Why NET String is immutable

Question

As we all know  String is immutable  What are the reasons for String being immutable and the introduction of StringBuilder class as mutable

User · Accepted Answer

Instances of immutable types are inherently thread-safe, since no thread can modify it, the risk of a thread modifying it in a way that interferes with another is removed (the reference itself is a different matter).
Similarly, the fact that aliasing can't produce changes (if x and y both refer to the same object a change to x entails a change to y) allows for considerable compiler optimisations.
Memory-saving optimisations are also possible. Interning and atomising being the most obvious examples, though we can do other versions of the same principle. I once produced a memory saving of about half a GB by comparing immutable objects and replacing references to duplicates so that they all pointed to the same instance (time-consuming, but a minute's extra start-up to save a massive amount of memory was a performance win in the case in question). With mutable objects that can't be done.
No side-effects can come from passing an immutable type as a method to a parameter unless it is out or ref (since that changes the reference, not the object). A programmer therefore knows that if string x = "abc" at the start of a method, and that doesn't change in the body of the method, then x == "abc" at the end of the method.
Conceptually, the semantics are more like value types; in particular equality is based on state rather than identity. This means that "abc" == "ab" + "c". While this doesn't require immutability, the fact that a reference to such a string will always equal "abc" throughout its lifetime (which does require immutability) makes uses as keys where maintaining equality to previous values is vital, much easier to ensure correctness of (strings are indeed commonly used as keys).
Conceptually, it can make more sense to be immutable. If we add a month onto Christmas, we haven't changed Christmas, we have produced a new date in late January. It makes sense therefore that Christmas.AddMonths(1) produces a new DateTime rather than changing a mutable one. (Another example, if I as a mutable object change my name, what has changed is which name I am using, "Jon" remains immutable and other Jons will be unaffected.
Copying is fast and simple, to create a clone just return this. Since the copy can't be changed anyway, pretending something is its own copy is safe.
[Edit, I'd forgotten this one]. Internal state can be safely shared between objects. For example, if you were implementing list which was backed by an array, a start index and a count, then the most expensive part of creating a sub-range would be copying the objects. However, if it was immutable then the sub-range object could reference the same array, with only the start index and count having to change, with a very considerable change to construction time.

In all, for objects which don't have undergoing change as part of their purpose, there can be many advantages in being immutable. The main disadvantage is in requiring extra constructions, though even here it's often overstated (remember, you have to do several appends before StringBuilder becomes more efficient than the equivalent series of concatenations, with their inherent construction).

It would be a disadvantage if mutability was part of the purpose of an object (who'd want to be modeled by an Employee object whose salary could never ever change) though sometimes even then it can be useful (in a many web and other stateless applications, code doing read operations is separate from that doing updates, and using different objects may be natural - I wouldn't make an object immutable and then force that pattern, but if I already had that pattern I might make my "read" objects immutable for the performance and correctness-guarantee gain).

Copy-on-write is a middle ground. Here the "real" class holds a reference to a "state" class. State classes are shared on copy operations, but if you change the state, a new copy of the state class is created. This is more often used with C++ than C#, which is why it's std:string enjoys some, but not all, of the advantages of immutable types, while remaining mutable.

User · Answer

Strings are passed as reference types in  NET   Reference types place a pointer on the stack  to the actual instance that resides on the managed heap   This is different to Value types  who hold their entire instance on the stack   When a value type is passed as a parameter  the runtime creates a copy of the value on the stack and passes that value into a method   This is why integers must be passed with a  ref  keyword to return an updated value   When a reference type is passed  the runtime creates a copy of the pointer on the stack   That copied pointer still points to the original instance of the reference type   The string type has an overloaded   operator which creates a copy of itself  instead of a copy of the pointer - making it behave more like a value type   However  if only the pointer was copied  a second string operation could accidently overwrite the value of a private member of another class causing some pretty nasty results   As other posts have mentioned  the StringBuilder class allows for the creation of strings without the GC overhead

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There are five common ways by which a class data store data that cannot be modified outside the storing class  control   As value-type primitives By holding a freely-shareable reference to class object whose properties of interest are all immutable By holding a reference to a mutable class object that will never be exposed to anything that might mutate any properties of interest As a struct  whether  mutable  or  immutable   all of whose fields are of types  1- 4  not  5   By holding the only extant copy of a reference to an object whose properties can only be mutated via that reference    Because strings are of variable length  they cannot be value-type primitives  nor can their character data be stored in a struct   Among the remaining choices  the only one which wouldn t require that strings  character data be stored in some kind of immutable object would be  5   While it would be possible to design a framework around option  5  that choice would require that any code which wanted a copy of a string that couldn t be changed outside its control would have to make a private copy for itself   While it hardly be impossible to do that  the amount of extra code required to do that  and the amount of extra run-time processing necessary to make defensive copies of everything  would far outweigh the slight benefits that could come from having string be mutable  especially given that there is a mutable string type  System Text StringBuilder  which accomplishes 99  of what could be accomplished with a mutable string

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Strings are not really immutable  They are just publicly immutable  It means you cannot modify them from their public interface  But in the inside the are actually mutable   If you don t believe me look at the String Concat definition using reflector   The last lines are     int length   str0 Length  string dest   FastAllocateString length   str1 Length   FillStringChecked dest  0  str0   FillStringChecked dest  length  str1   return dest    As you can see the FastAllocateString returns an empty but allocated string and then it is modified by FillStringChecked  Actually the FastAllocateString is an extern method and the FillStringChecked is unsafe so it uses pointers to copy the bytes   Maybe there are better examples but this is the one I have found so far

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Strings and other concrete objects are typically expressed as immutable objects to improve readability and runtime efficiency  Security is another  a process can t change your string and inject code into the string

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Immutable Strings also prevent concurrency-related issues

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string management is an expensive process  keeping strings immutable allows repeated strings to be reused  rather than re-created

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You never have to defensively copy immutable data   Despite the fact that you need to copy it to mutate it  often the ability to freely alias and never have to worry about unintended consequences of this aliasing can lead to better performance because of the lack of defensive copying

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Why are string types immutable in C      String is a reference type  so it is never copied  but passed by reference    Compare this to the C   std  string   object  which is not immutable   which   is passed by value  This means that if   you want to use a String as a key in a   Hashtable  you re fine in C    because   C   will copy the string to store the   key in the hashtable  actually   std  hash map  but still  for later   comparison  So even if you later   modify the std  string instance    you re fine  But in  Net  when you use   a String in a Hashtable  it will store   a reference to that instance  Now   assume for a moment that strings   aren t immutable  and see what   happens    1  Somebody inserts a value x with key  hello  into a Hashtable    2  The Hashtable computes the hash value for the String  and places a   reference to the string and the value   x in the appropriate bucket    3  The user modifies the String instance to be  bye     4  Now somebody wants the value in the hashtable associated with  hello   It   ends up looking in the correct bucket    but when comparing the strings it says    bye    hello   so no value is   returned    5  Maybe somebody wants the value  bye    bye  probably has a different   hash  so the hashtable would look in a   different bucket  No  bye  keys in   that bucket  so our entry still isn t   found       Making strings immutable means that   step 3 is impossible  If somebody   modifies the string he s creating a   new string object  leaving the old one   alone  Which means the key in the   hashtable is still  hello   and thus   still correct       So  probably among other things    immutable strings are a way to enable   strings that are passed by reference   to be used as keys in a hashtable or   similar dictionary object

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Just to throw this in  an often forgotten view is of security  picture this scenario if strings were mutable   string dir    C  SomePlainFolder      Kick off another thread GetDirectoryContents dir    void GetDirectoryContents string directory      if HasAccess directory          Here the other thread changed the string to  C  AllYourPasswords       return Contents directory         return null      You see how it could be very  very bad if you were allowed to mutate strings once they were passed

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Imagine being an OS working with a string that some other thread was modifying behind your back  How could you validate anything without making a copy

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Making strings immutable has many advantages   It provides automatic thread safety  and makes strings behave like an intrinsic type in a simple  effective manner   It also allows for extra efficiencies at runtime  such as allowing effective string interning to reduce resource usage   and has huge security advantages  since it s impossible for an third party API call to change your strings   StringBuilder was added in order to address the one major disadvantage of immutable strings - runtime construction of immutable types causes a lot of GC pressure and is inherently slow   By making an explicit  mutable class to handle this  this issue is addressed without adding unneeded complication to the string class

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Imagine you pass a mutable string to a function but don t expect it to be changed  Then what if the function changes that string  In C    for instance  you could simply do call-by-value  difference between std  string and std  string amp  parameter   but in C  it s all about references so if you passed mutable strings around every function could change it and trigger unexpected side effects   This is just one of various reasons  Performance is another one  interned strings  for example

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