com/en-us/dotnet/api/system.string.isinterned?view=netframework-4.7.2#System_String_IsInterned_System_String_

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