Why can t I have abstract static methods in C

Question

I ve been working with providers a fair bit lately  and I came across an interesting situation where I wanted to have an abstract class that had an abstract static method  I read a few posts on the topic  and it sort of made sense  but is there a nice clear explanation

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We actually override static methods  in delphi   it s a bit ugly  but it works just fine for our needs   We use it so the classes can have a list of their available objects without the class instance  for example  we have a method that looks like this   class function AvailableObjects  string  override  begin   Result     Object1  Object2   end     It s ugly but necessary  this way we can instantiate just what is needed  instead of having all the classes instantianted just to search for the available objects   This was a simple example  but the application itself is a client-server application which has all the classes available in just one server  and multiple different clients which might not need everything the server has and will never need an object instance   So this is much easier to maintain than having one different server application for each client   Hope the example was clear

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Another respondent  McDowell  said that polymorphism only works for object instances   That should be qualified  there are languages that do treat classes as instances of a  Class  or  Metaclass  type   These languages do support polymorphism for both instance and class  static  methods   C   like Java and C   before it  is not such a language  the static keyword is used explicitly to denote that the method is statically-bound rather than dynamic virtual

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Here is a situation where there is definitely a need for inheritance for static fields and methods   abstract class Animal     protected static string   legs     static Animal         legs new string 0          public static void printLegs           foreach  string leg in legs          print leg                 class Human  Animal     static Human         legs new string     left leg    right leg            class Dog  Animal     static Dog         legs new string     left foreleg    right foreleg    left hindleg    right hindleg            public static void main       Dog printLegs      Human printLegs          what is the output    does each subclass get its own copy of the array  legs

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Static methods cannot be inherited or overridden  and that is why they can t be abstract  Since static methods are defined on the type  not the instance  of a class  they must be called explicitly on that type  So when you want to call a method on a child class  you need to use its name to call it  This makes inheritance irrelevant  Assume you could  for a moment  inherit static methods  Imagine this scenario  public static class Base       public static virtual int GetNumber     return 5       public static class Child1   Base       public static override int GetNumber     return 1       public static class Child2   Base       public static override int GetNumber     return 2       If you call Base GetNumber    which method would be called  Which value returned  It s pretty easy to see that without creating instances of objects  inheritance is rather hard  Abstract methods without inheritance are just methods that don t have a body  so can t be called

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The abstract methods are implicitly virtual  Abstract methods require an instance  but static methods do not have an instance  So  you can have a static method in an abstract class  it just cannot be static abstract  or abstract static

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To add to the previous explanations  static method calls are bound to a specific method at compile-time  which rather rules out polymorphic behavior

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This question is 12 years old but it still needs to be given a better answer  As few noted in the comments and contrarily to what all answers pretend it would certainly make sense to have static abstract methods in C   As philosopher Daniel Dennett put it  a failure of imagination is not an insight into necessity  There is a common mistake in not realizing that C  is not only an OOP language  A pure OOP perspective on a given concept leads to a restricted and in the current case misguided examination  Polymorphism is not only about subtying polymorphism  it also includes parametric polymorphism  aka generic programming  and C  has been supporting this for a long time now  Within this additional paradigm  abstract classes  and most types  are not only used to type instances  They can also be used as bounds for generic parameters  something that has been understood by users of certain languages  like for example Haskell  but also more recently Scala  Rust or Swift  for years  In this context you may want to do something like this  void Catch lt TAnimal gt    where TAnimal   Animal       string scientificName   TAnimal ScientificName     abstract static property     Console WriteLine   quot Let s catch some  scientificName  quot               And here the capacity to express static members that can be specialized by subclasses totally makes sense  Unfortunately C  does not allow abstract static members but I d like to propose a pattern that can emulate them reasonably well  This pattern is not perfect  it imposes some restrictions on inheritance  but as far as I can tell it is typesafe  The main idea is to associate an abstract companion class  here SpeciesFor lt TAnimal gt   to the one that should contain abstract members  here Animal   public abstract class SpeciesFor lt TAnimal gt  where TAnimal   Animal       public static SpeciesFor lt TAnimal gt  Instance   get                   abstract  quot static quot  members      public abstract string ScientificName   get                    public abstract class Animal          Now we would like to make this work  void Catch lt TAnimal gt    where TAnimal   Animal       string scientificName   SpeciesFor lt TAnimal gt  Instance ScientificName      Console WriteLine   quot Let s catch some  scientificName  quot               Of course we have two problems to solve   How do we allow and force an implementer of a subclass of Animal to associate a specific instance of SpeciesFor lt TAnimal gt  to this subclass  How does the property SpeciesFor lt TAnimal gt  Instance retrieve this information   Here is how we can solve 1  public abstract class Animal lt TSelf gt  where TSelf   Animal lt TSelf gt        private Animal                  public abstract class OfSpecies lt TSpecies gt    Animal lt TSelf gt          where TSpecies   SpeciesFor lt TSelf gt   new                 protected OfSpecies        base                                By making the constructor of Animal lt TSelf gt  private we make sure that all its subclasses are also subclasses of inner class Animal lt TSelf gt  OfSpecies lt TSpecies gt   So these subclasses must specify a TSpecies type that has a new   bound  For 2 we can provide the following implementation  public abstract class SpeciesFor lt TAnimal gt  where TAnimal   Animal lt TAnimal gt        private static SpeciesFor lt TAnimal gt   instance       public static SpeciesFor lt TAnimal gt  Instance   gt   instance     MakeInstance         private static SpeciesFor lt TAnimal gt  MakeInstance                 Type t   typeof TAnimal           while  true                        if  t IsConstructedGenericType                      amp  amp  t GetGenericTypeDefinition      typeof Animal lt  gt  OfSpecies lt  gt                    return  SpeciesFor lt TAnimal gt  Activator CreateInstance t GenericTypeArguments 1                t   t BaseType              if  t    null                  throw new InvalidProgramException                            abstract  quot static quot  members      public abstract string ScientificName   get                    How can we be sure that the reflection code inside MakeInstance   never throws  As we ve already said  almost all classes within the hierarchy of Animal lt TSelf gt  are also subclasses of Animal lt TSelf gt  OfSpecies lt TSpecies gt   So we know that for these classes a specific TSpecies must be provided  This type is also necessarily constructible thanks to constraint   new    But this still leaves abstract types like Animal lt Something gt  that have no associated species  Now we can convince ourself that the curiously recurring template pattern where TAnimal   Animal lt TAnimal gt  makes it impossible to write SpeciesFor lt Animal lt Something gt  gt  Instance as type Animal lt Something gt  is never a subtype of Animal lt Animal lt Something gt  gt   Et voil    public class CatSpecies   SpeciesFor lt Cat gt           overriden  quot static quot  members      public override string ScientificName   gt   quot Felis catus quot       public override Cat CreateInVivoFromDnaTrappedInAmber               public override Cat Clone Cat a              public override Cat Breed Cat a1  Cat a2             public class Cat   Animal lt Cat gt  OfSpecies lt CatSpecies gt           overriden members      public override string CuteName   get              public class DogSpecies   SpeciesFor lt Dog gt           overriden  quot static quot  members      public override string ScientificName   gt   quot Canis lupus familiaris quot       public override Dog CreateInVivoFromDnaTrappedInAmber               public override Dog Clone Dog a              public override Dog Breed Dog a1  Dog a2             public class Dog   Animal lt Dog gt  OfSpecies lt DogSpecies gt           overriden members      public override string CuteName   get              public class Program       public static void Main                 ConductCrazyScientificExperimentsWith lt Cat gt             ConductCrazyScientificExperimentsWith lt Dog gt             ConductCrazyScientificExperimentsWith lt Tyranosaurus gt             ConductCrazyScientificExperimentsWith lt Wyvern gt                    public static void ConductCrazyScientificExperimentsWith lt TAnimal gt            where TAnimal   Animal lt TAnimal gt                   Look Ma  No animal instance polymorphism                   TAnimal a2039   SpeciesFor lt TAnimal gt  Instance CreateInVivoFromDnaTrappedInAmber            TAnimal a2988   SpeciesFor lt TAnimal gt  Instance CreateInVivoFromDnaTrappedInAmber            TAnimal a0400   SpeciesFor lt TAnimal gt  Instance Clone a2988           TAnimal a9477   SpeciesFor lt TAnimal gt  Instance Breed a0400  a2039           TAnimal a9404   SpeciesFor lt TAnimal gt  Instance Breed a2988  a9477                    Console WriteLine               quot The confederation of mad scientists is happy to announce the birth  quot                  quot of  a9404 CuteName   our new  SpeciesFor lt TAnimal gt  Instance ScientificName   quot             A limitation of this pattern is that it is not possible  as far as I can tell  to extend the class hierarchy in a satifying manner  For example we cannot introduce an intermediary Mammal class associated to a MammalClass companion  Another is that it does not work for static members in interfaces which would be more flexible than abstract classes

User · Answer

Static methods are not instantiated as such  they re just available without an object reference   A call to a static method is done through the class name  not through an object reference  and the Intermediate Language  IL  code to call it will call the abstract method through the name of the class that defined it  not necessarily the name of the class you used   Let me show an example   With the following code   public class A       public static void Test                  public class B   A       If you call B Test  like this   class Program       static void Main string   args                B Test              Then the actual code inside the Main method is as follows    entrypoint  maxstack 8 L0000  nop  L0001  call void ConsoleApplication1 A  Test   L0006  nop  L0007  ret    As you can see  the call is made to A Test  because it was the A class that defined it  and not to B Test  even though you can write the code that way   If you had class types  like in Delphi  where you can make a variable referring to a type and not an object  you would have more use for virtual and thus abstract static methods  and also constructors   but they aren t available and thus static calls are non-virtual in  NET   I realize that the IL designers could allow the code to be compiled to call B Test  and resolve the call at runtime  but it still wouldn t be virtual  as you would still have to write some kind of class name there   Virtual methods  and thus abstract ones  are only useful when you re using a variable which  at runtime  can contain many different types of objects  and you thus want to call the right method for the current object you have in the variable  With static methods you need to go through a class name anyway  so the exact method to call is known at compile time because it can t and won t change   Thus  virtual abstract static methods are not available in  NET

[c#] Why can't I have abstract static methods in C#?

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