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
long is a different type altogether -- it can only represent integers and not decimal values, hence the "Specified cast is not valid" error.
long rootid is nullable type? It should be then only it can accept nulls
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