Get email Headers from an Outlook MailItem

While working on a VSTO add-in for Microsoft Outlook, I came across the need to access the headers of an email, to parse out X-CustomProperty tags that were put in by an application that sent the emails. However, I couldn’t find a Headers property in the Outlook object model, so below is an attempt to create one.

The Regex used is courtesy of John Gietzen’s answer on this stackoverflow question.

using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using Microsoft.Office.Interop.Outlook;

public static class MailItemExtensions
{
    private const string HeaderRegex =
        @"^(?<header_key>[-A-Za-z0-9]+)(?<seperator>:[ \t]*)" +
            "(?<header_value>([^\r\n]|\r\n[ \t]+)*)(?<terminator>\r\n)";
    private const string TransportMessageHeadersSchema =
        "http://schemas.microsoft.com/mapi/proptag/0x007D001E";

    public static string[] Headers(this MailItem mailItem, string name)
    {
        var headers = mailItem.HeaderLookup();
        if (headers.Contains(name))
            return headers[name].ToArray();
        return new string[0];
    }

    public static ILookup<string, string> HeaderLookup(this MailItem mailItem)
    {
        var headerString = mailItem.HeaderString();
        var headerMatches = Regex.Matches
            (headerString, HeaderRegex, RegexOptions.Multiline).Cast<Match>();
        return headerMatches.ToLookup(
            h => h.Groups["header_key"].Value,
            h => h.Groups["header_value"].Value);
    }

    public static string HeaderString(this MailItem mailItem)
    {
        return (string)mailItem.PropertyAccessor
            .GetProperty(TransportMessageHeadersSchema);
    }
}

Sample usage:

string[] preparedByArray = mailItem.Headers("X-PreparedBy");
string preparedBy;
if (preparedByArray.Length == 1)
    preparedBy = preparedByArray[0];
else 
    preparedBy = "";

string allHeaders = mailItem.HeaderString();

Edit 2011/08/05:
Thanks to Rob for the heads-up. I updated the code to return a Lookup instead of a Dictionary – this handles the case of multiple headers with the same name.