C# Nullreferenceexception in DataTable.AsEnumerable() Where

Question

The Where clause has at least 1 Datarow as a result as I know for sure.

And I checked != null before calling 1) Select 2) DefaultIfEmpty method of course and it passed without a problem.

I also tried to use ?? false but don't know the exact mechanism it works and how to use it.

The columns are all Nullable.

I don't know what is wrong here...looking for solution all day.. Because I'm self-learning programmer, I'm really lack of fundamental knowledge.

dt.AsEnumerable().Where(dr => (dr.Field<string>("name_first_API").Contains("Ibuprofen")
                            || dr.Field<string>("name_first_API").Contains("Dexibuprofen")
                            || dr.Field<string>("name_first_API").Contains("Naproxen")
                            || dr.Field<string>("name_first_API").Contains("Meloxicam"))
                            && dr.Field<DateTime>("rx_visitdate").Date >= DateTime.Today.AddYears(-3)).
                  Select(dr => dr.Field<int>("howlongday_rx")).
                  DefaultIfEmpty(0).Sum();

@TimSchmelter, I also reviewed your answers for hours but cannot understand. The DataTable is not null for sure. Then how to avoid the fields are null ? Please kindly advise me. — Kay Lee, Jul 27 '18 at 07:33
For example: `...Where(dr=>dr.Field("name_first_API")?.Contains("Ibuprofen")==true || ...)` — Tim Schmelter, Jul 27 '18 at 07:35
@TimSchmelter, all are nullable as I wroted in OP. They are all from SQL table. — Kay Lee, Jul 27 '18 at 07:37
@TimSchmelter, it works ! :) Because I know how StackOverflow works, Before asking, I've searched for all day with this but, now you're my 1 sec savior...Thank you so much ! — Kay Lee, Jul 27 '18 at 07:45
For whom is looking for another solution, refer to HimBrombeere's answer. https://stackoverflow.com/questions/35134532/avoid-nullreferenceexception-in-linq-expression-on-datatable-column/35134640#35134640 — Kay Lee, Jan 24 '22 at 08:38

Tim Schmelter · Accepted Answer · 2018-07-27T08:31:16.900

3

If all can be null you could use:

string[] meds = { "Ibuprofen", "Dexibuprofen", "Naproxen", "Meloxicam" };
int sum = dt.AsEnumerable()
    .Select(dr => new
    {
        name = dr.Field<string>("name_first_API"),
        visit = dr.Field<DateTime?>("rx_visitdate"),
        howlong = dr.Field<int?>("howlongday_rx")
    })
    .Where(x => meds.Contains(x.name, StringComparer.InvariantCultureIgnoreCase)
                && x.visit?.Date >= DateTime.Today.AddYears(-3))
    .Sum(x => x.howlong ?? 0);

edited Jul 27 '18 at 08:31

answered Jul 27 '18 at 07:43

Tim Schmelter

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Oh, Teacher...are you writing in lightspeed ? :) I feel uncomfortable with high-level code...You've been and be my teacher always..Thank you ! – Kay Lee Jul 27 '18 at 07:50
you're happiness provider. how about happiness business? :) – Kay Lee Jul 27 '18 at 07:53

C# Nullreferenceexception in DataTable.AsEnumerable() Where

1 Answers1