C# Lamda GroupJoin的使用

            var group = countylist.GroupJoin(
                                        list,
                                       (countyinfo l1) => new { l1.Countycode },
                                       (Result l2) => new { l2.Countycode },
                                        (item1, item2) => new
                                        {
                                            item1,
                                            item2
                                        }) //2列表left jion
                                  .SelectMany(p => p.item2.DefaultIfEmpty(), (p, l2) => new
                                  {
                                      countyId=p.item1.Countycode,
                                      地区= p.item1.Countyname,
                                      //countyname = l2 == null ? string.Empty : l2.Countyname,
                                      低 = p.item2.Count(n => n.FX == "低"),
                                      合计 = p.item2.Count()
                                  }).Distinct().ToList();

参考：

https://www.cnblogs.com/zitjubiz/p/a-left-outer-join-using-linq-extension-methods.html