LINQ 多条件join on

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();