C# | String vs StringBuilder

23 Jan, 2019
StringBuilder is used to represent a mutable string of characters. Mutable means the string which can be changed. So String objects are immutable but StringBuilder is the mutable string type. It will not create a new modified instance of the current string object but do the modifications in the existing string object. The complete functionality of StringBuilder is provided by StringBuilder class which is present in System.Text namespace.
Need of the StringBuilder: As stated above that the String class objects are immutable which means that if the user will modify any string object it will result into the creation of a new string object. It makes the use of string costly. So when the user needs the repetitive operations on the string then the need of StringBuilder come into existence. It provides the optimized way to deal with the repetitive and multiple string manipulation operations.
Example:
// C# program to demonstrate the// difference between String,// StringBuilderusing System;using System.Text;using System.Collections;  class GFG {    // Concatenates to String    public static void concat1(String s1)    {          // taking a string which        // is to be Concatenate        String st = "forGeeks";          // using String.Concat method        // you can also replace it with        // s1 = s1 + "forgeeks";        s1 = String.Concat(s1, st);    }      // Concatenates to StringBuilder    public static void concat2(StringBuilder s2)    {          // using Append method        // of StringBuilder class        s2.Append("forGeeks");    }      // Main Method    public static void Main(String[] args)    {          String s1 = "Geeks";        concat1(s1); // s1 is not changed        Console.WriteLine("Using String Class: " + s1);          StringBuilder s2 = new StringBuilder("Geeks");        concat2(s2); // s2 is changed        Console.WriteLine("Using StringBuilder Class: " + s2);    }}
Output:
Using String Class: Geeks
Using StringBuilder Class: GeeksforGeeks

Explanation:
Use of concat1 Method: In this method, we are passing a string “Geeks” and performing “s1 = String.Concat(s1, st);” where st is “forGeeks” to be concatenated. The string passed from Main() is not changed, this is due to the fact that String is immutable. Altering the value of string creates another object and s1 in concat1() stores reference of the new string. But the references s1 in Main() and concat1() refer to different strings.
Use of concat2 Method: In this method, we are passing a string “Geeks” and performing “s2.Append(“forGeeks”)” which changes the actual value of the string (in Main) to “GeeksforGeeks”. This is due to the simple fact that StringBuilder is mutable and hence changes its value.
When to use which one:
If a string is going to remain constant throughout the program, then use String class object because a String object is immutable.
If a string can change (example: lots of logic and operations in the construction of the string) then using a StringBuilder is the best option.
Converting String to StringBuilder:To convert a String class object to StringBuilder class object, just pass the string object to the StringBuilder class constructor.
Example:
// C# program to demonstrate the// conversion from String to StringBuilder.using System;using System.Text;  class GFG {      // Main Method    public static void Main(String[] args)    {        String str = "Geeks";          // conversion from String object        // to StringBuilder        StringBuilder sbl = new StringBuilder(str);        sbl.Append("ForGeeks");        Console.WriteLine(sbl);    }}
Output:
GeeksForGeeks
 Converting StringBuilder to String:This conversions can be performed using ToString() method.
Example:
// C# program to demonstrate the// conversion from String to StringBuilderusing System;using System.Text;  class GFG {      // Main Method    public static void Main(String[] args)    {          StringBuilder sbdr = new StringBuilder("Builder");          // conversion from StringBuilder        // object to String using ToString method        String str1 = sbdr.ToString();          Console.Write("StringBuilder object to String: ");        Console.WriteLine(str1);    }}
Output:
StringBuilder object to String: Builder
CSharp-string
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