Programatically add file in folder in sharepoint Document Library 

                    SPSite site = SPContext.Current.Site;
                    SPWeb oweb = site.OpenWeb();
                    oweb.AllowUnsafeUpdates = true;
                    SPList olist = oweb.Lists[listid];
                    SPListItem oitem = olist.GetItemById(Convert.ToInt32(itemid));
                    SPList list = oweb.Lists["SharedDocuments"];
                    SPFolder folder = list.RootFolder.SubFolders["Folder1"];
                    byte[] file = oitem.File.OpenBinary();
                    string dsturl = list.RootFolder.SubFolders["Folder1"].Url + "/" + oitem.File.Name;
                    SPFile myfile = folder.Files.Add(dsturl, file, true);
                    list.Update();