Linq to DataTable - Cannot cast DBNull

Question

New to Linq, so apologies if this is basic. This query is throwing up the error {"Cannot cast DBNull.Value to type 'System.Int64'. Please use a nullable type."} when I enumerate the results.

    private void AddLevels(long rootid)
    {
        var results = from row in data.AsEnumerable()
                      where row.Field<long>("ParentID") == rootid
                      select row;

        foreach (DataRow row in results)
        {
            //do stuff
        }

    }

The column ParentID does accept nulls - do I need to handle these separately?

EDIT2: Actual solution below that still uses Linq.

EDIT: I solved this by scrapping Linq and just using a DataTable.Select statement instead. If anyone's got input on the performance difference I'd be interested.

Donut · Accepted Answer · 2011-08-17T11:00:16.833

15

Use this line in your query:

where row.Field<decimal?>("ParentID") == rootid

decimal? is syntactic sugar for System.Nullable<decimal>, which is essentially the same as decimal, except that it also allows for null values.

long is a different type altogether -- it can only represent integers and not decimal values, hence the "Specified cast is not valid" error.

edited Aug 17 '11 at 11:00

answered Aug 17 '11 at 10:37

Donut

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Donut - thanks. That's changed the error to _Specified cast is not valid._ – David M Aug 17 '11 at 10:39
@David M -- Hrm, it may not be as simple as this; since there's no implicit conversion from `DBNull` to `long?`. – Donut Aug 17 '11 at 10:44
@David what type is ParentID in your database? – Manatherin Aug 17 '11 at 10:51
@Manatherin in the DataTable it's a System.Decimal; in the source DB (Oracle) it's Number(22,0). The rootid is taken from a non-nullable column, if that's relevant? – David M Aug 17 '11 at 10:56
1

@David M Ah, well that's your problem. Hang on, I'll update my answer – Donut Aug 17 '11 at 10:58
@David M Updated -- basically, just use `decimal?` instead of `long?` – Donut Aug 17 '11 at 11:00
@David worth noting with Donuts change that rootid is probably meant to be a decimal type too, assuming it is meant to match against that column – Manatherin Aug 17 '11 at 11:02
@Donut - Perfect! Helping me learn C# one StackOverflow answer at a time. – David M Aug 17 '11 at 11:03
@Manatherin - yep, I've updated rootid as well. Thanks for pointing it out. – David M Aug 17 '11 at 11:04

score 0 · Answer 2 · answered Aug 17 '11 at 10:36

0

long rootid is nullable type? It should be then only it can accept nulls

answered Aug 17 '11 at 10:36

Zenwalker

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Linq to DataTable - Cannot cast DBNull

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