Why static classes cant implement interfaces? [duplicate]

Possible Duplicate:
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.

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.

Maybe our experience will help. Rather than SqlRepository as a static class, we use AutoFac for injection and hide the container behind a static class. Then each entity has a static repository property:

public class Part : inheritence...
{
    public static IPartRepository Repository
    {
        get { return IoCContainer.GetInstance<IRepository<Part>>(); }
    }
    // ... more part-y stuff
}

This way we can swap out the implementation and callers always know where to get it:

Part p = Part.Repository.Get(id);

In another project there is a PartRepository registered with the container:

public class PartRepository : IPartRepository
{
    // IPartRepository implementation that talks to injected DAL
}

In yet another project we have mocks for testing, including repositories pre-loaded with known entires:

public class MockPartRepository : Dictionary<Part, int>, IPartRepository
{
    // IPartRepository implementation based on dictionary
}

...and it is registered with the container for unit testing. The SAME call gets the repository:

Part p = Part.Repository.Get(id);

By definition, interfaces create a contract for instances to fulfill. Since you cannot instantiate a static class, static classes cannot implement interfaces.

There is no need to have a static repository. Simply make it non-static and instantiate it when you need it.