List对象中的组合、查询、扩展

var Pnts = segs.Select( c =>
                                             pntTsLst.Where                                                            //根据条件组合另一个List， pntTsLst是一个List<KEYPOINT>
                                             (d => d.KEYPOINT_UUID == c.END_POINT_ID)
                                                                                                                          .Select(
                                                                                                                                           k =>new { id=k.KEYPOINT_UUID,name=k.NAME + (k.DESIGNATOR.IsEmptyOrNull() ? "" : $"({k.DESIGNATOR})"),seq=c.SEQNUM+1 }  //从另一个List中取值并添加其他属性
                                                                                                                                      ).First()      //First非常重要，作用于where，把整个最外层括号中的数据类型变为属性，否则是个两重List结构                            
                                             ).ToList();

//Pnts 是一个自定义对象{id,name,seq} 的list

或者，返回一个扩展的对象

var lst = Pnts.Select(c => new {
                                                      keypt =pntTsLst.Where(d => d.KEYPOINT_UUID == c.id).First(),
                                                      seq =c.seq
                                                 }

).ToList();

lst 是一个扩展对象{KEYPOINT,seq}的list