validate barcodes so that only valid 13-digit numeric barcodes

 string barcodes = string.Join(",",     result.data         .Where(a => !string.IsNullOrEmpty(a.BarcodeValue)) // ignore null/e...

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leading zeros removed but then still ensure it becomes a fixed 13-digit barcode

 string barcodes = string.Join(",",     result.data.Select(a =>     {         if (long.TryParse(a.BarcodeValue, out long num)) ...

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Remove the leading zero(s) before joining, you can parse to a number and back to string.

 string barcodes = string.Join(",",     result.data.Select(a =>     {         if (long.TryParse(a.BarcodeValue, out long num)) ...

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