In C why is String a reference type that behaves like a value type

Question

A String is a reference type even though it has most of the characteristics of a value type such as being immutable and having    overloaded to compare the text rather than making sure they reference the same object   Why isn t string just a value type then

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Isn t just as simple as Strings are made up of characters arrays  I look at strings as character arrays    Therefore they are on the heap because the reference memory location is stored on the stack and points to the beginning of the array s memory location on the heap  The string size is not known before it is allocated    perfect for the heap    That is why a string is really immutable because when you change it even if it is of the same size the compiler doesn t know that and has to allocate a new array and assign characters to the positions in the array  It makes sense if you think of strings as a way that languages protect you from having to allocate memory on the fly  read C like programming

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In a very simple words any value which has a definite size can be treated as a value type

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The distinction between reference types and value types are basically a performance tradeoff in the design of the language  Reference types have some overhead on construction and destruction and garbage collection  because they are created on the heap  Value types on the other hand have overhead on method calls  if the data size is larger than a pointer   because the whole object is copied rather than just a pointer  Because strings can be  and typically are  much larger than the size of a pointer  they are designed as reference types  Also  as Servy pointed out  the size of a value type must be known at compile time  which is not always the case for strings   The question of mutability is a separate issue  Both reference types and value types can be either mutable or immutable  Value types are typically immutable though  since the semantics for mutable value types can be confusing   Reference types are generally mutable  but can be designed as immutable if it makes sense  Strings are defined as immutable because it makes certain optimizations possible  For example  if the same string literal occurs multiple times in the same program  which is quite common   the compiler can reuse the same object   So why is      overloaded to compare strings by text  Because it is the most useful semantics  If two strings are equal by text  they may or may not be the same object reference due to the optimizations  So comparing references are pretty useless  while comparing text are almost always what you want   Speaking more generally  Strings has what is termed value semantics  This is a more general concept than value types  which is a C  specific implementation detail  Value types have value semantics  but reference types may also have value semantics  When a type have value semantics  you can t really tell if the underlying implementation is a reference type or value type  so you can consider that an implementation detail

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It is not a value type because performance  space and time   would be terrible if it were a value type and its value had to be copied every time it were passed to and returned from methods  etc   It has value semantics to keep the world sane  Can you imagine how difficult it would be to code if  string s    hello   string t    hello   bool b    s    t     set b to be false  Imagine how difficult coding just about any application would be

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This is a late answer to an old question  but all other answers are missing the point  which is that  NET did not have generics until  NET 2 0 in 2005  String is a reference type instead of a value type because it was of crucial importance for Microsoft to ensure that strings could be stored in the most efficient way in non-generic collections  such as System Collections ArrayList  Storing a value-type in a non-generic collection requires a special conversion to the type object which is called boxing  When the CLR boxes a value type  it wraps the value inside a System Object and stores it on the managed heap  Reading the value from the collection requires the inverse operation which is called unboxing  Both boxing and unboxing have non-negligible cost  boxing requires an additional allocation  unboxing requires type checking  Some answers claim incorrectly that string could never have been implemented as a value type because its size is variable  Actually it is easy to implement string as a fixed-length data structure containing two fields  an integer for the length of the string  and a pointer to a char array  You can also use a Small String Optimization strategy on top of that  If generics had existed from day one I guess having string as a value type would probably have been a better solution  with simpler semantics  better memory usage and better cache locality  A List lt string gt  containing only small strings could have been a single contiguous block of memory

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Also  the way strings are implemented  different for each platform  and when you start stitching them together   Like using a StringBuilder   It allocats a buffer for you to copy into  once you reach the end  it allocates even more memory for you  in the hopes that if you do a large concatenation performance won t be hindered   Maybe Jon Skeet can help up out here

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Strings aren t value types since they can be huge  and need to be stored on the heap  Value types are  in all implementations of the CLR as of yet  stored on the stack  Stack allocating strings would break all sorts of things  the stack is only 1MB for 32-bit and 4MB for 64-bit  you d have to box each string  incurring a copy penalty  you couldn t intern strings  and memory usage would balloon  etc      Edit  Added clarification about value type storage being an implementation detail  which leads to this situation where we have a type with value sematics not inheriting from System ValueType  Thanks Ben

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It is mainly a performance issue   Having strings behave LIKE value type helps when writing code  but having it BE a value type would make a huge performance hit   For an in-depth look  take a peek at a nice article on strings in the  net framework

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How can you tell string is a reference type  I m not sure that it matters how it is implemented  Strings in C  are immutable precisely so that you don t have to worry about this issue

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Actually strings have very few resemblances to value types  For starters  not all value types are immutable  you can change the value of an Int32 all you want and it it would still be the same address on the stack   Strings are immutable for a very good reason  it has nothing to do with it being a reference type  but has a lot to do with memory management  It s just more efficient to create a new object when string size changes than to shift things around on the managed heap  I think you re mixing together value reference types and immutable objects concepts   As far as       Like you said      is an operator overload  and again it was implemented for a very good reason to make framework more useful when working with strings

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Not only strings are immutable reference types   Multi-cast delegates too  That is why it is safe to write  protected void OnMyEventHandler          delegate handler   this MyEventHandler       if  null    handler                 handler this  new EventArgs                I suppose that strings are immutable because this is the most safe method to work with them and allocate memory   Why they are not Value types  Previous authors are right about stack size etc  I would also add that making strings a reference types allow to save on assembly size when you use the same constant string in the program  If you define  string s1    my string     some code here string s2    my string     Chances are that both instances of  my string  constant will be allocated in your assembly only once    If you would like to manage strings like usual reference type  put the string inside a new StringBuilder string s   Or use MemoryStreams   If you are to create a library  where you expect a huge strings to be passed in your functions  either define a parameter as a StringBuilder or as a Stream

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At the risk of getting yet another mysterious down-vote   the fact that many mention the stack and memory with respect to value types and primitive types is because they must fit into a register in the microprocessor  You cannot push or pop something to from the stack if it takes more bits than a register has    the instructions are  for example  pop eax  -- because eax is 32 bits wide on a 32-bit system   Floating-point primitive types are handled by the FPU  which is 80 bits wide   This was all decided long before there was an OOP language to obfuscate the definition of primitive type and I assume that value type is a term that has been created specifically for OOP languages

[c#] In C#, why is String a reference type that behaves like a value type?

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