如果使用null检查,LINQ2SQL不返回行

Question

我有以下LINQ2SQL查询:

var map =
                dbContext.TCPDriverMappings.FirstOrDefault(
                      c => c.DriverFacilityId == tcpDms.FacilityId &&
                                 c.DriverControlledParameterId == controlledParamId &&
                                 c.DriverValue == value);

所有类型都是字符串.

在我的数据库中,我有一行,必须通过查询返回.

当value="0", controlledParamId =null and FacilityId ="abc"此查询返回null时,但是当我写下以下内容时:

var test = dbContext.TCPDriverMappings.FirstOrDefault(
                          c => c.DriverFacilityId == "abc" &&
                                     c.DriverControlledParameterId == null &&
                                     c.DriverValue == "0");

测试没有 null

我究竟做错了什么？

PS我也试过c.DriverControlledParameterId.Equals(controlledParamId)但它也行不通.

Answer 1

问题是,LINQ2SQL对表达式有一个特殊的处理c.DriverControlledParameterId == null.它被翻译成SQL DriverControlledParameterId IS NULL.
但是c.DriverControlledParameterId = controlledParamId被翻译成SQL DriverControlledParameterId = :p1,即使controlledParamId是null.在SQL中DriverControlledParameterId = NULL是未定义的,因此永远不会TRUE.

如何解决:null具体处理案例:

TCPDriverMapping test;
if(controlledParamId == null)
    test = dbContext.TCPDriverMappings.FirstOrDefault(
               c => c.DriverFacilityId == "abc" &&
                    c.DriverControlledParameterId == null &&
                    c.DriverValue == "0");
else 
    test = dbContext.TCPDriverMappings.FirstOrDefault(
               c => c.DriverFacilityId == "abc" &&
                    c.DriverControlledParameterId == controlledParamId &&
                    c.DriverValue == "0");

或者像这样:

var test = dbContext.TCPDriverMappings.FirstOrDefault(
               c => c.DriverFacilityId == "abc" &&
                    ((controlledParamId == null &&
                      c.DriverControlledParameterId == null) || 
                     c.DriverControlledParameterId == controlledParamId) &&
                    c.DriverValue == "0");

或者像这样:

IQueryable<TCPDriverMapping> query =
    dbContext.TCPDriverMappings.Where(c => c.DriverFacilityId == "abc" &&
                                           c.DriverValue == "0");
if(controlledParamId == null)
    query = query.Where(c => c.DriverControlledParameterId == null);
else
    query = query.Where(c => c.DriverControlledParameterId == controlledParamId);

var test = query.FirstOrDefault();

我会使用第三个选项.在我看来,这比选项2更具可读性,并且没有像第一个那样重复的代码.