Question

I’m writing two LINQ queries where I use my first query’s result set in my second query.

But in some cases when there is no data in the database table my first query returns null,
and because of this my second query fails since wsdetails.location and wsdetails.worklocation are null causing an exception.

Exception:

Object reference not set to an instance of an object

My code is this:

        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();

As a workaround I can check

if wsdetails == null

and change my second LINQ logic, but I believe there are some ways to handle null values in LINQ itself like the ?? operator.

But I tried this and it didn’t work for me.

Any help?

Answer 1

The problem is EF can’t translate the null-coalescing operator to SQL. Personally I don’t see what’s wrong with checking the result with an if statement before executing the next query. However, if you don’t want to do that, then because your result is always going to be a single query why not do something like:

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";