List<long> key = rateCardData.Select(x => x.RateTableCategory2.RateCategoryID2).ToList();
The cast to value type 'Int64' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.
List<long> key = rateCardData.Select(x=>.RateTableCategory2.RateCategoryID2).ToList<long>();
List<long> key =(List<long>) rateCardData.Select(x=>.RateTableCategory2.RateCategoryID2).ToList();
var
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