Code:
String filename=@"C:\Test.csv";
if(sec.Positions.LastPositionClosed!=null)
{
  using (StreamWriter writer=new StreamWriter( File.Open(fileName, FileMode.OpenOrCreate, FileAccess.Write)))
            {
                writer.BaseStream.Seek(0, SeekOrigin.End);
                writer.Write(sec.Positions.LastPositionClosed.EntryPrice);
writer.Write(";");
writer.Write(sec.Positions.LastPositionClosed.ExitPrice);
writer.WriteLine();
            }
}