C#.NET – String Interning

If there are two identical string literals in one compilation unit then the code we generate ensures that only one string object is created by the CLR for all instances of that literal within the assembly. This process know as “string interning”

CLR keeps string storage by maintaining a table called the intern pool, which contains a single reference to each unique literal string declared or created programmatically in the program. Hence, an instance of a literal string with a particular value only exists once in the system

Advantage :

1. It decreases memory consumption 
2. It decreases time required to compare two strings 

Disadvantage :

1. After declaring new string literal, It needs to search all strings objects in memory to check if any one matches with in assembly

Example 1 :

                string str1=”christopher”;

                string str2=”christopher”;

    Console.WriteLine(str1 == str2); // true

    Console.WriteLine(Object.ReferenceEquals(str1,str2); // true

Here both strings (i.e. str1, str2) are having same identical text/literals. So it creates only one memory location by CLR and both strings will be pointed to same memory location (This process known as string intern).

Example 2:

                string str1=”christopher nolan”;

                string str2=”christopher” + “nolan”;

    Console.WriteLine(str1 == str2); // true

    Console.WriteLine(Object.ReferenceEquals(str1,str2); // true

Here both strings (i.e. str1, str2) are having same identical text/literals. So it creates only one memory location by CLR and both strings will be pointed to same memory location (This process known as string intern).

Example 3:

                string lastname=”nolan”

                string str1=”christopher nolan”;

                string str2=”christopher ” + lastname;

    Console.WriteLine(str1 == str2); // true

    Console.WriteLine(Object.ReferenceEquals(str1,str2); // false

Here second condition returns false, because we are assigning string object instead of string text/literal.

   If we change str2 assignment as follows

string str2=string.Intern(”christopher ” + lastname);

Console.WriteLine(Object.ReferenceEquals(str1,str2); // true

Now it returns true.

Example 4:

                string str1=”christopher nolan”;

                string str2=new StringBuilder().Append(“Christopher “).Append(“nolan”).ToString();

string str3=String.Intern(str2);

Console.WriteLine((object)str1 == (object)str2); // false

Console.WriteLine((object)str1 == (object)str3); // true

Here first condition returns false, because we are assigning string object instead of string text/literal.

Example 5:

                string str1=””;

                string str2=””;

StringBuilder sb = new StringBuilder().Append(String.Empty);

str2 = String.Intern(sb.ToString());

Console.WriteLine((object)str1 == (object)str2); // sometimes true, sometimes false

Regard to interning empty string,

.NET Framework 1.0, 1.1, and 3.5 SP1 returns true

.NET Framework 2.0 SP1, 3.0 returns false

Happy Coding :)