reference the "self" type in a static method - C#

Question

I am trying to create a static method that would return an instance of the class, something like:

class A {
   public static A getInstance() {
      return new A();
   }
}

The problem I am having is that if I have a subclass B derived from A, I would like B.getInstance() to return an instance of B, and not A. In PHP world, you could use a keyword "self" to reference to your own type, so your getInstance() would look like:

public static function getInstance() {
   return new self();
}

What's the best way to go about this?

Only way I've ever done is use the CRTP to pass the derived type to the base type via generics. It's frowned upon here, so I won't discuss it further. Perhaps reflection and `Activator.CreateInstance()` will be your best bet? — siride, Aug 23 '11 at 14:49

score 8 · Answer 1 · answered Aug 23 '11 at 14:50

8

You can't, basically. If a call to a static member which is only declared in a base class is actually expressed in terms of the derived class, like this:

// Urgh
Encoding ascii = ASCIIEncoding.ASCII;

// Worse yet (and yes, I've seen this)
Encoding ascii = UTF8Encoding.ASCII;

then the compiler silently transforms that to:

Encoding ascii = Encoding.ASCII;

The fact that the original source contained the name of a derived class is not preserved in the compiled code, so it can't be acted on at execution time.

answered Aug 23 '11 at 14:50

Jon Skeet

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Makes sense. PHP can do this since it's an interpreted language - the feature is actually called "late static binding" (http://php.net/manual/en/language.oop5.late-static-bindings.php). Thank you! – m1tk4 Aug 23 '11 at 15:03
I think `UTF8Encoding.Default` is even worse. At least with `.ASCII`, you should expect that it's actually ASCII. With `.Default`, it looks like the default variant of UTF-8, or something like that. – svick Aug 23 '11 at 20:52
@svick: True. `Default` is named badly to start with, mind you - it should be `SystemDefault` to distinguish it from "the default encoding used for most API calls" which is UTF-8. – Jon Skeet Aug 23 '11 at 20:55

score 7 · Answer 2 · answered Aug 23 '11 at 14:51

7

How about using generics to provide the type to produce:

public static T getInstance<T>() where T : A, new()
{
    return new T();
}

answered Aug 23 '11 at 14:51

George Duckett

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score 5 · Accepted Answer · edited Jul 19 '12 at 07:17

5

You can hide and so in a way override the getInstance method in the inheriting type like this:

class B : A {
    public static new B getInstance() {
        return new B();
    }
}

The new keyword specifies that the hiding of the inherited method was intentional, otherwise you get a compiler warning.

Complete example:

using System;

class A {
    public static A getInstance() { return new A(); }
    public void PrintSelf() {Console.WriteLine(this.GetType().Name);}
}

class B : A {   
    public static new B getInstance() {
        return new B();
    }
}

public class MyClass {
    public static void Main() {
        A a = A.getInstance();
        B b = B.getInstance();
        a.PrintSelf();
        b.PrintSelf();
    }
}

edited Jul 19 '12 at 07:17

Dio F

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answered Aug 23 '11 at 14:52

Paul Sasik

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This will actually do for my purposes. Thank you! – m1tk4 Aug 23 '11 at 15:05
3

Just remember that if you forget to do this anywhere, the compiler won't warn you... you'll just silently be given the "wrong" type. – Jon Skeet Aug 23 '11 at 15:31

score 1 · Answer 4 · answered Aug 23 '11 at 14:49

1

There's no way to do this statically -- you just have to use the actual name.

However, I'll bet PHP does this dynamically (i.e., with reflection) -- in which case you can always use GetCurrentMethod to get information about the method and itse enclosing type.

That way you can do something like:

using System.Reflection;

class Test
{
    static object MakeTest()
    {
        return Activator.CreateInstance(
            MethodBase.GetCurrentMethod().DeclaringType);
    }

    void Hello() { Console.WriteLine("Hi!"); }

    static void Main()
    {
        dynamic test = MakeTest();
        test.Hello();
    }
}

answered Aug 23 '11 at 14:49

user541686

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1

I don't think this will work because the `DeclaringType` will be the base class (since the static method is there) and not the derived class. – siride Aug 23 '11 at 14:56
@siride: Great point -- I think I might have misunderstood the question then. It seems like the OP is looking for Python's class methods, which C# doesn't quite have. – user541686 Aug 23 '11 at 14:58
1

The PHP feature that allowed me to do this is called "late static binding" - http://php.net/manual/en/language.oop5.late-static-bindings.php . Apparently, there is no equivalent in C# world. – m1tk4 Aug 23 '11 at 15:02

score 1 · Answer 5 · answered Aug 23 '11 at 14:53

1

Since others say there's no way to do this, I'll just offer up CRTP. It has its faults (Eric Lippert has a post about it here somewhere), but it will solve this problem

class Base<T> where T : Base<T>, new() {
    public static T GetInstance() {
        return new T();
    }
}

class Derived : Base<Derived> {
}

Derived d = Derived.GetInstance();

This does make Base<T> hard to use, however. It's best if Base<T> is abstract. You can also create a side class like this that can actually be instantiated and used:

class Base : Base<Base> {
}

It looks weird, but it works. Unfortunately, Base and Derived will not be related anymore (except by Base<T>) so you can't cast between them, as such.

answered Aug 23 '11 at 14:53

siride

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The post you're thinking of is probably this one: http://blogs.msdn.com/b/ericlippert/archive/2011/02/03/curiouser-and-curiouser.aspx which was based on this SO answer: http://stackoverflow.com/questions/3783321/why-does-this-generic-constraint-compile-when-it-seems-to-have-a-circular-referen/3789193#3789193 – Eric Lippert Aug 23 '11 at 17:17
@Eric Lippert: that's the one. I both agree and disagree. You are logically correct, but sometimes it's just easier to use CRTP and demand that users do the right things where it could break. – siride Aug 24 '11 at 01:33

score 1 · Answer 6 · answered Aug 23 '11 at 14:54

You can not override static member function in derived class. If you choose static approach, you need just add the same method to B class.

Sure you can use also different design patterns, like Factory for example, but I think this is out of scope of the question.

reference the "self" type in a static method - C#

6 Answers6