First deny permissions to All users for the list item and grant only one user to access the item

in my event receiver i have to write the below functionality:

 Deny permission for all but one user .

i tried below : but it throws an error  on this line :

                                                        SPRoleDefinition biffroledefi = CurrentWeb.RoleDefinitions[SPRoleType.None];

how can i deny access  for the currentlistitem to the all users in my  site collection and then i want to add only one person to access this listitem. 

CurrentWeb.RoleDefinitions[SPRoleType.None];

try
            {
                string currentuserIdd = properties.CurrentUserId.ToString();

...............

...........

CurrentWeb.AllowUnsafeUpdates = true;

                                CurrentListItem.BreakRoleInheritance(false);

                                
                                   SPRoleAssignment AllBIFRoleassignment = new SPRoleAssignment(myusersGroup);
                                                        SPRoleDefinition biffroledefi = CurrentWeb.RoleDefinitions[SPRoleType.None);
                                                        AllBIFRoleassignment.RoleDefinitionBindings.Add(biffroledefi);
                                                        CurrentListItem.RoleAssignments.Add(AllBIFRoleassignment);

                                                        // added on 7 pm 04-june-2014

                                                        SPRoleAssignment mroleAssignmentforCurrUser = new SPRoleAssignment(currUser);
                                                        SPRoleDefinition mroleDefinitionforCurrUser = CurrentWeb.RoleDefinitions["Full Control"];
                                                        mroleAssignmentforCurrUser.RoleDefinitionBindings.Add(mroleDefinitionforCurrUser);
                                                        CurrentListItem.RoleAssignments.Add(mroleAssignmentforCurrUser);

                                                        this.EventFiringEnabled = false;

                                                        CurrentListItem["Author"] = currentuserIdd;
                                                        CurrentListItem.Update();

                                                        CurrentListItem["Editor"] = currentuserIdd;
                                                        CurrentListItem.Update();
                                                        this.EventFiringEnabled = true;
                                                        CurrentWeb.AllowUnsafeUpdates = false;