Why can't I mark this method as static?

Question

I have a BaseImageView (platform is irrelevant here), which has a property Effect, which is a command object.

I would like all my commands to implement IImageEffect, which has an static method ApplyEffect, taking an image and returning the image with the effect.

But the compiler complains that I cannot mark an interface method as static, but this seems like the logical option (as there is no state information needed for the effect).

I ask this because I would need 100 effect objects to apply 1 effect to 100 different images, while I could get away with passing the class and calling the static method, this would do away with the 100 instances.

Is there a way I can have a static interface, or a way that I can pass a class and have it call the static ApplyEffect of that type?

No, interface members cannot be static. How would the CLR know which implementation you wanted to call, if you called `IImageEffect.ApplyEffect`? (There's an argument for static interface members with generic type arguments, but that's more subtle.) — Jon Skeet, Sep 21 '16 at 10:34
@JonSkeet I would pass a class to the `Effect` property, and then call `ApplyEffect` on that class, so it would become `EffectClass.ApplyEffect`. Then it would know what specific class I want to call it on. — vrwim, Sep 21 '16 at 10:35
Sorry, this is very hard to understand without a concrete example *in the question*. If you just want a static method that always does the same thing, just put it in a class instead of an interface... — Jon Skeet, Sep 21 '16 at 10:36
@MortenToudahl I don't want to instantiate a new instance of the same class each time I use that effect, so I thought I'd make it static, but I just realised that this is why the singleton pattern was invented. I'll go ahead and use that. — vrwim, Sep 21 '16 at 11:41
@vrwim I think that is misuse of the singleton pattern. Singletons are for when more than one instance would break the app. ie, two drivers for one piece of hardware running. Like 2 x video card drivers. Or for managing configurations - accessing/updating the same configuration in two different objects could make something go wrong. Take a look at this, and read the articles linked to too: http://stackoverflow.com/questions/135347/most-common-examples-of-misuse-of-singleton-class — Morten Toudahl, Sep 22 '16 at 10:12
@MortenToudahl I have a method that only takes an image and only outputs an image, it wouldn't break the app if I had 2 instances. I just deem it useless to have multiple at the same time. What do you suggest I use? Let it be an interface and just instantiate a new instance each time I use it? Or cache an instance and reuse that one? Or simply use a singleton? — vrwim, Sep 22 '16 at 10:15
@vrwim Create a static class just to handle effects? Adhering with the single responsibility principle — Morten Toudahl, Sep 22 '16 at 10:18

score 0 · Answer 1 · answered Sep 22 '16 at 10:20

I would suggest that you either make an abstract class where you implement the static method, like you wanted to do with the interface.

It could look like this

class Program
{

    static void Main(string[] args)
    {
        BaseImageView.ApplyEffect();
        // Or
        EffectClass.ApplyEffect();
    }

}

public abstract class EffectClass
{
    public static void ApplyEffect()
    {
        // Effect of awesomeness
    }
}

public class BaseImageView : EffectClass
{

}

Or create a static class to apply the effect with, if you don't want to inherrit from an abstract class.

public static class EffectClass
{
    public static void ApplyEffect()
    {
        // Effect of awesomeness
    }
}

Why can't I mark this method as static?

1 Answers1