为什么静态类不能实现接口？

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.

Updated in 2022: .NET 7 does support static interface methods.

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.