實體框架-處理Linq中的空值

Question

我正在編寫兩個LINQ查詢，其中在第二個查詢中使用第一個查詢的結果集。

但是在某些情況下，當數據庫表中沒有數據時，我的第一個查詢返回null，因此，由於wsdetails.location和wsdetails.worklocation為null導致異常，因此我的第二次查詢失敗。

例外：

你調用的對象是空的

我的代碼是這樣的：

        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();

作為一種解決方法，我可以檢查

if wsdetails == null

並更改我的第二個LINQ邏輯，但是我相信有一些方法可以處理LINQ本身中的null值，例如?? 操作員。

但是我嘗試了一下，但對我沒有用。

有什么幫助嗎？

Answer 1

代替“二進制”運算符?? 也許您應該使用舊的三元數， ? : ? : ，如

wsdetails != null ? wsdetails.location : null

Answer 2

問題是EF無法將null-coalescing運算符轉換為SQL。 我個人看不到執行下一個查詢之前用if語句檢查結果有什么問題。 但是，如果您不想這樣做，那么由於您的結果始終是單個查詢，為什么不這樣做：

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

嘗試不評估第一個查詢，而在第二個查詢中使用它。 這將導致一個SQL語句。

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();