Preprocessing is a reverse engineering option you can enable or disable.
- Select . 
                    The Reverse Engineering VB .NET dialog box is displayed. 
 
- Select files to reverse engineer in the Selection tab.
- Select reverse engineering options in the Options tab.
- Select the Enable preprocessing check box in the Preprocessing tab.
- Select symbols in the list of symbols.
- Click OK to start reverse engineering. 
                    When preprocessing is over the code is passed to reverse engineering.