Difference Between String And StringBuilder In C#

Immutability of string object Means, If any of your operation on the string instance changes it’s value, it will end up in the creation of new instance in a different address location with modified value. Mutability of StringBuilder is just opposite for this.It won’t create new instance when their content changes as string does.Instead it makes the new changes in the same instance.

Mutability and Immutability of these two can be understood from following C# Program.

//for String Class
using System;
//for StringBuilder Class
using System.Text;

class Program
{
  static void Main(string[] args)
  {
    String str = "My first string was ";
    str += "Hello World";
    //Now str="My first string was Hello World"
    StringBuilder sbr = new StringBuilder("My Favourite Programming Font is ");
    sbr.Append("Inconsolata");
    //Now sbr="My Favourite Programming Font is Inconsolata"
    Console.ReadKey();
  }
}

in line 11 content of str changes, so new instance is created in a different memory location with new value as shown in above image.

Even Though line 14 changes the value of stringbuilder variable sbr it won’t create a new instead it will keep appending new strings to existing instance.see how it looks in terms of memory

Because of this behaviour of StringBuilder it also known as Mutable String.

I am not convinced

If you say I’m not convinced. let us check these behaviours using  C# code snippet. For that I am using C# class (in System.Runtime.Serialization Namespace). Actually it will return an unique integer value for instances that we created in our programs.With the help of this class we can check whether new instance is created or not for various operations on string and stringbuilder .Consider following program

using System;
using System.Text;
using System.Runtime.Serialization;

class Program
{
  static void Main(string[] args)
  {
    ObjectIDGenerator idGenerator = new ObjectIDGenerator();
    bool blStatus = new bool();
    //just ignore this blStatus Now.
    String str = "My first string was ";
    Console.WriteLine("str = {0}", str);
    Console.WriteLine("Instance Id : {0}", idGenerator.GetId(str, out blStatus));
    //here blStatus get True for new instace otherwise it will be false
    Console.WriteLine("this instance is new : {0}\n", blStatus);
    str += "Hello World";
    Console.WriteLine("str = {0}", str);
    Console.WriteLine("Instance Id : {0}", idGenerator.GetId(str, out blStatus));
    Console.WriteLine("this instance is new : {0}\n", blStatus);
    //Now str="My first string was Hello World"
    StringBuilder sbr = new StringBuilder("My Favourate Programming Font is ");
    Console.WriteLine("sbr = {0}", sbr);
    Console.WriteLine("Instance Id : {0}", idGenerator.GetId(sbr, out blStatus));
    Console.WriteLine("this instance is new : {0}\n", blStatus);
    sbr.Append("Inconsolata");
    Console.WriteLine("sbr = {0}", sbr);
    Console.WriteLine("Instance Id : {0}", idGenerator.GetId(sbr, out blStatus));
    Console.WriteLine("this instance is new : {0}\n", blStatus);
    //Now sbr="My Favourate Programming Font is Inconsolata"
    Console.ReadKey();
  }
}

           Let’s discuss performance difference between string and stringbuilder.The following code box will explain things in a better way. Before that if you don’t know how to measure execution time in C# you can read my article from here .

using System;
using System.Text;
using System.Diagnostics;

class Program
{
  static void Main(string[] args)
  {
    Stopwatch Mytimer = new Stopwatch();
    string str = string.Empty;
    Mytimer.Start();
    for (int i = 0; i < 10000; i++)
    {
        str += i.ToString();
    }
    Mytimer.Stop();
    Console.WriteLine("Time taken by string : {0}", Mytimer.Elapsed);
    StringBuilder sbr = new StringBuilder(string.Empty);
    //restart timer from zero
    Mytimer.Restart();
    for (int i = 0; i < 10000; i++)
    {
        sbr.Append(i.ToString());
    }
    Mytimer.Stop();
    Console.WriteLine("Time taken by stringbuilder : {0}", Mytimer.Elapsed);
    Console.ReadKey();
  }
}

Output of this program was

This output clearly shows their performance difference.StringBuilder is about 70X faster than String in my laptop. it might be different in your case but generally speaking stringbuilder gives 10x times speed than string.

One Last Thing

New Instance of string will be created only when it’s value changes

One more thing,If you do an operation on a string variable, Creation of new instance occurs only when it’s current value changes.try following code,

using System;
using System.Text;
using System.Runtime.Serialization;

class Program
{
  static void Main(string[] args)
  {
    ObjectIDGenerator idGenerator = new ObjectIDGenerator();
    bool blStatus = new bool();
    string str = "Fashion Fades,Style Remains Same";
    Console.WriteLine("initial state");
    Console.WriteLine("str = {0}", str);
    Console.WriteLine("instance id : {0}", idGenerator.GetId(str, out blStatus));
    Console.WriteLine("this is new instance : {0}", blStatus);
    //a series of operations that won't change value of str
    str += "";
    //try to replace character 'x' which is not present in str so no change
    str = str.Replace('x', 'Q');
    //trim removes whitespaces from both ends so no change
    str = str.Trim();
    str = str.Trim();
    Console.WriteLine("\nfinal state");
    Console.WriteLine("str = {0}", str);
    Console.WriteLine("instance id : {0}", idGenerator.GetId(str, out blStatus));
    Console.WriteLine("this is new instance : {0}", blStatus);
    Console.ReadKey();
  }
}

Output!

initial and final id is 1-means new instance is not created for these operation since it does not change the value of str. You may wonder about how compiler shows this much intelligence.

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