Linq query return one column list -- specified cast is not valid

Question

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Linq query return one column list from table

How to return handle null values , using which data type

C#

DataTable PreviousQuarter = GroupFive.Tables["PreviousQuarter_108_49_100_51"];
                LastQuarter.Merge(PreviousQuarter);
                
                var AHCT = (from ah in PreviousQuarter.AsEnumerable()
                                  where
                                    ah.Field<int>("ParameterID") == 10 || ah.Field<int>("ParameterID") == 11
                                     || ah.Field<int>("ParameterID") == 12 || ah.Field<int>("ParameterID") == 13
                                  select ah.Field<int>("Tot_Tkts_Closed")).ToList();
                               
                if (AHCT == null)
                {
                    Response = "";
                }

What I have tried:

C#

DataTable PreviousQuarter = GroupFive.Tables["PreviousQuarter_108_49_100_51"];
                LastQuarter.Merge(PreviousQuarter);
                
                var AHCT = (from ah in PreviousQuarter.AsEnumerable()
                                  where
                                    ah.Field<int>("ParameterID") == 10 || ah.Field<int>("ParameterID") == 11
                                     || ah.Field<int>("ParameterID") == 12 || ah.Field<int>("ParameterID") == 13
                                  select ah.Field<int>("Tot_Tkts_Closed")).ToList();
                               
                if (AHCT == null)
                {
                    Response = "";
                }

Posted 26-Sep-19 3:58am

manuel_joseph

Updated 26-Sep-19 6:57am

Andy Lanng

v2

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Andy Lanng · Answer 1 · 2019-09-26T06:57:00

"int" is a struct and cannot be null. Nullable<int> is a class that can be null. The common shortcut is int? which works almost the same as int.

Some differences are that you need to use the Value property to get the non-null value. It returns the default<int> when it's null which is zero (0).

C#

DataTable PreviousQuarter = GroupFive.Tables["PreviousQuarter_108_49_100_51"];
                LastQuarter.Merge(PreviousQuarter);
                
                var AHCT = (from ah in PreviousQuarter.AsEnumerable()
                                  where
                                    ah.Field<int?>("ParameterID").Value == 10 || ah.Field<int?>("ParameterID") == 11
                                     || ah.Field<int?>("ParameterID").Value == 12 || ah.Field<int?>("ParameterID") == 13
                                  select ah.Field<int?>("Tot_Tkts_Closed")).ToList();
                               
                if (AHCT == null)
                {
                    Response = "";
                }