What s the difference between struct and class in NET

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What s the difference between struct and class in  NET

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------------------------ ------------------------------------------------------------------------------------------------------ ---------------------------------------------------------------------------------------------------                                                                            Struct                                                                                                Class                                                  ------------------------ ------------------------------------------------------------------------------------------------------ ---------------------------------------------------------------------------------------------------    Type                     Value-type                                                                                             Reference-type                                                                                        Where                    On stack   Inline in containing type                                                                   On Heap                                                                                               Deallocation             Stack unwinds   containing type gets deallocated                                                       Garbage Collected                                                                                     Arrays                   Inline  elements are the actual instances of the value type                                            Out of line  elements are just references to instances of the reference type residing on the heap     Aldel Cost               Cheap allocation-deallocation                                                                          Expensive allocation-deallocation                                                                     Memory usage             Boxed when cast to a reference type or one of the interfaces they implement                            No boxing-unboxing                                                                                                             Unboxed when cast back to value type                                                                                                                                                                                                   Negative impact because boxes are objects that are allocated on the heap and are garbage-collected                                                                                                          Assignments              Copy entire data                                                                                       Copy the reference                                                                                    Change to an instance    Does not affect any of its copies                                                                      Affect all references pointing to the instance                                                        Mutability               Should be immutable                                                                                    Mutable                                                                                               Population               In some situations                                                                                     Majority of types in a framework should be classes                                                    Lifetime                 Short-lived                                                                                            Long-lived                                                                                            Destructor               Cannot have                                                                                            Can have                                                                                              Inheritance              Only from an interface                                                                                 Full support                                                                                          Polymorphism             No                                                                                                     Yes                                                                                                   Sealed                   Yes                                                                                                    When have sealed keyword                                                                              Constructor              Can not have explicit parameterless constructors                                                       Any constructor                                                                                       Null-assignments         When marked with nullable question mark                                                                Yes    When marked with nullable question mark in C  8                                                Abstract                 No                                                                                                     When have abstract keyword                                                                            Member Access Modifiers  public  private  internal                                                                              public  protected  internal  protected internal  private protected                                   ------------------------ ------------------------------------------------------------------------------------------------------ ---------------------------------------------------------------------------------------------------

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Events declared in a class have their    and -  access automatically locked via a lock this  to make them thread safe  static events are locked on the typeof the class   Events declared in a struct do not have their    and -  access automatically locked  A lock this  for a struct would not work since you can only lock on a reference type expression  Creating a struct instance cannot cause a garbage collection  unless the constructor directly or indirectly creates a reference type instance  whereas creating a reference type instance can cause garbage collection  A struct always has a built-in public default constructor   class DefaultConstructor       static void Eg                 Direct     yes   new   Direct       Always compiles OK         InDirect maybe   new InDirect       Compiles if constructor exists and is accessible                         This means that a struct is always instantiable whereas a class might not be since all its constructors could be private   class NonInstantiable       private NonInstantiable      OK                struct Direct       private Direct      Compile-time error                A struct cannot have a destructor  A destructor is just an override of object Finalize in disguise  and structs  being value types  are not subject to garbage collection   struct Direct        Direct         Compile-time error   class InDirect        InDirect         Compiles OK    And the CIL for  Indirect   looks like this    method family hidebysig virtual instance void         Finalize   cil managed                 end of method Indirect  Finalize  A struct is implicitly sealed  a class isn t   A struct can t be abstract  a class can   A struct can t call   base   in its constructor whereas a class with no explicit base class can   A struct can t extend another class  a class can   A struct can t declare protected members  for example  fields  nested types  a class can   A struct can t declare abstract function members  an abstract class can   A struct can t declare virtual function members  a class can   A struct can t declare sealed function members  a class can   A struct can t declare override function members  a class can   The one exception to this rule is that a struct can override the virtual methods of System Object  viz  Equals    and GetHashCode    and ToString

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Instances of classes are stored on the managed heap  All variables  containing  an instance are simply a reference to the instance on the heap  Passing an object to a method results in a copy of the reference being passed  not the object itself   Structures  technically  value types  are stored wherever they are used  much like a primitive type  The contents may be copied by the runtime at any time and without invoking a customised copy-constructor  Passing a value type to a method involves copying the entire value  again without invoking any customisable code   The distinction is made better by the C   CLI names   ref class  is a class as described first   value class  is a class as described second  The keywords  class  and  struct  as used by C  are simply something that must be learned

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A short summary of each   Classes Only    Can support inheritance Are reference  pointer  types The reference can be null Have memory overhead per new instance   Structs Only    Cannot support inheritance Are value types Are passed by value  like integers  Cannot have a null reference  unless Nullable is used  Do not have a memory overhead per new instance - unless  boxed    Both Classes and Structs    Are compound data types typically used to contain a few variables that have some logical relationship Can contain methods and events Can support interfaces

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Well  for starters  a struct is passed by value rather than by reference   Structs are good for relatively simple data structures  while classes have a lot more flexibility from an architectural point of view via polymorphism and inheritance   Others can probably give you more detail than I  but I use structs when the structure that I am going for is simple

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In addition to all differences described in the other answers    Structs cannot have an explicit parameterless constructor whereas a class can Structs cannot have destructors  whereas a class can Structs can t inherit from another struct or class whereas a class can inherit from another class   Both structs and classes can implement from an interface     If you are after a video explaining all the differences  you can check out Part 29 - C  Tutorial - Difference between classes and structs in C

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From Microsoft s Choosing Between Class and Struct         As a rule of thumb  the majority of types in a framework should be   classes  There are  however  some situations in which the   characteristics of a value type make it more appropriate to use   structs         CONSIDER a struct instead of a class          If instances of the type are small and commonly short-lived or are commonly embedded in other objects          X AVOID a struct unless the type has all of the following   characteristics          It logically represents a single value  similar to primitive types     int  double  etc       It has an instance size under 16 bytes     It is immutable   cannot be changed    It will not have to be boxed frequently

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Every variable or field of a primitive value type or structure type holds a unique instance of that type  including all its fields  public and private   By contrast  variables or fields of reference types may hold null  or may refer to an object  stored elsewhere  to which any number of other references may also exist  The fields of a struct will be stored in the same place as the variable or field of that structure type  which may be either on the stack or may be part of another heap object    Creating a variable or field of a primitive value type will create it with a default value  creating a variable or field of a structure type will create a new instance  creating all fields therein in the default manner   Creating a new instance of a reference type will start by creating all fields therein in the default manner  and then running optional additional code depending upon the type   Copying one variable or field of a primitive type to another will copy the value  Copying one variable or field of structure type to another will copy all the fields  public and private  of the former instance to the latter instance  Copying one variable or field of reference type to another will cause the latter to refer to the same instance as the former  if any    It s important to note that in some languages like C    the semantic behavior of a type is independent of how it is stored  but that isn t true of  NET  If a type implements mutable value semantics  copying one variable of that type to another copies the properties of the first to another instance  referred to by the second  and using a member of the second to mutate it will cause that second instance to be changed  but not the first  If a type implements mutable reference semantics  copying one variable to another and using a member of the second to mutate the object will affect the object referred to by the first variable  types with immutable semantics do not allow mutation  so it doesn t matter semantically whether copying creates a new instance or creates another reference to the first   In  NET  it is possible for value types to implement any of the above semantics  provided that all of their fields can do likewise  A reference type  however  can only implement mutable reference semantics or immutable semantics  value types with fields of mutable reference types are limited to either implementing mutable reference semantics or weird hybrid semantics

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As previously mentioned  Classes are reference type while Structs are value types with all the consequences   As a thumb of rule Framework Design Guidelines recommends using Structs instead of classes if     It has an instance size under 16 bytes It logically represents a single value  similar to primitive types  int  double  etc   It is immutable It will not have to be boxed frequently

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Structure vs Class  A structure is a value type so it is stored on the stack  but a class is a reference type and is stored on the heap   A structure doesn t support inheritance  and polymorphism  but a class supports both   By default  all the struct members are public but class members are by default private in nature   As a structure is a value type  we can t assign null to a struct object  but it is not the case for a class

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In  NET  there are two categories of types  reference types and value types   Structs are value types and classes are reference types   The general difference is that a reference type lives on the heap  and a value type lives inline  that is  wherever it is your variable or field is defined   A variable containing a value type contains the entire value type value  For a struct  that means that the variable contains the entire struct  with all its fields   A variable containing a reference type contains a pointer  or a reference to somewhere else in memory where the actual value resides   This has one benefit  to begin with    value types always contains a value reference types can contain a null-reference  meaning that they don t refer to anything at all at the moment   Internally  reference types are implemented as pointers  and knowing that  and knowing how variable assignment works  there are other behavioral patterns    copying the contents of a value type variable into another variable  copies the entire contents into the new variable  making the two distinct  In other words  after the copy  changes to one won t affect the other copying the contents of a reference type variable into another variable  copies the reference  which means you now have two references to the same somewhere else storage of the actual data  In other words  after the copy  changing the data in one reference will appear to affect the other as well  but only because you re really just looking at the same data both places   When you declare variables or fields  here s how the two types differ    variable  value type lives on the stack  reference type lives on the stack as a pointer to somewhere in heap memory where the actual memory lives  though note Eric Lipperts article series  The Stack Is An Implementation Detail   class struct-field  value type lives completely inside the type  reference type lives inside the type as a pointer to somewhere in heap memory where the actual memory lives

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Besides the basic difference of access specifier  and few mentioned above I would like to add some of the major differences including few of the mentioned above with a code sample with output  which will give a more clear idea of the reference and value   Structs     Are value types and do not require heap allocation  Memory allocation is different and is stored in stack Useful for small data structures  Affect performance  when we pass value to method  we pass the entire data structure and all is passed to the stack  Constructor simply returns the struct value itself  typically in a temporary location on the stack   and this value is then copied as necessary The variables each have their own copy of the data  and it is not possible for operations on one to affect the other  Do not support user-specified inheritance  and they implicitly inherit from type object   Class    Reference Type value Stored in Heap Store a reference to a dynamically allocated object Constructors are invoked with the new operator  but that does not allocate memory on the heap Multiple variables may have a reference to the same object It is possible for operations on one variable to affect the object referenced by the other variable   Code Sample      static void Main string   args                  Struct         myStruct objStruct   new myStruct            objStruct x   10          Console WriteLine  Initial value of Struct Object is      objStruct x           Console WriteLine            methodStruct objStruct           Console WriteLine            Console WriteLine  After Method call value of Struct Object is      objStruct x           Console WriteLine               Class         myClass objClass   new myClass 10           Console WriteLine  Initial value of Class Object is      objClass x           Console WriteLine            methodClass objClass           Console WriteLine            Console WriteLine  After Method call value of Class Object is      objClass x           Console Read              static void methodStruct myStruct newStruct                newStruct x   20          Console WriteLine  Inside Struct Method            Console WriteLine  Inside Method value of Struct Object is      newStruct x             static void methodClass myClass newClass                newClass x   20          Console WriteLine  Inside Class Method            Console WriteLine  Inside Method value of Class Object is      newClass x             public struct myStruct               public int x          public myStruct int xCons                        this x   xCons                      public class myClass               public int x          public myClass int xCons                        this x   xCons                    Output  Initial value of Struct Object is  10  Inside Struct Method Inside Method value of Struct Object is  20  After Method call value of Struct Object is  10  Initial value of Class Object is  10  Inside Class Method Inside Method value of Class Object is  20  After Method call value of Class Object is  20  Here you can clearly see the difference between call by value and call by reference

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Structs are the actual value - they can be empty but never null   This is true  however also note that as of  NET 2 structs support a Nullable version and C  supplies some syntactic sugar to make it easier to use   int  value   null  value    1

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I   visualizations  and here I ve created a one to show the basic differences between structs and classes      For more information look below    Classes and structs  official documentation   Choosing Between Class and Struct  official documentation

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Difference between Structs and Classes   Structs are value type whereas Classes are reference type  Structs are stored on the stack whereas Classes are stored on the heap  Value types hold their value in memory where they are declared  but reference type holds a reference to an object memory  Value types destroyed immediately after the scope is lost whereas reference type only the variable destroy after the scope is lost  The object is later destroyed by the garbage collector  When you copy struct into another struct  a new copy of that struct gets created modified of one struct won t affect the value of the other struct  When you copy a class into another class  it only copies the reference variable  Both the reference variable point to the same object on the heap  Change to one variable will affect the other reference variable  Structs can not have destructors  but classes can have destructors  Structs can not have explicit parameterless constructors whereas classes can  Structs don t support inheritance  but classes do  Both support inheritance from an interface  Structs are sealed type

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To add to the other answers  there is one fundamental difference that is worth noting  and that is how the data is stored within arrays as this can have a major effect on performance     With a struct  the array contains the instance of the struct With a class  the array contains a pointer to an instance of the class elsewhere in memory   So an array of structs looks like this in memory   struct  struct  struct  struct  struct  struct  struct  struct   Whereas an array of classes looks like this   pointer  pointer  pointer  pointer  pointer  pointer  pointer  pointer   With an array of classes  the values you re interested in are not stored within the array  but elsewhere in memory   For a vast majority of applications this difference doesn t really matter  however  in high performance code this will affect locality of data within memory and have a large impact on the performance of the CPU cache  Using classes when you could should have used structs will massively increase the number of cache misses on the CPU   The slowest thing a modern CPU does is not crunching numbers  it s fetching data from memory  and an L1 cache hit is many times faster than reading data from RAM   Here s some code you can test  On my machine  iterating through the class array takes  3x longer than the struct array       private struct PerformanceStruct               public int i1          public int i2             private class PerformanceClass               public int i1          public int i2             private static void DoTest                 var structArray   new PerformanceStruct 100000000           var classArray   new PerformanceClass structArray Length            for  var i   0  i  lt  structArray Length  i                          structArray i    new PerformanceStruct                classArray i    new PerformanceClass                       long total   0          var sw   new Stopwatch            sw Start            for  var loops   0  loops  lt  100  loops            for  var i   0  i  lt  structArray Length  i                          total    structArray i  i1   structArray i  i2                     sw Stop            Console WriteLine   Struct Time   sw ElapsedMilliseconds             sw   new Stopwatch            sw Start            for  var loops   0  loops  lt  100  loops            for  var i   0  i  lt  classArray Length  i                          total    classArray i  i1   classArray i  i2                     Console WriteLine   Class Time   sw ElapsedMilliseconds

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There is one interesting case of  class vs struct  puzzle - situation when you need to return several results from the method  choose which to use  If you know the  ValueTuple story - you know that ValueTuple  struct  was added because it should  be more effective then Tuple  class   But what does it mean in numbers  Two tests  one is struct class that have 2 fields  other with struct class that have 8 fields  with dimension more then 4 - class should become more effective then struct in terms processor ticks  but of course GC load also should be considered    P S  Another benchmark for specific case  sturct or class with collections  is there  https   stackoverflow com a 45276657 506147  BenchmarkDotNet v0 10 10  OS Windows 10 Redstone 2  1703  Creators Update   10 0 15063 726  Processor Intel Core i5-2500K CPU 3 30GHz  Sandy Bridge   ProcessorCount 4 Frequency 3233540 Hz  Resolution 309 2586 ns  Timer TSC  NET Core SDK 2 0 3    Host     NET Core 2 0 3  Framework 4 6 25815 02   64bit RyuJIT   Clr       NET Framework 4 7  CLR 4 0 30319 42000   64bit RyuJIT-v4 7 2115 0   Core      NET Core 2 0 3  Framework 4 6 25815 02   64bit RyuJIT               Method    Job   Runtime       Mean       Error      StdDev        Min        Max     Median   Rank    Gen 0   Allocated   ------------------  -----  --------  ---------  ----------  ----------  ---------  ---------  ---------  -----  -------  ----------     TestStructReturn    Clr       Clr   17 57 ns   0 1960 ns   0 1834 ns   17 25 ns   17 89 ns   17 55 ns      4   0 0127        40 B      TestClassReturn    Clr       Clr   21 93 ns   0 4554 ns   0 5244 ns   21 17 ns   23 26 ns   21 86 ns      5   0 0229        72 B    TestStructReturn8    Clr       Clr   38 99 ns   0 8302 ns   1 4097 ns   37 36 ns   42 35 ns   38 50 ns      8   0 0127        40 B     TestClassReturn8    Clr       Clr   23 69 ns   0 5373 ns   0 6987 ns   22 70 ns   25 24 ns   23 37 ns      6   0 0305        96 B     TestStructReturn   Core      Core   12 28 ns   0 1882 ns   0 1760 ns   11 92 ns   12 57 ns   12 30 ns      1   0 0127        40 B      TestClassReturn   Core      Core   15 33 ns   0 4343 ns   0 4063 ns   14 83 ns   16 44 ns   15 31 ns      2   0 0229        72 B    TestStructReturn8   Core      Core   34 11 ns   0 7089 ns   1 4954 ns   31 52 ns   36 81 ns   34 03 ns      7   0 0127        40 B     TestClassReturn8   Core      Core   17 04 ns   0 2299 ns   0 2150 ns   16 68 ns   17 41 ns   16 98 ns      3   0 0305        96 B     Code test   using System  using System Text  using System Collections Generic  using BenchmarkDotNet Attributes  using BenchmarkDotNet Attributes Columns  using BenchmarkDotNet Attributes Exporters  using BenchmarkDotNet Attributes Jobs  using DashboardCode Routines Json   namespace Benchmark          Config typeof MyManualConfig         RankColumn  MinColumn  MaxColumn  StdDevColumn  MedianColumn       ClrJob  CoreJob       HtmlExporter  MarkdownExporter       MemoryDiagnoser      public class BenchmarkStructOrClass               static TestStruct testStruct   new TestStruct            static TestClass testClass   new TestClass            static TestStruct8 testStruct8   new TestStruct8            static TestClass8 testClass8   new TestClass8             Benchmark          public void TestStructReturn                         testStruct TestMethod                        Benchmark          public void TestClassReturn                         testClass TestMethod                         Benchmark          public void TestStructReturn8                         testStruct8 TestMethod                        Benchmark          public void TestClassReturn8                         testClass8 TestMethod                       public class TestStruct                       public int Number   5              public struct StructType lt T gt                                public T Instance                  public List lt string gt  List                             public int TestMethod                                 var s   Method1 1                   return s Instance                             private StructType lt int gt  Method1 int i                                return Method2   i                              private StructType lt int gt  Method2 int i                                return Method3   i                              private StructType lt int gt  Method3 int i                                return Method4   i                              private StructType lt int gt  Method4 int i                                var x   new StructType lt int gt                     x List   new List lt string gt                     x Instance     i                  return x                                   public class TestClass                       public int Number   5              public class ClassType lt T gt                                public T Instance                  public List lt string gt  List                             public int TestMethod                                 var s   Method1 1                   return s Instance                             private ClassType lt int gt  Method1 int i                                return Method2   i                              private ClassType lt int gt  Method2 int i                                return Method3   i                              private ClassType lt int gt  Method3 int i                                return Method4   i                              private ClassType lt int gt  Method4 int i                                var x   new ClassType lt int gt                     x List   new List lt string gt                     x Instance     i                  return x                                   public class TestStruct8                       public int Number   5              public struct StructType lt T gt                                public T Instance1                  public T Instance2                  public T Instance3                  public T Instance4                  public T Instance5                  public T Instance6                  public T Instance7                  public List lt string gt  List                             public int TestMethod                                 var s   Method1 1                   return s Instance1                             private StructType lt int gt  Method1 int i                                return Method2   i                              private StructType lt int gt  Method2 int i                                return Method3   i                              private StructType lt int gt  Method3 int i                                return Method4   i                              private StructType lt int gt  Method4 int i                                var x   new StructType lt int gt                     x List   new List lt string gt                     x Instance1     i                  return x                                   public class TestClass8                       public int Number   5              public class ClassType lt T gt                                public T Instance1                  public T Instance2                  public T Instance3                  public T Instance4                  public T Instance5                  public T Instance6                  public T Instance7                  public List lt string gt  List                             public int TestMethod                                 var s   Method1 1                   return s Instance1                             private ClassType lt int gt  Method1 int i                                return Method2   i                              private ClassType lt int gt  Method2 int i                                return Method3   i                              private ClassType lt int gt  Method3 int i                                return Method4   i                              private ClassType lt int gt  Method4 int i                                var x   new ClassType lt int gt                     x List   new List lt string gt                     x Instance1     i                  return x

User · Answer

Just to make it complete  there is another difference when using the Equals method  which is inherited by all classes and structures    Lets s say we have a class and a structure   class A    public int a  b    struct B    public int a  b      and in the Main method  we have 4 objects   static void Main    A c1   new A    c2   new A      c1 a   c1 b   c2 a   c2 b   1    B s1   new B    s2   new B      s1 a   s1 b   s2 a   s2 b   1      Then   s1 Equals s2     true s1 Equals c1     false c1 Equals c2     false c1    c2    false   So  structures are suited for numeric-like objects  like points  save x and y coordinates   And classes are suited for others  Even if 2 people have same name  height  weight     they are still 2 people

User · Answer

In  NET the struct and class declarations differentiate between reference types and value types   When you pass round a reference type there is only one actually stored   All the code that accesses the instance is accessing the same one   When you pass round a value type each one is a copy  All the code is working on its own copy   This can be shown with an example   struct MyStruct        string MyProperty   get  set       void ChangeMyStruct MyStruct input         input MyProperty    new value              Create value type MyStruct testStruct   new MyStruct   MyProperty    initial value       ChangeMyStruct testStruct       Value of testStruct MyProperty is still  initial value     - the method changed a new copy of the structure    For a class this would be different  class MyClass        string MyProperty   get  set       void ChangeMyClass MyClass input         input MyProperty    new value              Create reference type MyClass testClass   new MyClass   MyProperty    initial value      ChangeMyClass testClass       Value of testClass MyProperty is now  new value      - the method changed the instance passed    Classes can be nothing - the reference can point to a null   Structs are the actual value - they can be empty but never null  For this reason structs always have a default constructor with no parameters - they need a  starting value

[.net] What's the difference between struct and class in .NET?

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