Question

Is it possible for a class to inherit from a nested class, or to implement a nested interface in C#?

class Outer : Outer.Inner {
    class Inner { ... }
    ...
}

Answer 1

In the way that you wrote, no (see this). But if your inner interface is in another class, then you can.

public class SomeClass : SomeOtherClass.ISomeInterface {
    public void DoSomething() {}
}

public class SomeOtherClass {
    public interface ISomeInterface {
        void DoSomething(); 
    }
}