Detecting if a page has already been initialized and using it instead of creating a new instance

See more:

C#

WPF

The C# WPF application I am writing is heavily dependant on pages hosted inside a frame control. The pages are accessed by clicking their respective menu items with frame.Navigate(new pageName()); But this means that each time the user wants to go back to a page (which is quite a lot of times) I have to reload all of the data that was in the interface before the page was switched. Currently I am using a separate XML file containing all the data for each page before it is changed which is then checked by the page' loaded event and reloaded into the controls. Is there a better and easier way to access pages over and over again without having to create a new instance of it?

I could initialize all of the pages with public pageName pagename = new PageName(); but it would probably use up unnecessary memory and it would be wasted if the user never accessed one of the pages.

I could carry on initializing a new instance of the page in its menu click event but check if it is already initialized and use that instead before creating a new one.

I could continue using my method of saving to a file and loading from it on the next load of the page.

I think the second one is best but I have no idea how to check if the page I want has already been initialized and can be used. I use this for getting the current instance of a window: var MainWindow = Application.Current.Windows.Cast().FirstOrDefault(window => window is MainWindow) as MainWindow; but it doesn't work for pages. Basically I just want to know how I can use the current instance of a page if it has already been initialized instead of creating a new one each time it needs to be accessed.