传递信息的方法有很多。我建议两种方法
1.关于linq

_listTesta = (from c in _listTesta
              join ct in _listTestB
                         on c.Id equals ct.Id into g
              from ct in g.DefaultIfEmpty()
              where c.Id != null
              select new Testa
              {
                  Name = ct == null ? c.Name : ct.Name,
                  Family = ct == null ? c.Family : ct.Family,

                  Id = ct == null ? c.Id : ct.Id,
              }).ToArray();

示例数据：

List<Testa> _t = new List<Testa>();
List<TestB> _ta = new List<TestB>();

var _listTesta = Enumerable.Range(1000, 50).Select(i => new Testa(i, "n" +
i, "nf" + i)).ToArray();

var _listTestB = Enumerable.Range(1000, 20).Select(i => new TestB(i, "nb" +
i, "fb" + i)).ToArray();

2.用自动机
Automapperautomapper-net-core