Override created by or modified by property in SharePoint

SPCreatedModifiedByOverride.cs

using (SPSite site = new SPSite(SPContext.Current.Web.Url))
            {
                using (SPWeb web = site.OpenWeb())
                {
                    web.AllowUnsafeUpdates = true;
                    SPList list = web.Lists["Shared Documents"];
                    SPListItemCollection items = list.Items;
                    SPListItemCollection oListCollection = list.Items;
                    foreach (SPListItem oListItem in oListCollection)
                    {
                        SPUser user = web.EnsureUser("New Account");
                        SPFieldUserValue oUser = new SPFieldUserValue(web, user.ID, user.LoginName);

                        oListItem["Author"] = oUser;
                        oListItem["Editor"] = oUser;               
                        oListItem.Update(); 
                    }
                    web.AllowUnsafeUpdates = false;
                }
            }