How to define the default implementation of an interface in c#?

Question

There is some black magic code in c# where you can define the default implementation of an interface.

So you can write

var instance = new ISomeInterface();

Any pointers?

UPDATE 1: Note that I did not ask if this is a good idea. Just how was it possible to do it.

UPDATE 2: to anyone seeing the accepted answer.

"this should be treated merely as a curiosity." from Marc Gravel "Newing up" Interfaces
"It's a bad idea to use a tool designed for COM interop to do something completely and utterly different. That makes your code impossible to understand for the next guy who has to maintain it" from Eric Lippert "Newing up" Interfaces
"While it may work, if it were ever found in production code by a rational coder, it would be refactored to use a base class or dependency injection instead." from Stephen Cleary in a comment below.

Some articles on this: http://stackoverflow.com/questions/1093536 and http://blogs.msdn.com/b/ericlippert/archive/2007/04/20/chained-user-defined-explicit-conversions-in-c-part-three.aspx — Eric Lippert, Jul 18 '10 at 15:47

score 34 · Accepted Answer · answered Jul 17 '10 at 11:40

34

Here comes the black magic:

class Program
{
    static void Main()
    {
        IFoo foo = new IFoo("black magic");
        foo.Bar();
    }
}

[ComImport]
[Guid("C8AEBD72-8CAF-43B0-8507-FAB55C937E8A")]
[CoClass(typeof(FooImpl))]
public interface IFoo
{
    void Bar();
}

public class FooImpl : IFoo
{
    private readonly string _text;
    public FooImpl(string text)
    {
        _text = text;
    }

    public void Bar()
    {
        Console.WriteLine(_text);
    }
}

Notice that not only you can instantiate an interface but also pass arguments to its constructor :-)

answered Jul 17 '10 at 11:40

Darin Dimitrov

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1

And with the keyword "CoClass" the google gods are with me again. http://ayende.com/Blog/archive/2009/08/15/instantiating-interfaces.aspx http://marcgravell.blogspot.com/2009/08/who-says-you-cant-instantiate-interface.html – Simon Jul 17 '10 at 12:13
@Simon, those are two blogs I would recommend you tuning your RSS reader to :-) – Darin Dimitrov Jul 17 '10 at 12:15
@Daren already have them. but all my combinations of "interface default implementation" came up with naught. Signal to noise ratio was not good :) – Simon Jul 17 '10 at 12:23
1

@daren kind of funny that the answers saying "it is not possible" are geting upvoted after you posted your answer. – Simon Jul 17 '10 at 12:24
If I ever saw this in code by a colleague I'd sack them. – jbrown Aug 16 '16 at 10:04

Stephen Cleary · Answer 2 · 2010-07-17T11:22:43.467

3

Only if ISomeInterface is a class.

Update (for clarification):

Jon Skeet has a talk where he mentions default implementations for interfaces. They are not part of the C# language, though. The talk is about what Jon Skeet would like to see in a future version of C#.

For now, the only default implementations are done via (possibly abstract) base classes.

edited Jul 17 '10 at 11:22

answered Jul 17 '10 at 11:14

Stephen Cleary

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I downvoted all the incorrect answers. because they were incorrect. no rationality about it. See Darins answer – Simon Jul 17 '10 at 12:05
2

I saw the "correct" answer. While it may work, if it were ever found in production code by a rational coder, it would be refactored to use a base class or dependency injection instead. – Stephen Cleary Jul 17 '10 at 12:22
@Setphen Agreed. edited the question so hopefully no one thinks it is a good idea. – Simon Jul 17 '10 at 12:36

score 1 · Answer 3 · answered Jul 17 '10 at 11:20

Maybe you refer to Dependency Injection? Where when using DI framework (such as Ninject or Unity), you can define default instance for each interface and then using it like this:

(assuming you have IWeapon interface and Sword implements it)

IKernel kernel = new StandardKernel();
kernel.Bind<IWeapon>().To<Sword>();
var weapon = kernel.Get<IWeapon>();

But Ninject (and most other IoC frameworks) can do some more clever things, like: let's say we have the class Warrior that takes IWeapon as a parameter in its constructor. We can get an instance of Warrior from Ninject:

var warrior = kernel.Get<Warrior>();

Ninject will pass the IWeapon implementation we specified to the Warrior constructor method and return the new instance.

Other than that, I don't know of any in-language feature that allows this kind of behavior.

good suggestion but not what i was looking for. See Darins Answer — Simon, Jul 17 '10 at 12:04
Yes, I missed the part where you mentioned "black magic" in your question :-) Thanks for asking this -- I learned something new! — arikfr, Jul 17 '10 at 12:27

How to define the default implementation of an interface in c#?

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