Private nested static class - Good or bad practice?

Question

Would it be considered a bad practice to nest a private static class inside of a non-static class?

public class Outer
{
    private static class Inner
    {


    }
}

The idea here is that all instances of 'Outer' would share access to the static state. Another way to do it might be to just let the Inner class be non-static and use a static instance of it:

public class Outer
{
    private static innerInstance = new Inner(); 

    private class Inner
    {


    }
}

Similar effect. What are the pros / cons or other considerations with this approach?

I must admit that I almost never use nested classes, whether static or not, but I am interested in this particular concept..

Answer 1

Both approaches are entirely valid.

I wish developers would use private nested classes more often. In conjunction with c#'s partial keyword, it makes writing very complex classes much more maintainable. Imagine needing to build a class that has the complexity of a small application - much easier when you actually can build an entire private application, with classes that are totally internal to your complex outer class!

One very common case I've seen is enumerables - these can be quite complex, especially when you start building custom iterators that can be chained, like LINQ. Hiding the complexity inside the individual classes is the very definition of encapsulation.