Creating a fixed width file in C#

string a = String.Format("|{0,5}|{1,5}|{2,5}", 1, 20, 300);
string b = String.Format("|{0,-5}|{1,-5}|{2,-5}", 1, 20, 300);

// 'a' will be equal to "|    1|   20|  300|"
// 'b' will be equal to "|1    |20   |300  |"

<WriteFixedWidth Table="orders" StartAt="1" Output="Return">
  <Position Start="1" Length="17" Name="Unique Identifier"/>
  <Position Start="18" Length="3" Name="Error Flag"/>
  <Position Start="21" Length="16" Name="Account Number" Justification="right"/>
  <Position Start="37" Length="8" Name="Member Number"/>
  <Position Start="45" Length="4" Name="Product"/>
  <Position Start="49" Length="3" Name="Paytype"/>
  <Position Start="52" Length="9" Name="Transit Routing Number"/>
</WriteFixedWidth>

public void WriteFixedWidth(System.Xml.Linq.XElement CommandNode, DataTable Table, Stream outputStream)
    {
        StreamWriter Output = new StreamWriter(outputStream);
        int StartAt = CommandNode.Attribute("StartAt") != null ? int.Parse(CommandNode.Attribute("StartAt").Value) : 0;

        var positions = from c in CommandNode.Descendants(Namespaces.Integration + "Position")
                        orderby int.Parse(c.Attribute("Start").Value) ascending
                        select new
                        {
                            Name = c.Attribute("Name").Value,
                            Start = int.Parse(c.Attribute("Start").Value) - StartAt,
                            Length = int.Parse(c.Attribute("Length").Value),
                            Justification = c.Attribute("Justification") != null ? c.Attribute("Justification").Value.ToLower() : "left"
                        };

        int lineLength = positions.Last().Start + positions.Last().Length;
        foreach (DataRow row in Table.Rows)
        {
            StringBuilder line = new StringBuilder(lineLength);
            foreach (var p in positions)
                line.Insert(p.Start,
                    p.Justification == "left" ? (row.Field<string>(p.Name) ?? "").PadRight(p.Length,' ')
                                              : (row.Field<string>(p.Name) ?? "").PadLeft(p.Length,' ')
                    );
            Output.WriteLine(line.ToString());
        }
        Output.Flush();
    }

handle.WriteLine(s20.PadRight(20));
handle.WriteLine(s80.PadRight(80));
handle.WriteLine(s10.PadRight(10));
handle.WriteLine(s2.PadRight(2));