Why can't I declare C# methods virtual and sta

问题:

I have a helper class that is just a bunch of static methods and would like to subclass the helper class. Some behavior is unique depending on the subclass so I would like to call a virtual method from the base class, but since all the methods are static I can't create a plain virtual method (need object reference in order to access virtual method).

Is there any way around this? I guess I could use a singleton.. HelperClass.Instance.HelperMethod() isn't so much worse than HelperClass.HelperMethod(). Brownie points for anyone that can point out some languages that support virtual static methods.

Edit: OK yeah I'm crazy. Google search results had me thinking I wasn't for a bit there.

回答1:

Virtual static methods don't make sense. If I call HelperClass.HelperMethod();, why would I expect some random subclass' method to be called? The solution really breaks down when you have 2 subclasses of HelperClass - which one would you use?

If you want to have overrideable static-type methods you should probably go with:

• A singleton, if you want the same subclass to be used globally.
• A tradition class hierarchy, with a factory or dependency injection, if you want different behavior in different parts of your application.

Choose whichever solution makes more sense in your situation.

回答2:

I don't think you are crazy. You just want to use what is impossible currently in .NET.

Your request for virtual static method would have so much sense if we are talking about generics. For example my future request for CLR designers is to allow me to write intereface like this:

public interface ISumable<T>
{
  static T Add(T left, T right);
}

and use it like this:

public T Aggregate<T>(T left, T right) where T : ISumable<T>
{
  return T.Add(left, right);
}

But it's impossible right now, so I'm doing it like this:

    public static class Static<T> where T : new()
    {
      public static T Value = new T();
    }

    public interface ISumable<T>
    {
      T Add(T left, T right);
    }

    public T Aggregate<T>(T left, T right) where T : ISumable<T>, new()
    {
      return Static<T>.Value.Add(left, right);
    }

回答3:

Indeed, this can be done in Delphi. An example:

type
  TForm1 = class(TForm)
    procedure FormShow(Sender: TObject);
  end;

  TTestClass = class
  public
    class procedure TestMethod(); virtual;
  end;

  TTestDerivedClass = class(TTestClass)
  public
    class procedure TestMethod(); override;
  end;

  TTestMetaClass = class of TTestClass;

var
  Form1: TForm1;

implementation

{$R *.dfm}

class procedure TTestClass.TestMethod();
begin
  Application.MessageBox('base', 'Message');
end;

class procedure TTestDerivedClass.TestMethod();
begin
  Application.MessageBox('descendant', 'Message');
end;


procedure TForm1.FormShow(Sender: TObject);
var
  sample: TTestMetaClass;
begin
  sample := TTestClass;
  sample.TestMethod;
  sample := TTestDerivedClass;
  sample.TestMethod;
end;

Quite interesting. I no longer use Delphi, but I recall being able to very easily create different types of controls on a custom designer canvas using the metaclass feature: the control class, eg. TButton, TTextBox etc. was a parameter, and I could call the appropriate constructor using the actual metaclass argument.

Kind of the poor man's factory pattern :)

回答4:

You can achieve the same effect by just having a regular static method and then shadow it with the new keyword

public class Base 
{
    //Other stuff

    public static void DoSomething()
    {
        Console.WriteLine("Base");
    }
}

public class SomeClass : Base
{
    public new static void DoSomething()
    {
        Console.WriteLine("SomeClass");
    }
}
public class SomeOtherClass : Base
{
}

Then you can call the methods like so

Base.DoSomething(); //Base
SomeClass.DoSomething(); //SomeClass
SomeOtherClass.DoSomething(); //Base

回答5:

I come from Delphi and this is a feature among many that I sorely miss in c#. Delphi would allow you to create typed type references and you could pass the type of a derived class wherever the type of a parent class was needed. This treatment of types as objects had powerful utility. In particular allowing run time determination of meta data. I am horribly mixing syntax here but in c# it would look something like:

    class Root {
       public static virtual string TestMethod() {return "Root"; }
    }
    TRootClass = class of TRoot; // Here is the typed type declaration

    class Derived : Root {
       public static overide string TestMethod(){ return "derived"; }
    }

   class Test {
        public static string Run(){
           TRootClass rc;
           rc = Root;
           Test(rc);
           rc = Derived();
           Test(rc);
        }
        public static Test(TRootClass AClass){
           string str = AClass.TestMethod();
           Console.WriteLine(str);
        }
    } 

would produce: Root derived

回答6:

a static method exists outside of an instance of a class. It cannot use any non-static data.

a virtual method will be "overwritten" by an overloaded function depending of the type of an instance.

so you have a clear contradiction between static and virtual.

This is not a problem of support, It is a concept.

Update: I was proven wrong here(see comments):

So I doubt you will find any OOP-Language which will support virtual static methods.

回答7:

You are not crazy. What you are referring to is called Late Static Binding; it's been recently added to PHP. There's a great thread that describes it - here: When would you need to use late static binding?

回答8:

I heard that Delphi suports something like this. It seems it does it by making classes object instances of a metaclass.

I've not seen it work, so I'm not sure that it works, or what's the point for that.

P.S. Please correct me if I'm wrong, since it's not my domain.

回答9:

Because a virtual method uses the defined type of the instantiated object to determine which implementation to execute, (as opposed to the declared type of the reference variable)

... and static, of course, is all about not caring if there's even an instantiated instance of the class at all...

So these are incompatible.

Bottom line, is if you want to change behavior based on which subclass an instance is, then the methods should have been virtual methods on the base class, not static methods.

But, as you already have these static methods, and now need to override them, you can solve your problem by this: Add virtual instance methods to the base class that simply delegate to the static methods, and then override those virtual instance wrapper methods (not the static ones) in each derived subclass, as appropriate...

回答10:

It is actually possible to combine virtual and static for a method or a member by using the keyword new instead of virtual.

Here is an example:

class Car
{
    public static int TyreCount = 4;
    public virtual int GetTyreCount() { return TyreCount; }
}
class Tricar : Car
{
    public static new int TyreCount = 3;
    public override int GetTyreCount() { return TyreCount; }
}

...

Car[] cc = new Car[] { new Tricar(), new Car() };
int t0 = cc[0].GetTyreCount(); // t0 == 3
int t1 = cc[1].GetTyreCount(); // t1 == 4

Obviously the TyreCount value could have been set in the overridden GetTyreCount method, but this avoids duplicating the value. It is possible to get the value both from the class and the class instance.

Now can someone find a really intelligent usage of that feature?

回答11:

Mart got it right with the 'new' keyword. I actually got here because I needed this type of functionality and Mart's solution works fine. In fact I took it one better and made my base class method abstract to force the programmer to supply this field.

My scenario was as follows:

I have a base class HouseDeed. Each House type is derived from HouseDeed must have a price.

Here is the partial base HouseDeed class:

public abstract class HouseDeed : Item
{
    public static int m_price = 0;
    public abstract int Price { get; }
    /* more impl here */
}

Now lets look at two derived house types:

public class FieldStoneHouseDeed : HouseDeed
{
    public static new int m_price = 43800;
    public override int Price { get { return m_price; } }
    /* more impl here */
}

and...

public class SmallTowerDeed : HouseDeed
{
    public static new int m_price = 88500;
    public override int Price { get { return m_price; } }
    /* more impl here */
}

As you can see I can access the price of the house via type SmallTowerDeed.m_price, and the instance new SmallTowerDeed().Price And being abstract, this mechanism nags the programmer into supplying a price for each new derived house type.

Someone pointed how 'static virtual' and 'virtual' are conceptually at odds with one another. I disagree. In this example, the static methods do not need access to the instance data, and so the requirements that (1) the price be available via the TYPE alone, and that (2) a price be supplied are met.

回答12:

An override method provides a new implementation of a member that is inherited from a base class. The method that is overridden by an override declaration is known as the overridden base method. The overridden base method must have the same signature as the override method. You cannot override a non-virtual or static method. The overridden base method must be virtual, abstract, or override.

An override declaration cannot change the accessibility of the virtual method. Both the override method and the virtual method must have the same access level modifier.

You cannot use the new, static, or virtual modifiers to modify an override method.

An overriding property declaration must specify exactly the same access modifier, type, and name as the inherited property, and the overridden property must be virtual, abstract, or override.

回答13:

There is a way to force an inheritance of "abstract static" methods from an abstract generic class. See as follow :

public abstract class Mother<T> where T : Mother<T>, new()
{
    public abstract void DoSomething();

    public static void Do()
    {
        (new T()).DoSomething();
    }

}

public class ChildA : Mother<ChildA>
{
    public override void DoSomething() { /* Your Code */ }
}

public class ChildB : Mother<ChildB>
{
    public override void DoSomething() { /* Your Code */ }
}

Example (using the previous Mother):

public class ChildA : Mother<ChildA>
{
    public override void DoSomething() { Console.WriteLine("42"); }
}

public class ChildB : Mother<ChildB>
{
    public override void DoSomething() { Console.WriteLine("12"); }
}

public class Program
{
    static void Main()
    {
        ChildA.Do();  //42
        ChildB.Do();  //12
        Console.ReadKey();
    }
}

It's not that great since you can inherit from only one abstract class and it will ask you to be lenient with your new() implementation.

More, I think it will be costly memory-wise depending on the size of your inherited classes. In case you have memory issue, you would have to set every properties/variables after your new in a public method which is an awful way to have default values.

回答14:

You can use the new keyword

namespace AspDotNetStorefront
{
    // This Class is need to override StudioOnlineCommonHelper Methods in a branch
    public class StudioOnlineCommonHelper : StudioOnlineCore.StudioOnlineCommonHelper
    {
        //
        public static new void DoBusinessRulesChecks(Page page)
        {
            StudioOnlineCore.StudioOnlineCommonHelper.DoBusinessRulesChecks(page);
        }
    }
}