What is the difference between a mutable and immutable string in C

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String is immutable  i e  strings cannot be altered  When you alter a string  by adding to it for example   you are actually creating a new string   But StringBuilder is not immutable  rather  it is mutable   so if you have to alter a string many times  such as multiple concatenations  then use StringBuilder

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In  NET  System String  aka string  is a immutable object  That means when you create an object you can not change it s value afterwards  You can only recreate a immutable object    System Text StringBuilder is mutable equivalent of System String and you can chane its value  For Example   class Program       static void Main string   args                 System String str    inital value           str     nsecond value           str     nthird value            StringBuilder sb   new StringBuilder            sb Append  initial value            sb AppendLine  second value            sb AppendLine  third value              Generates following MSIL   If you investigate the code  You will see that whenever you chane an object of System String you are actually creating new one  But in System Text StringBuilder whenever you change the value of text you dont recreate the object    method private hidebysig static void  Main string   args  cil managed      entrypoint      Code size       62  0x3e     maxstack  2    locals init   0  string str              1  class  mscorlib System Text StringBuilder sb    IL 0000   nop   IL 0001   ldstr       inital value    IL 0006   stloc 0   IL 0007   ldstr        nsecond value    IL 000c   stloc 0   IL 000d   ldstr        nthird value    IL 0012   stloc 0   IL 0013   newobj     instance void  mscorlib System Text StringBuilder   ctor     IL 0018   stloc 1   IL 0019   ldloc 1   IL 001a   ldstr       initial value    IL 001f   callvirt   instance class  mscorlib System Text StringBuilder  mscorlib System Text StringBuilder  Append string    IL 0024   pop   IL 0025   ldloc 1   IL 0026   ldstr       second value    IL 002b   callvirt   instance class  mscorlib System Text StringBuilder  mscorlib System Text StringBuilder  AppendLine string    IL 0030   pop   IL 0031   ldloc 1   IL 0032   ldstr       third value    IL 0037   callvirt   instance class  mscorlib System Text StringBuilder  mscorlib System Text StringBuilder  AppendLine string    IL 003c   pop   IL 003d   ret      end of method Program  Main

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The data value may not be changed   Note  The variable value may be changed  but the original immutable data value was discarded and a new data value was created in memory

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Here is the example for Immutable string and Mutable string builder          Console WriteLine  Mutable String Builder            Console WriteLine                                                  Console WriteLine            StringBuilder sb   new StringBuilder  Very Good Morning            Console WriteLine sb ToString             sb Remove 0  5           Console WriteLine sb ToString              Console WriteLine             Console WriteLine  Immutable String            Console WriteLine                                                  Console WriteLine            string s    Very Good Morning           Console WriteLine s           s Substring 0  5           Console WriteLine s           Console ReadLine

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To clarify there is no such thing as a mutable string in C   or  NET in general    Other langues support mutable strings  string which can change  but the  NET framework does not   So the correct answer to your question is ALL string are immutable in C    string has a specific meaning   string  lowercase keyword is merely a shortcut for an object instantiated from System String class   All objects created from string class are ALWAYS immutable   If you want a mutable representation of text then you need to use another class like StringBuilder  StringBuilder allows you to iteratively build a collection of  words  and then convert that to a string  once again immutable

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None  actually  The String class is mutable   unsafe       string foo   string Copy  I am immutable         fixed  char  pChar   foo                char  pFoo   pChar           pFoo 5                 pFoo 6                    Console WriteLine foo       I am   mutable       This kind of logic is done all the time in the String and StringBuilder classes  actually  They just allocate a new string each time you call Concat  Substring  etc  and use pointer arithmetic to copy over to the new string  Strings just don t mutate themselves  hence why they are considered  immutable      By the way  do not attempt this with string literals or you will badly mess up your program   string bar    I am a string     fixed  char  pChar   bar        char  pBar   pChar       pBar 2            string baz    I am a string     Console WriteLine baz       I  m a string     This is because string literals are interned in the desktop  NET Framework  in other words  bar and baz point to the exact same string  so mutating one will mutate the other  This is all fine and dandy though if you re using an unmanaged platform like WinRT  which lacks string interning

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An object is mutable if  once created  its state can be changed by calling various operations on it  otherwise it is immutable   Immutable String  In C   and  NET  a string is represented by class System String  The string keyword is an alias for this class    The System String class is immutable  i e once created its state cannot be altered   So all the operations you perform on a string like Substring  Remove  Replace  concatenation using     operator etc will create a new string and return it   See the following program for demonstration -  string str    mystring   string newString   str Substring 2   Console WriteLine newString   Console WriteLine str     This will print  string  and  mystring  respectively   For the benefits of immutability and why string are immutable check Why  NET String is immutable    Mutable String  If you want to have a string which you want to modify often you can use the StringBuilder class  Operations on a StringBuilder instance will modify the same object    For more advice on when to use StringBuilder refer to When to use StringBuilder

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From http   yassershaikh com what-is-the-difference-between-strings-and-stringbuilder-in-c-net   Short Answer   String is immutable     whereas StringBuilder is mutable   What does that mean   Wiki says   In object-oriented  an immutable object is an object whose state cannot be modified after it is created  This is in contrast to a mutable object  which can be modified after it is created   From the StringBuilder Class documentation   The String object is immutable  Every time you use one of the methods in the System String class  you create a new string object in memory  which requires a new allocation of space for that new object   In situations where you need to perform repeated modifications to a string  the overhead associated with creating a new String object can be costly   The System Text StringBuilder class can be used when you want to modify a string without creating a new object  For example  using the StringBuilder class can boost performance when concatenating many strings together in a loop

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Mutable and immutable are English words meaning  quot can change quot  and  quot cannot change quot  respectively   The meaning of the words is the same in the IT context  i e   a mutable string can be changed  and an immutable string cannot be changed   The meanings of these words are the same in C     NET as in other programming languages   environments  though  obviously  the names of the types may differ  as may other details   For the record   String is the standard C     Net immutable string type StringBuilder is the standard C     Net mutable string type  To  quot effect a change quot  on a string represented as a C  String  you actually create a new String object   The original String is not changed     because it is unchangeable  In most cases it is better to use String because it is easier reason about them  e g  you don t need to consider the possibility that some other thread might  quot change my string quot   However  when you need to construct or modify a string using a sequence of operations  it may be more efficient to use a StringBuilder   An example is when you are concatenating many string fragments to form a large string   If you do this as a sequence of String concatenations  you copy O N 2  characters  where N is the number of component strings  If use a StringBuilder you only copy O N  characters    And finally  for those people who assert that a StringBuilder is not a string because it is not immutable  the Microsoft documentation describes StringBuilder thus    quot Represents a mutable string of characters  This class cannot be inherited  quot

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String in C  is immutable  If you concatenate it with any string  you are actually making a new string  that is new string object   But StringBuilder creates mutable string

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Strings are mutable because  NET use string pool behind the scene  It means    string name    My Country   string name2    My Country     Both name  and name2 are referring to same memory location from string pool  Now suppose you want to change name2 to    name2    My Loving Country     It will look in to string pool for the string  My Loving Country   if found you will get the reference of it other wise new string    My Loving Country  will be created in string pool and name2 will get reference of it  But it this whole process  My Country  was not changed because other variable like name is still using it  And that is the reason why string are IMMUTABLE   StringBuilder works in different manner and don t use string pool  When we create any instance of StringBuilder    var address    new StringBuilder 500     It allocate memory chunk of size 500 bytes for this instance and all operation just modify this memory location and this memory not shared with any other object  And that is the reason why StringBuilder is MUTABLE   I hope it will help

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Immutable     When you do some operation on a object  it creates a new object hence state is not modifiable as in case of string   Mutable   When you perform some operation on a object  object itself modified no new obect created as in case of StringBuilder

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in implementation detail   CLR2 s System String is mutable     StringBuilder Append calling String AppendInplace   private method   CLR4 s System String is immutable     StringBuilder have Char array with chunking

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All string objects are immutable in C   Objects of the class string  once created  can never represent any value other than the one they were constructed with  All operations that seem to  change  a string instead produce a new one  This is inefficient with memory  but extremely useful with regard to being able to trust that a string won t change out form under you- because as long as you don t change your reference  the string being referred to will never change   A mutable object  by contrast  has data fields that can be altered  One or more of its methods will change the contents of the object  or it has a Property that  when written into  will change the value of the object   If you have a mutable object- the most similar one to String is StringBuffer- then you have to make a copy of it if you want to be absolutely sure it won t change out from under you  This is why mutable objects are dangerous to use as keys into any form of Dictionary or set- the objects themselves could change  and the data structure would have no way of knowing  leading to corrupt data that would  eventually  crash your program   However  you can change its contents- so it s much  much more memory efficient than making a complete copy because you wanted to change a single character  or something similar   Generally  the right thing to do is use mutable objects while you re creating something  and immutable objects once you re done  This applies to objects that have immutable forms  of course  most of the collections don t  It s often useful to provide read-only forms of collections  though  which is the equivalent of immutable  when sending the internal state of your collection to other contexts- otherwise  something could take that return value  do something to it  and corrupt your data

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StringBuilder is a better option to concat a huge data string because the StringBuilder is a mutable string type and StringBuilder object is an immutable type  that means  StringBuilder never create a new instance of object while concat the string   If we are using string instead of StringBuilder to achieve for concatenation then it will create new instance in memory every time

[c#] What is the difference between a mutable and immutable string in C#?

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