Question

我正在编写两个LINQ查询，我在第二个查询中使用了我的第一个查询结果集。

但是在某些情况下，当数据库表中没有数据时，我的第一个查询返回null，因此，我的第二个查询失败，因为wsdetails.location和wsdetails.worklocation null导致异常。

例外：

未将对象引用设置为对象的实例

我的代码是：

        var wsdetails = (from assetTable in Repository.Asset
                         join userAsset in Repository.UserAsset on
                         assetTable.Asset_Id equals userAsset.Asset_Id
                         join subLocationTable in Repository.SubLocation on
                         assetTable.Sub_Location_Id equals subLocationTable.Sub_Location_ID
                         where userAsset.User_Id == userCode
                         && assetTable.Asset_TypeId == 1 && assetTable.Asset_SubType_Id == 1
                         select new { workstation = subLocationTable.Sub_Location_Name, location = assetTable.Location_Id }).FirstOrDefault();


            result = (from emp in this.Repository.Employee
                      join designation in this.Repository.Designation on
                      emp.DesignationId equals designation.Id
                      where emp.Code == userCode
                      select new EmployeeDetails
                      {                             
                          firstname = emp.FirstName,
                          lastname = emp.LastName,                              
                          designation = designation.Title,
                          LocationId = wsdetails.location,
                          WorkStationName = wsdetails.workstation
                      }).SingleOrDefault();

作为一种解决方法，我可以检查

if wsdetails == null

并更改了我的第二个LINQ逻辑，但我相信有一些方法可以像null运算符一样处理LINQ本身的??值。

但是我尝试过这个并不适合我。

任何帮助？

Answer 1

而不是“二元”运算符??也许您应该使用旧的三元运算符? :，就像在

中一样

wsdetails != null ? wsdetails.location : null

Answer 2

问题是EF无法将null-coalescing operator转换为SQL。我个人认为在执行下一个查询之前用if语句检查结果有什么问题。但是，如果您不想这样做，那么因为您的结果始终是单个查询，为什么不执行以下操作：

var wsdetails = (from assetTable in Repository.Asset 
                 join userAsset in Repository.UserAsset on 
                 assetTable.Asset_Id equals userAsset.Asset_Id 
                 join subLocationTable in Repository.SubLocation on 
                 assetTable.Sub_Location_Id equals subLocationTable.Sub_Location_ID 
                 where userAsset.User_Id == userCode 
                 && assetTable.Asset_TypeId == 1 && assetTable.Asset_SubType_Id == 1 
                 select new { workstation = subLocationTable.Sub_Location_Name, location = assetTable.Location_Id }).FirstOrDefault();  

result = (from emp in this.Repository.Employee 
          join designation in this.Repository.Designation on 
          emp.DesignationId equals designation.Id 
          where emp.Code == userCode 
          select new EmployeeDetails 
          {                              
              firstname = emp.FirstName, 
              lastname = emp.LastName,                               
              designation = designation.Title
           }).SingleOrDefault();

result.LocationId = wsdetails != null ? wsdetails.location : "someDefaultValue";
result.WorkStationName = wsdetails != null ? wsdetails.workstation ?? "someDefaultValue";

Answer 3

尝试不评估第一个查询，并在第二个查询中使用它。这应该会产生一个SQL语句。

var wsdetails = (from assetTable in Repository.Asset
                         join userAsset in Repository.UserAsset on
                         assetTable.Asset_Id equals userAsset.Asset_Id
                         join subLocationTable in Repository.SubLocation on
                         assetTable.Sub_Location_Id equals subLocationTable.Sub_Location_ID
                         where userAsset.User_Id == userCode
                         && assetTable.Asset_TypeId == 1 && assetTable.Asset_SubType_Id == 1
                         select new { workstation = subLocationTable.Sub_Location_Name, location = assetTable.Location_Id });
// wsdetails is still an IEnumerable/IQueryable


        result = (from emp in this.Repository.Employee
                  join designation in this.Repository.Designation on
                  emp.DesignationId equals designation.Id
                  where emp.Code == userCode
                  select new EmployeeDetails
                  {                             
                      firstname = emp.FirstName,
                      lastname = emp.LastName,                              
                      designation = designation.Title,
                      LocationId = wsdetails.First().location,
                      WorkStationName = wsdetails.First().workstation
                  }).SingleOrDefault();

实体框架 - 处理Linq中的空值

3 个答案: