Class/Object basics..

[Craig Lister]

I'd like to have an XML document that my app reads and writes to on a
regular basis. It will read the XML document on opening, write to it when
closing, and while running, it will do updates and add and delete nodes.

So...

I think I should create a 'Settings' class, with methods for reading,
writing, getting a value.. etc..

So, my question is: Where would I declare the object. Or should I say,
create it, so that I have access to it from anywhere in my main class? So
that at any point, I can say:

SettingsObject.Flush(); // Update the settings file with the latest changes
SettingsObject.ShutDown() // Do all the end-of-running things, and close the
XML file.

[Bruce Wood]

I would make it a Singleton. Look up the Singleton design pattern. Jon
Skeet has a page on this:

http://www.yoda.arachsys.com/csharp/singleton.html

Your "Settings" class could use composition: it could contain an XML
document as a private field, and then provide a "nice" interface to the
rest of the application that talks to the app in higher-level terms (so
that the app doesn't have to know that the settings are stored in XML,
and you could change the representation / location of the settings
later).