Should I file bankruptcy before the foreclosure or after?

When is a good time to file for Bankruptcy? before or after foreclosure? I recceived a letter in the mail from an attorney. My lender wants to start the foreclosure proceedings. At the moment, I am 8 months behind on my mortgage. I want to dump the house as I owe 20% more than it is worth. What happens if I file for bankruptcy right away? Does that cover the foreclosure too?? Or would the bank c