how to set default culture info for entire c# application

Not for entire application or particular class.

CurrentUICulture and CurrentCulture are settable per thread as discussed here Is there a way of setting culture for a whole application? All current threads and new threads?. You can't change InvariantCulture at all.

Sample code to change cultures for current thread:

CultureInfo ci = new CultureInfo(theCultureString);
Thread.CurrentThread.CurrentCulture = ci;
Thread.CurrentThread.CurrentUICulture = ci;

For class you can set/restore culture inside critical methods, but it would be significantly safe to use appropriate overrides for most formatting related methods that take culture as one of arguments:

(3.3).ToString(new CultureInfo("fr-FR"))

With 4.0, you will need to manage this yourself by setting the culture for each thread as Alexei describes. But with 4.5, you can define a culture for the appdomain and that is the preferred way to handle this. The relevant apis are CultureInfo.DefaultThreadCurrentCulture and CultureInfo.DefaultThreadCurrentUICulture.