Error in LINQ Left JOIN

cp.DefaultIfEmpty() returns a sequence which will have a single null value in if cp was empty.

That means you have to account for the fact that the p in

from p in cp.DefaultIfEmpty()

may be null. Now, you haven't really said what you want to happen in that case. You might want something like this:

var qry = from c in dc.category_feature_Name_trans_SelectAll_Active()
          join p in dc.product_category_feature_trans_SelectAll()
          on c.cft_id equals p.cft_id into cp
          from p in cp.DefaultIfEmpty()                      
          select new
          {
              c.cft_id,
              c.feature_id,
              c.feature_name,
              product_id = p == null ? null : p.product_id,
              value = p == null ? null : p.value 
          };

... or you may want some different handling. We don't know the types of p.product_id or p.value, which doesn't help. (For example, you'll need a bit more work with the above code if product_id is a value type.)

I am facing the same issue while using the LEFT JOIN to multiple tables in LINQ query and I was facing null reference exception. I used the trick to check the null value using the "?" Please correct me if my approach is incorrect.

var deviceResultDetails = from pa in programAliasrecords
                          join pgm in Newprogramrecords on pa.program_id equals pgm.id into pgm_join
                          from pgm2 in pgm_join.DefaultIfEmpty()
                          join latest_fw in firmwareWithIdrecords on pgm2?.latest_firmware_id equals latest_fw?.id into latest_fw_join
                          from latest_fw2 in latest_fw_join.DefaultIfEmpty()
                          join dev_fw in firmwareWithIdrecords on pgm2?.firmware_group_id equals dev_fw?.firmware_group_id into dev_fw_join
                          from dev_fw2 in dev_fw_join.DefaultIfEmpty()
                          join vv in vulnerabilityrecords on pgm2?.id equals vv?.program_id into vv_join
                          from vv2 in vv_join.DefaultIfEmpty()
                          where 
                          dev_fw2?.version == row.firmware    
                          && pa?.keyword == row.model      

                             select new _deviceResults
                             {
                                 model = row.model, 
                                 serial = row.serial, 
                                 firmware = row.firmware, 
                                 firmware_date = dev_fw2.br_date == null ? null: dev_fw2.br_date,
                                 latest_firmware = latest_fw2.version == null ? null : latest_fw2.version,
                                 latest_firmware_date = latest_fw2.br_date == null ? null : latest_fw2.br_date,

                                 status = Convert.ToInt32(vulnerability_count) > 0 && pgm2.out_of_support == "TRUE" ? "Vulnerable (End of Suport)" :
                                          Convert.ToInt32(vulnerability_count) > 0 && pgm2.out_of_support == " " ? "Vulnerable (Upgradeable)" :
                                                 pgm2.out_of_support == "TRUE" ? "Out-of-support" :
                                                 Convert.ToInt32(dev_fw2.revs_out_of_date) > 1 ? "Out-of-date" :
                                                 pgm2.id == "NonHP" ? "NonHP" :
                                                 pgm2.id == "NoFirmware" ? "No Firmware" :
                                                 pgm2.id == "HPink" || pgm2?.id == "HPOther" ? "Not evaluated (model not included yet)" :
                                                 pgm2.id == " " ? "Not evaluated (model not recognized)" :
                                                 dev_fw2.version == " " ? "Not evaluated (firmware version missing)" :
                                                 dev_fw2.id == " " ? "Not evaluated (firmware version mismatch)" :    // && dev_fw.id  in (select version from firmware) 
                                                 dev_fw2.id == " " ? "Not evaluated (firmware version unrecognized)" :
                                                 dev_fw2.br_date == " " || pgm2.id == " " || dev_fw2.revs_out_of_date == " " ? "Not evaluated" : "OK",

                                 out_of_support = pgm2.out_of_support == "" ? "false" : pgm2.out_of_support,
                                 revs_out_of_date = dev_fw2.revs_out_of_date == null ? null : dev_fw2.revs_out_of_date,
                                 program_id = pgm2.id == null ? null :  pgm2.id,
                                 firmware_id = dev_fw2.id == null ? null : latest_fw2.br_date
                             };

You are making left join, so p can be null. You need to account for that.

Here is the query that should work, although I don't know for sure what kind of value is p.value. The query will work if value is a reference type. If value is value type, than use the cast similar to product_id cast.

var qry = from c in dc.category_feature_Name_trans_SelectAll_Active()
            join p in dc.product_category_feature_trans_SelectAll()
            on c.cft_id equals p.cft_id into cp
            from p in cp.DefaultIfEmpty()
            select new
            {
                c.cft_id,
                c.feature_id,
                c.feature_name,
                product_id = p == null ? (int?)null : p.product_id,
                value = p == null ? null : p.value,
            };