untitled

for (int row = 4; row <= lastRow; row++)
{
    var source = worksheet.Cell(row, 1).GetString();
    var destination = worksheet.Cell(row, 3).GetString();

    // Skip rows without both Source and Destination
    if (string.IsNullOrWhiteSpace(source) ||
        string.IsNullOrWhiteSpace(destination))
    {
        continue;
    }

    var sourceCheck = ValidateSourceDestination(source);
    var destinationCheck = ValidateSourceDestination(destination);

    output.AppendLine($"Row {row}");
    output.AppendLine($"  Source      : {source}");
    output.AppendLine($"  Source Check: {sourceCheck}");
    output.AppendLine($"  Destination : {destination}");
    output.AppendLine($"  Dest Check  : {destinationCheck}");
    output.AppendLine();
}