storing data

Question

I'm not a professional programmer and new to c# and hoping to find a little direction.

I have a multi dimensional data array which I get from third party using their API and then populate a listbox with the data using listbox.datasource and databind() which is fine but when a selectedindexchange event post back to the page the array is now empty so I need a way to store the data instead of constatly making api calls.  I know that I can create an xml file but dont want to have to delete and rewrite all entries when the data is refreshed.

Any suggestions would be greatly appreciated!

Thanks,
Greg