When should I use a struct rather than a class in C

Question

When should you use struct and not class in C   My conceptual model is that structs are used in times when the item is merely a collection of value types   A way to logically hold them all together into a cohesive whole   I came across these rules here    A struct should represent a single value  A struct should have a memory footprint less than 16 bytes  A struct should not be changed after creation    Do these rules work  What does a struct mean semantically

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With the exception of the valuetypes that are used directly by the runtime and various others for PInvoke purposes  you should only use valuetypes in 2 scenarios    When you need copy semantics  When you need automatic initialization  normally in arrays of these types

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Structs are good for atomic representation of data  where the said data can be copied multiple times by the code  Cloning an object is in general more expensive than copying a struct  as it involves allocating the memory  running the constructor and deallocating garbage collection when done with it

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NET supports value types and reference types  in Java  you can define only reference types   Instances of reference types get allocated in the managed heap and are garbage collected when there are no outstanding references to them  Instances of value types  on the other hand  are allocated in the stack  and hence allocated memory is reclaimed as soon as their scope ends  And of course  value types get passed by value  and reference types by reference  All C  primitive data types  except for System String  are value types   When to use struct over class   In C   structs are value types  classes are reference types  You can create value types  in C   using the enum keyword and the struct keyword  Using a value type instead of a reference type will result in fewer objects on the managed heap  which results in lesser load on the garbage collector  GC   less frequent GC cycles  and consequently better performance  However  value types have their downsides too  Passing around a big struct is definitely costlier than passing a reference  that s one obvious problem  The other problem is the overhead associated with boxing unboxing  In case you re wondering what boxing unboxing mean  follow these links for a good explanation on boxing and unboxing  Apart from performance  there are times when you simply need types to have value semantics  which would be very difficult  or ugly  to implement if reference types are all you have  You should use value types only  When you need copy semantics or need automatic initialization  normally in arrays of these types

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A struct is a value type   If you assign a struct to a new variable  the new variable will contain a copy of the original   public struct IntStruct       public int Value  get  set       Excecution of the following results in 5 instances of the struct stored in memory   var struct1   new IntStruct     Value   0       original var struct2   struct1      A copy is made var struct3   struct2      A copy is made var struct4   struct3      A copy is made var struct5   struct4      A copy is made     NOTE  A  copy  will occur when you pass a struct into a method parameter     To avoid the  copy   use the ref keyword      Although structs are designed to use less system resources    than classes   If used incorrectly  they could use significantly more    A class is a reference type   When you assign a class to a new variable  the variable contains a reference to the original class object   public class IntClass       public int Value  get  set       Excecution of the following results in only one instance of the class object in memory   var class1   new IntClass     Value   0    var class2   class1      A reference is made to class1 var class3   class2      A reference is made to class1 var class4   class3      A reference is made to class1 var class5   class4      A reference is made to class1     Structs may increase the likelihood of a code mistake   If a value object is treated like a mutable reference object  a developer may be surprised when changes made are unexpectedly lost   var struct1   new IntStruct     Value   0    var struct2   struct1  struct2 Value   1     At this point  a developer may be surprised when     struct1 Value is 0 and not 1

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Here is a basic rule    If all member fields are value types create a struct  If any one member field is a reference type  create a class  This is because the reference type field will need the  heap allocation anyway    Exmaples  public struct MyPoint        public int X     Value Type     public int Y     Value Type    public class MyPointWithName        public int X     Value Type     public int Y     Value Type     public string Name     Reference Type

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The C  struct is a lightweight alternative to a class  It can do almost the same as a class  but it s less  expensive  to use a struct rather than a class  The reason for this is a bit technical  but to sum up  new instances of a class is placed on the heap  where newly instantiated structs are placed on the stack  Furthermore  you are not dealing with references to structs  like with classes  but instead you are working directly with the struct instance  This also means that when you pass a struct to a function  it is by value  instead of as a reference  There is more about this in the chapter about function parameters   So  you should use structs when you wish to represent more simple data structures  and especially if you know that you will be instantiating lots of them  There are lots of examples in the  NET framework  where Microsoft has used structs instead of classes  for instance the Point  Rectangle and Color struct

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I use structs for packing or unpacking any sort of binary communication format  That includes reading or writing to disk  DirectX vertex lists  network protocols  or dealing with encrypted compressed data   The three guidelines you list haven t been useful for me in this context  When I need to write out four hundred bytes of stuff in a Particular Order  I m gonna define a four-hundred-byte struct  and I m gonna fill it with whatever unrelated values it s supposed to have  and I m going to set it up whatever way makes the most sense at the time   Okay  four hundred bytes would be pretty strange-- but back when I was writing Excel files for a living  I was dealing with structs of up to about forty bytes all over  because that s how big some of the BIFF records ARE

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A class is a reference type  When an object of the class is created  the variable to which the object is assigned holds only a reference to that memory  When the object reference is assigned to a new variable  the new variable refers to the original object  Changes made through one variable are reflected in the other variable because they both refer to the same data  A struct is a value type  When a struct is created  the variable to which the struct is assigned holds the struct s actual data  When the struct is assigned to a new variable  it is copied  The new variable and the original variable therefore contain two separate copies of the same data  Changes made to one copy do not affect the other copy  In general  classes are used to model more complex behavior  or data that is intended to be modified after a class object is created  Structs are best suited for small data structures that contain primarily data that is not intended to be modified after the struct is created   Classes and Structs  C  Programming Guide

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Following are the rules defined at Microsoft website     CONSIDER defining a struct instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects    AVOID defining 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  It will not have to be boxed frequently  for further reading

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MYTH  1  STRUCTS ARE LIGHTWEIGHT CLASSES  This myth comes in a variety of forms  Some people believe that value types can   t or shouldn   t have methods or other significant behavior   they should be used as simple data transfer types  with just public fields or simple properties  The DateTime type is a good counterexample to this  it makes sense for it to be a value type  in terms of being a fundamental unit like a number or a character  and it also makes sense for it to be able to perform calculations based on its value  Looking at things from the other direction  data transfer types should often be reference types anyway   the decision should be based on the desired value or reference type semantics  not the simplicity of the type   Other people believe that value types are    lighter    than reference types in terms of performance  The truth is that in some cases value types are more performant    they don   t require garbage collection unless they   re boxed  don   t have the type identification overhead  and don   t require dereferencing  for example  But in other ways  reference types are more performant   parameter passing  assigning values to variables  returning values  and similar operations only require 4 or 8 bytes to becopied  depending on whether you   re running the 32-bit or 64-bit CLR  rather than copying all the data  Imagine if ArrayList were somehow a    pure    value type  and passing an ArrayList expression to a method involved copying all its data  In almost all cases  performance isn   t really determined by this sort of decision anyway  Bottlenecks are almost never where you think they   ll be  and before you make a design decision based on performance  you should measure the different options   It   s worth noting that the combination of the two beliefs doesn   t work either  It doesn   t matter how many methods a type has  whether it   s a class or a struct    the memory taken per instance isn   t affected   There   s a cost in terms of the memory taken up for the code itself  but that   s incurred once rather than for each instance     MYTH  2  REFERENCE TYPES LIVE ON THE HEAP  VALUE TYPES LIVE ON THE STACK  This one is often caused by laziness on the part of the person repeating it  The first part is correct   an instance of a reference type is always created on the heap  It   s the second part that causes problems  As I   ve already noted  a variable   s value lives wherever it   s declared  so if you have a class with an instance variable of type int  that variable   s value for any given object will always be where the rest of the data for the object is   on the heap  Only local variables  variables declared within methods  and method parameters live on the stack  In C  2 and later  even some local variables don   t really live on the stack  as you   ll see when we look at anonymous methods in chapter 5  ARE THESE CONCEPTS RELEVANT NOW  It   s arguable that if you   re writing managed code  you should let the runtime worry about how memory is best used  Indeed  the language specification makes no guarantees about what lives where  a future runtime may be able to create some objects on the stack if it knows it can get away with it  or the C  compiler could generate code that hardly uses the stack at all  The next myth is usually just a terminology issue    MYTH  3  OBJECTS ARE PASSED BY REFERENCE IN C  BY DEFAULT  This is probably the most widely propagated myth  Again  the people who make this claim often  though not always  know how C  actually behaves  but they don   t know what    pass by reference    really means  Unfortunately  this is confusing for people who do know what it means   The formal definition of pass by reference is relatively complicated  involving l-values and similar computer-science terminology  but the important thing is that if you pass a variable by reference  the method you   re calling can change the value of the caller   s variable by changing its parameter value  Now  remember that the value of a reference type variable is the reference  not the object itself  You can change the contents of the object that a parameter refers to without the parameter itself being passed by reference  For instance  the following method changes the contents of the StringBuilder object in question  but the caller   s expression will still refer to the same object as before   void AppendHello StringBuilder builder        builder Append  hello        When this method is called  the parameter value  a reference to a StringBuilder  is passed by value  If you were to change the value of the builder variable within the method   for example  with the statement builder   null    that change wouldn   t be seen by the caller  contrary to the myth   It   s interesting to note that not only is the    by reference    bit of the myth inaccurate  but so is the    objects are passed    bit  Objects themselves are never passed  either by reference or by value  When a reference type is involved  either the variable is passed by reference or the value of the argument  the reference  is passed by value  Aside from anything else  this answers the question of what happens when null is used as a by-value argument   if objects were being passed around  that would cause issues  as there wouldn   t be an object to pass  Instead  the null reference is passed by value in the same way as any other reference would be   If this quick explanation has left you bewildered  you might want to look at my article     Parameter passing in C       http   mng bz otVt   which goes into much more detail   These myths aren   t the only ones around  Boxing and unboxing come in for their fair share of misunderstanding  which I   ll try to clear up next    Reference  C  in Depth 3rd Edition by Jon Skeet

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Nah - I don t entirely agree with the rules  They are good guidelines to consider with performance and standardization  but not in light of the possibilities   As you can see in the responses  there are a lot of creative ways to use them  So  these guidelines need to just be that  always for the sake of performance and efficiency   In this case  I use classes to represent real world objects in their larger form  I use structs to represent smaller objects that have more exact uses  The way you said it   a more cohesive whole   The keyword being cohesive  The classes will be more object oriented elements  while structs can have some of those characteristics  though on a smaller scale  IMO   I use them a lot in Treeview and Listview tags where common static attributes can be accessed very quickly  I have always struggled to get this info another way  For example  in my database applications  I use a Treeview where I have Tables  SPs  Functions  or any other objects  I create and populate my struct  put it in the tag  pull it out  get the data of the selection and so forth  I wouldn t do this with a class   I do try and keep them small  use them in single instance situations  and keep them from changing  It s prudent to be aware of memory  allocation  and performance  And testing is so necessary

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The source referenced by the OP has some credibility    but what about Microsoft - what is the stance on struct usage  I sought some extra learning from Microsoft  and here is what I found        Consider defining a structure instead of a class if instances of the   type are small and commonly short-lived or are commonly embedded in   other objects       Do not define a structure unless the type has all of the following characteristics            It logically represents a single value  similar to primitive types  integer  double  and so on     It has an instance size smaller than 16 bytes    It is immutable    It will not have to be boxed frequently         Microsoft consistently violates those rules  Okay   2 and  3 anyway  Our beloved dictionary has 2 internal structs      StructLayout LayoutKind Sequential       default for structs private struct Entry     lt Tkey  TValue gt            View code at  Reference Source     Serializable  StructLayout LayoutKind Sequential   public struct Enumerator        IEnumerator lt KeyValuePair lt TKey  TValue gt  gt   IDisposable       IDictionaryEnumerator  IEnumerator           View code at  Reference Source      Reference Source  The  JonnyCantCode com  source got 3 out of 4 - quite forgivable since  4 probably wouldn t be an issue  If you find yourself boxing a struct  rethink your architecture     Let s look at why Microsoft would use these structs    Each struct  Entry and Enumerator  represent single values  Speed Entry is never passed as a parameter outside of the Dictionary class  Further investigation shows that in order to satisfy implementation of IEnumerable  Dictionary uses the Enumerator struct which it copies every time an enumerator is requested    makes sense  Internal to the Dictionary class  Enumerator is public because Dictionary is enumerable and must have equal accessibility to the IEnumerator interface implementation - e g  IEnumerator getter      Update - In addition  realize that when a struct implements an interface - as Enumerator does - and is cast to that implemented type  the struct becomes a reference type and is moved to the heap  Internal to the Dictionary class  Enumerator is still a value type  However  as soon as a method calls GetEnumerator    a reference-type IEnumerator is returned   What we don t see here is any attempt or proof of requirement to keep structs immutable or maintaining an instance size of only 16 bytes or less      Nothing in the structs above is declared readonly - not immutable Size of these struct could be well over 16 bytes Entry has an undetermined lifetime  from Add    to Remove    Clear    or garbage collection     And      4  Both structs store TKey and TValue  which we all know are quite capable of being reference types  added bonus info   Hashed keys notwithstanding  dictionaries are fast in part because instancing a struct is quicker than a reference type  Here  I have a Dictionary lt int  int gt  that stores 300 000 random integers with sequentially incremented keys      Capacity  312874   MemSize   2660827 bytes   Completed Resize   5ms   Total time to fill   889ms   Capacity   number of elements available before the internal array must be resized     MemSize   determined by serializing the dictionary into a MemoryStream and getting a byte length  accurate enough for our purposes      Completed Resize   the time it takes to resize the internal array from 150862 elements to 312874 elements  When you figure that each element is sequentially copied via Array CopyTo    that ain t too shabby     Total time to fill  admittedly skewed due to logging and an OnResize event I added to the source  however  still impressive to fill 300k integers while resizing 15 times during the operation  Just out of curiosity  what would the total time to fill be if I already knew the capacity  13ms    So  now  what if Entry were a class  Would these times or metrics really differ that much        Capacity   312874   MemSize   2660827 bytes   Completed Resize   26ms   Total time to fill   964ms   Obviously  the big difference is in resizing  Any difference if Dictionary is initialized with the Capacity  Not enough to be concerned with     12ms     What happens is  because Entry is a struct  it does not require initialization like a reference type  This is both the beauty and the bane of the value type  In order to use Entry as a reference type  I had to insert the following code          Added to satisfy initialization of entry elements --     this is where the extra time is spent resizing the Entry array        for  int i   0   i  lt  prime   i          destinationArray i    new Entry                                                                  The reason I had to initialize each array element of Entry as a reference type can be found at MSDN  Structure Design  In short      Do not provide a default constructor for a structure       If a structure defines a default constructor  when arrays of the   structure are created  the common language runtime automatically   executes the default constructor on each array element       Some compilers  such as the C  compiler  do not allow structures to   have default constructors    It is actually quite simple and we will borrow from Asimov s Three Laws of Robotics    The struct must be safe to use The struct must perform its function efficiently  unless this would violate rule  1 The struct must remain intact during its use unless its destruction is required to satisfy rule  1        what do we take away from this  in short  be responsible with the use of value types  They are quick and efficient  but have the ability to cause many unexpected behaviors if not properly maintained  i e  unintentional copies

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Use a struct when you want value semantics as opposed to reference semantics    Edit  Not sure why folks are downvoting this but this is a valid point  and was made before the op clarified his question  and it is the most fundamental basic reason for a struct   If you need reference semantics you need a class not a struct

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In addition to the  it is a value  answer  one specific scenario for using structs is when you know that you have a set of data that is causing garbage collection issues  and you have lots of objects  For example  a large list array of Person instances  The natural metaphor here is a class  but if you have large number of long-lived Person instance  they can end up clogging GEN-2 and causing GC stalls  If the scenario warrants it  one potential approach here is to use an array  not list  of Person structs  i e  Person    Now  instead of having millions of objects in GEN-2  you have a single chunk on the LOH  I m assuming no strings etc here - i e  a pure value without any references   This has very little GC impact   Working with this data is awkward  as the data is probably over-sized for a struct  and you don t want to copy fat values all the time  However  accessing it directly in an array does not copy the struct - it is in-place  contrast to a list indexer  which does copy   This means lots of work with indexes   int index       int id   peopleArray index  Id    Note that keeping the values themselves immutable will help here  For more complex logic  use a method with a by-ref parameter   void Foo ref Person person            Foo ref peopleArray index      Again  this is in-place - we have not copied the value   In very specific scenarios  this tactic can be very successful  however  it is a fairly advanced scernario that should be attempted only if you know what you are doing and why  The default here would be a class

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I made a small benchmark with BenchmarkDotNet to get a better understanding of  struct  benefit in numbers  I m testing looping through array  or list  of structs  or classes   Creating those arrays or lists is out of the benchmark s scope - it is clear that  class  is more heavy will utilize more memory  and will involve GC   So the conclusion is  be careful with LINQ and hidden structs boxing unboxing and using structs for microoptimizations strictly stay with arrays    P S  Another benchmark about passing struct class through call stack is there https   stackoverflow com a 47864451 506147   BenchmarkDotNet v0 10 8  OS Windows 10 Redstone 2  10 0 15063  Processor Intel Core i5-2500K CPU 3 30GHz  Sandy Bridge   ProcessorCount 4 Frequency 3233542 Hz  Resolution 309 2584 ns  Timer TSC    Host    Clr 4 0 30319 42000  64bit RyuJIT-v4 7 2101 1   Clr      Clr 4 0 30319 42000  64bit RyuJIT-v4 7 2101 1   Core      NET Core 4 6 25211 01  64bit RyuJIT             Method    Job   Runtime        Mean       Error      StdDev         Min         Max      Median   Rank    Gen 0   Allocated   ----------------  -----  --------  ----------  ----------  ----------  ----------  ----------  ----------  -----  -------  ----------      TestListClass    Clr       Clr    5 599 us   0 0408 us   0 0382 us    5 561 us    5 689 us    5 583 us      3        -         0 B     TestArrayClass    Clr       Clr    2 024 us   0 0102 us   0 0096 us    2 011 us    2 043 us    2 022 us      2        -         0 B     TestListStruct    Clr       Clr    8 427 us   0 1983 us   0 2204 us    8 101 us    9 007 us    8 374 us      5        -         0 B    TestArrayStruct    Clr       Clr    1 539 us   0 0295 us   0 0276 us    1 502 us    1 577 us    1 537 us      1        -         0 B      TestLinqClass    Clr       Clr   13 117 us   0 1007 us   0 0892 us   13 007 us   13 301 us   13 089 us      7   0 0153        80 B     TestLinqStruct    Clr       Clr   28 676 us   0 1837 us   0 1534 us   28 441 us   28 957 us   28 660 us      9        -        96 B      TestListClass   Core      Core    5 747 us   0 1147 us   0 1275 us    5 567 us    5 945 us    5 756 us      4        -         0 B     TestArrayClass   Core      Core    2 023 us   0 0299 us   0 0279 us    1 990 us    2 069 us    2 013 us      2        -         0 B     TestListStruct   Core      Core    8 753 us   0 1659 us   0 1910 us    8 498 us    9 110 us    8 670 us      6        -         0 B    TestArrayStruct   Core      Core    1 552 us   0 0307 us   0 0377 us    1 496 us    1 618 us    1 552 us      1        -         0 B      TestLinqClass   Core      Core   14 286 us   0 2430 us   0 2273 us   13 956 us   14 678 us   14 313 us      8   0 0153        72 B     TestLinqStruct   Core      Core   30 121 us   0 5941 us   0 5835 us   28 928 us   30 909 us   30 153 us     10        -        88 B     Code    RankColumn  MinColumn  MaxColumn  StdDevColumn  MedianColumn       ClrJob  CoreJob       HtmlExporter  MarkdownExporter       MemoryDiagnoser      public class BenchmarkRef               public class C1                       public string Text1              public string Text2              public string Text3                     public struct S1                       public string Text1              public string Text2              public string Text3                     List lt C1 gt  testListClass   new List lt C1 gt             List lt S1 gt  testListStruct   new List lt S1 gt             C1   testArrayClass          S1   testArrayStruct          public BenchmarkRef                         for int i 0 i lt 1000 i                                  testListClass Add new C1    Text1  i ToString    Text2 null  Text3  i ToString                       testListStruct Add new S1   Text1   i ToString    Text2   null  Text3   i ToString                                 testArrayClass   testListClass ToArray                testArrayStruct   testListStruct ToArray                        Benchmark          public int TestListClass                         var x   0              foreach var i in testListClass                                x    i Text1 Length   i Text3 Length                            return x                      Benchmark          public int TestArrayClass                         var x   0              foreach  var i in testArrayClass                                x    i Text1 Length   i Text3 Length                            return x                      Benchmark          public int TestListStruct                         var x   0              foreach  var i in testListStruct                                x    i Text1 Length   i Text3 Length                            return x                      Benchmark          public int TestArrayStruct                         var x   0              foreach  var i in testArrayStruct                                x    i Text1 Length   i Text3 Length                            return x                      Benchmark          public int TestLinqClass                         var x   testListClass Select i  gt  i Text1 Length   i Text3 Length  Sum                return x                      Benchmark          public int TestLinqStruct                         var x   testListStruct Select i   gt  i Text1 Length   i Text3 Length  Sum                return x

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First  Interop scenarios or when you need to specify the memory layout  Second  When the data is almost the same size as a reference pointer anyway

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Structure types in C  or other  net languages are generally used to hold things that should behave like fixed-sized groups of values   A useful aspect of structure types is that the fields of a structure-type instance can be modified by modifying the storage location in which it is held  and in no other way   It s possible to code a structure in such a way that the only way to mutate any field is to construct a whole new instance and then use a struct assignment to mutate all the fields of the target by overwriting them with values from the new instance  but unless a struct provides no means of creating an instance where its fields have non-default values  all of its fields will be mutable if and if the struct itself is stored in a mutable location   Note that it s possible to design a structure type so that it will essentially behave like a class type  if the structure contains a private class-type field  and redirects its own members to that of the wrapped class object   For example  a PersonCollection might offer properties SortedByName and SortedById  both of which hold an  immutable  reference to a PersonCollection  set in their constructor  and implement GetEnumerator by calling either creator GetNameSortedEnumerator or creator GetIdSortedEnumerator   Such structs would behave much like a reference to a PersonCollection  except that their GetEnumerator methods would be bound to different methods in the PersonCollection   One could also have a structure wrap a portion of an array  e g  one could define an ArrayRange lt T gt  structure which would hold a T   called Arr  an int Offset  and an int Length  with an indexed property which  for an index idx in the range 0 to Length-1  would access Arr idx Offset     Unfortunately  if foo is a read-only instance of such a structure  current compiler versions won t allow operations like foo 3   4  because they have no way to determine whether such operations would attempt to write to fields of foo   It s also possible to design a structure to behave a like a value type which holds a variable-sized collection  which will appear to be copied whenever the struct is  but the only way to make that work is to ensure that no object to which the struct holds a reference will ever be exposed to anything which might mutate it   For example  one could have an array-like struct which holds a private array  and whose indexed  put  method creates a new array whose content is like that of the original except for one changed element   Unfortunately  it can be somewhat difficult to make such structs perform efficiently   While there are times that struct semantics can be convenient  e g  being able to pass an array-like collection to a routine  with the caller and callee both knowing that outside code won t modify the collection  may be better than requiring both caller and callee to defensively copy any data they re given   the requirement that class references point to objects that will never be mutated is often a pretty severe constraint

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I think a good first approximation is  never    I think a good second approximation is  never    If you are desperate for perf  consider them  but then always measure

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Briefly  use struct if   your object properties fields do not need to be changed  I mean you just want to give them an initial value and then read them   properties and fields in your object are value type and they are not so large    If that s the case  you can take advantage of structs for a better performance and optimized memory allocation as they use only stacks rather than both stacks and heaps  in classes

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I do not agree with the rules given in the original post  Here are my rules   1  You use structs for performance when stored in arrays   see also When are structs the answer    2  You need them in code passing structured data to from C C    3  Do not use structs unless you need them     They behave different from  normal objects   reference types  under assignment and when passing as arguments  which can lead to unexpected behavior  this is particularly dangerous if the person looking at the code does not know they are dealing with a struct  They cannot be inherited  Passing structs as arguments is more expensive than classes

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Whenever you    don t need polymorphism  want value semantics  and  want to avoid heap allocation and the associated garbage collection overhead      The caveat  however  is that structs  arbitrarily large  are more expensive to pass around than class references  usually one machine word   so classes could end up being faster in practice

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I was just dealing with Windows Communication Foundation  WCF  Named Pipe and I did notice that it does make sense to use Structs in order to ensure that exchange of data is of value type instead of reference type

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Struct can be used to improve garbage collection performance   While you usually don t have to worry about GC performance  there are scenarios where it can be a killer   Like large caches in low latency applications   See this post for an example   http   00sharp wordpress com 2013 07 03 a-case-for-the-struct

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Structures are in most ways like classes objects  Structure can contain functions  members and can be inherited  But structures are in C  used just for data holding  Structures does take less RAM than classes and are easier for garbage collector to collect  But when you use functions in your structure  then compiler actually takes that structure very similarly as class object  so if you want something with functions  then use class object

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You need to use a  struct  in situations where you want to explicitly specify memory layout using the StructLayoutAttribute - typically for PInvoke   Edit  Comment points out that you can use class or struct with StructLayoutAttribute and that is certainly true  In practice  you would typically use a struct - it is allocated on the stack vs the heap which makes sense if you are just passing an argument to an unmanaged method call

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I rarely use a struct for things  But that s just me  It depends whether I need the object to be nullable or not   As stated in other answers  I use classes for real-world objects  I also have the mindset of structs are used for storing small amounts of data

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My rule is   1  Always use class   2  If there is any performance issue  I try to change some class to struct depending on the rules which  IAbstract mentioned  and then do a test to see if these changes can improve performance

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From the C  Language specification      1 7 Structs       Like classes  structs are data structures that can contain data members and function members  but unlike classes  structs are   value types and do not require heap allocation  A variable of a struct   type directly stores the data of the struct  whereas a variable of a   class type stores a reference to a dynamically allocated object    Struct types do not support user-specified inheritance  and all struct   types implicitly inherit from type object       Structs are particularly useful for small data structures that have   value semantics  Complex numbers  points in a coordinate system  or   key-value pairs in a dictionary are all good examples of structs  The   use of structs rather than classes for small data structures can make   a large difference in the number of memory allocations an application   performs  For example  the following program creates and initializes   an array of 100 points  With Point implemented as a class  101   separate objects are instantiated   one for the array and one each for   the 100 elements    class Point      public int x  y      public Point int x  int y          this x   x        this y   y          class Test      static void Main           Point   points   new Point 100         for  int i   0  i  lt  100  i    points i    new Point i  i            An alternative is to make Point a struct   struct Point      public int x  y      public Point int x  int y          this x   x        this y   y           Now  only one object is instantiated   the one for the array   and the Point instances are stored in-line in the array   Struct constructors are invoked with the new operator  but that does not imply that memory is being allocated  Instead of dynamically allocating an object and returning a reference to it  a struct constructor simply returns the struct value itself  typically in a temporary location on the stack   and this value is then copied as necessary   With classes  it is possible for two variables to reference the same object and thus possible for operations on one variable to affect the object referenced by the other variable  With structs  the variables each have their own copy of the data  and it is not possible for operations on one to affect the other  For example  the output produced by the following code fragment depends on whether Point is a class or a struct   Point a   new Point 10  10   Point b   a  a x   20  Console WriteLine b x     If Point is a class  the output is 20 because a and b reference the same object  If Point is a struct  the output is 10 because the assignment of a to b creates a copy of the value  and this copy is unaffected by the subsequent assignment to a x   The previous example highlights two of the limitations of structs  First  copying an entire struct is typically less efficient than copying an object reference  so assignment and value parameter passing can be more expensive with structs than with reference types  Second  except for ref and out parameters  it is not possible to create references to structs  which rules out their usage in a number of situations

[c#] When should I use a struct rather than a class in C#?

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