Copy File via Save Dialog Box

Question

The program that i wanted to do is that the application uses a Save Dialog box then chooses the directory where the user wants to copy a file... after that, the file is copied to the selected directory. Although i'm having an error "the file does not exist" when i use the Save Dialog box. But when i try not to use a dialog box and use a specific directory where I want to save a copied file, it works fine... Can anybody provide me a working code for it?