c#根据某一个属性使用groupJoin拼接不同list

var resultList = userList.GroupJoin(departList, u => u.departmentId, a => a.id, (u, a) => new
            {
                id = u.id,
                serialNumber = u.serialNumber,
                nickName = u.nickName,
                departName = a.FirstOrDefault(x => x.id == u.departmentId) == null ? "Null" : a.FirstOrDefault(x => x.id == u.departmentId).name
            }).Select(o => o).ToList();
          

  public class UserInfo
    {
        public string id { get; set; }//设备id
        public string serialNumber { get; set; }//设备编号
        public string nickName { get; set; }//设备名称
        public string portrait { get; set; }//角色
        public string departmentId { get; set; }//部门ID
        public string terminalType { get; set; }//终端类型：1 一期；2 二期；5 三期
    }

 public class DepartmentInfo
    {
        public string id { get; set; }//部门id
        public string name { get; set; }//部门名称
        public string parentId { get; set; }//上级机构ID
        public string sort { get; set; }//排序(同部门下的显示顺序)
    }