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

User · Answer

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<TAnimal>() where TAnimal : Animal
{
    string scientificName = TAnimal.ScientificName; // abstract static property
    Console.WriteLine($"Let's catch some {scientificName}");
    …
}

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<TAnimal>) to the one that should contain abstract members (here Animal):

public abstract class SpeciesFor<TAnimal> where TAnimal : Animal
{
    public static SpeciesFor<TAnimal> Instance { get { … } }

    // abstract "static" members

    public abstract string ScientificName { get; }
    
    …
}

public abstract class Animal { … }

Now we would like to make this work:

void Catch<TAnimal>() where TAnimal : Animal
{
    string scientificName = SpeciesFor<TAnimal>.Instance.ScientificName;
    Console.WriteLine($"Let's catch some {scientificName}");
    …
}

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<TAnimal> to this subclass?
How does the property SpeciesFor<TAnimal>.Instance retrieve this information?

Here is how we can solve 1:

public abstract class Animal<TSelf> where TSelf : Animal<TSelf>
{
    private Animal(…) {}
    
    public abstract class OfSpecies<TSpecies> : Animal<TSelf>
        where TSpecies : SpeciesFor<TSelf>, new()
    {
        protected OfSpecies(…) : base(…) { }
    }
    
    …
}

By making the constructor of Animal<TSelf> private we make sure that all its subclasses are also subclasses of inner class Animal<TSelf>.OfSpecies<TSpecies>. 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<TAnimal> where TAnimal : Animal<TAnimal>
{
    private static SpeciesFor<TAnimal> _instance;

    public static SpeciesFor<TAnimal> Instance => _instance ??= MakeInstance();

    private static SpeciesFor<TAnimal> MakeInstance()
    {
        Type t = typeof(TAnimal);
        while (true)
        {
            if (t.IsConstructedGenericType
                    && t.GetGenericTypeDefinition() == typeof(Animal<>.OfSpecies<>))
                return (SpeciesFor<TAnimal>)Activator.CreateInstance(t.GenericTypeArguments[1]);
            t = t.BaseType;
            if (t == null)
                throw new InvalidProgramException();
        }
    }

    // abstract "static" 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<TSelf> are also subclasses of Animal<TSelf>.OfSpecies<TSpecies>. 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<Something> that have no associated species. Now we can convince ourself that the curiously recurring template pattern where TAnimal : Animal<TAnimal> makes it impossible to write SpeciesFor<Animal<Something>>.Instance as type Animal<Something> is never a subtype of Animal<Animal<Something>>.

Et voilà:

public class CatSpecies : SpeciesFor<Cat>
{
    // overriden "static" members

    public override string ScientificName => "Felis catus";
    public override Cat CreateInVivoFromDnaTrappedInAmber() { … }
    public override Cat Clone(Cat a) { … }
    public override Cat Breed(Cat a1, Cat a2) { … }
}

public class Cat : Animal<Cat>.OfSpecies<CatSpecies>
{
    // overriden members

    public override string CuteName { get { … } }
}

public class DogSpecies : SpeciesFor<Dog>
{
    // overriden "static" members

    public override string ScientificName => "Canis lupus familiaris";
    public override Dog CreateInVivoFromDnaTrappedInAmber() { … }
    public override Dog Clone(Dog a) { … }
    public override Dog Breed(Dog a1, Dog a2) { … }
}

public class Dog : Animal<Dog>.OfSpecies<DogSpecies>
{
    // overriden members

    public override string CuteName { get { … } }
}

public class Program
{
    public static void Main()
    {
        ConductCrazyScientificExperimentsWith<Cat>();
        ConductCrazyScientificExperimentsWith<Dog>();
        ConductCrazyScientificExperimentsWith<Tyranosaurus>();
        ConductCrazyScientificExperimentsWith<Wyvern>();
    }
    
    public static void ConductCrazyScientificExperimentsWith<TAnimal>()
        where TAnimal : Animal<TAnimal>
    {
        // Look Ma! No animal instance polymorphism!
        
        TAnimal a2039 = SpeciesFor<TAnimal>.Instance.CreateInVivoFromDnaTrappedInAmber();
        TAnimal a2988 = SpeciesFor<TAnimal>.Instance.CreateInVivoFromDnaTrappedInAmber();
        TAnimal a0400 = SpeciesFor<TAnimal>.Instance.Clone(a2988);
        TAnimal a9477 = SpeciesFor<TAnimal>.Instance.Breed(a0400, a2039);
        TAnimal a9404 = SpeciesFor<TAnimal>.Instance.Breed(a2988, a9477);
        
        Console.WriteLine(
            "The confederation of mad scientists is happy to announce the birth " +
            $"of {a9404.CuteName}, our new {SpeciesFor<TAnimal>.Instance.ScientificName}.");
    }
}

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

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

User · Answer

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

User · Answer

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

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

User · Answer

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

User · Answer

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

User · Answer

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

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

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