var  tmp = from a in DT1.AsEnumerable()
join b in DT2.AsEnumerable()
on new { bm = a.Field<string>("编码"), lx = "类型" } equals new { bm = b.Field<string>("编码"), lx= b.Field<string>("类型") }
into ab
from c in ab.DefaultIfEmpty()
select new
{
编码 = a.Field<string>("编码"),
类型 = c == null ? "" : c.Field<string>("类型")
};
dt_结果 = tmp.MyCopyToDataTable();