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Why Doesn’t C# Allow Static Methods to Implement an Interface?
In my application I want to use a Repository that will do the raw data access (TestRepository, SqlRepository, FlatFileRepository etc). Because such a repository would be used throughout the runtime of my application it seemed like a sensible thing to me to make it a static class so I could go
SqlRepository.GetTheThingById(5); without it having to be regenerated all the time. Because I want my repositories to be interchangeable, I want them to implement a common interface: IRepository. But when I try to do so, I get:
Static classes cannot implement interfaces
Why can't they? How do you suggest I change my design then? Is there a pattern I could use?
UPDATE
Five years later: this question is visited 20k+ times, I learned about the disadvantages of the repository pattern, learned about IoC and realise my question was poorly formulated.
I wasn't really asking what the C# specification of an interface is, rather why it was deliberately restricting me in this specific way.
The practical answer is that the syntax for calling a method on an instance or on a type are different. But the question is closed.
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Static classes cannot be inherited whereas an interface must be inherited in order to use it. Making an interface static, would render it pretty much useless. You'd need to inherit it in a class to implement it's members but static would prevent you from doing exactly that.
Polymorphism works through instances - whereas static members explicitly don't use instances. that's the reason interfaces don't have static methods.
Static classes are sealed and therefore cannot be inherited. They cannot inherit from any class except Object. Static classes cannot contain an instance constructor. However, they can contain a static constructor.
It's not allowed because inheritance is about creating related classes of objects, and you're not creating any objects at all with static classes.
Interfaces can't have static methods. A class that implements an interface needs to implement them all as instance methods. Static classes can't have instance methods. QED.
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