score:14

Accepted answer

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.

score:0

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


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