How to Select Nullable Value from a coloumn in the List

Question

How can I select the Nullable Value from a column from the list.

Say for example I have a dataset converted into a list like below instead of passing a value in the client id(nullable column). I need to pass null. I've made the following attempts myself:

var reqlist = (from list in tableList.AsEnumerable()
                where (list.Field<int?>("ClientID") == clientID)
                       && list.Field<bool>("VisibleToAdmin") == true
                       && list.Field<bool>("Required") == true
                select list.Field<string>("FieldName"));

1.

var reqlist = (from list in tableList.AsEnumerable()
                 where (list.Field<int?>("ClientID") == null)
                        && list.Field<bool>("VisibleToAdmin") == true
                        && list.Field<bool>("Required") == true
                 select list.Field<string>("FieldName"));

2.

var reqlist = (from list in tableList.AsEnumerable()
                 where (list.Field<int?>("ClientID") == (int?)(null))
                        && list.Field<bool>("VisibleToAdmin") == true
                        && list.Field<bool>("Required") == true
                 select list.Field<string>("FieldName"));

3.

var reqlist = (from list in tableList.AsEnumerable()
                 where (list.Field<int?>("ClientID") == (bool?)(null))
                        && list.Field<bool>("VisibleToAdmin") == true
                        && list.Field<bool>("Required") == true
                 select list.Field<string>("FieldName"));

4.

var reqlist = (from list in tableList.AsEnumerable()
                 where (list.IsNull("ClientID"))
                        && list.Field<bool>("VisibleToAdmin") == true
                        && list.Field<bool>("Required") == true
                 select list.Field<string>("FieldName"));

With all of the above methods, an InvalidCastException is thrown.

@DACrosby: +1 for your edit. Does wonders for legibility. (@mohamed: hint, hint ;) — stakx - no longer contributing, Jul 01 '13 at 07:07
Are you sure that you are using the correct types for the fields in your `Field` calls? It occurs to me that getting this wrong may throw this kind of exception as well... — Chris, Jul 01 '13 at 07:54

score 0 · Answer 1 · answered Jul 01 '13 at 07:55

0

It's completely legal to compare nullable value with null:

list.Field<int?>("ClientID") == null
// same as 
!list.Field<int?>("ClientID").HasValue

Looks like you have DbNull.Value in either VisibleToAdmin or Required field. So, you should use nullable bool to get those fields values:

int? clientID = null;
var reqlist = from r in tableList.AsEnumerable()
              where r.Field<int?>("ClientID") == clientID &&
                    r.Field<bool?>("VisibleToAdmin") == true &&
                    r.Field<bool?>("Required") == true
              select r.Field<string>("FieldName");

answered Jul 01 '13 at 07:55

Sergey Berezovskiy

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1

Thanks lazyberezovsky.It is Working Well. – mohamed Kalithu Jul 01 '13 at 10:34
@mohamedKalithu that's good. Please mark answer as accepted for others to see that problem was solved – Sergey Berezovskiy Jul 01 '13 at 10:36

How to Select Nullable Value from a coloumn in the List

1 Answers1