c#

In .NET, you can 'nest' namespaces - so the System.Xml namespace is nested in the System namespace.

However, for the purposes of C#'s 'using' directive, each namespace is treated separately. So, if you want to use a class in the System.Xml namespace (XmlDocument say) without having to 'fully qualify' its name, then you must use:

  using System.Xml;

You might reasonably have thought that if you did this:

  using System;

then you'd be able to do this:

  Xml.XmlDocument doc = new Xml.Document();

but, if you try to do that, you'll get a compiler error.

Curiously, though, you can do this sort of thing in VB.NET which is a much more forgiving language than C#. For example the following compiles fine: