Multiple POST parameters, some containing XML

I'd Like to be able to have my web service accept multiple POST parameters, some of which will be XML. Is this possible? The code below will generate a server error:

WebResponse resp = (WebResponse)req.GetResponse();

string programId = "1";
        string statusMessages = statusMessagesXML.ToString(SaveOptions.DisableFormatting);


    string postData = "programId=" + programId;
    postData += "&statusMessages=" + HttpUtility.UrlEncode(statusMessages);
    string data = postData;

    HttpWebRequest req = (HttpWebRequest)WebRequest.Create(uri);
    req.ContentType = "application/x-www-form-urlencoded";
    req.ContentLength = data.Length;
    req.Method = "POST";
    StreamWriter writer = new StreamWriter(req.GetRe开发者_JAVA百科questStream());
    writer.Write(data);
    writer.Flush();
    writer.Close();

    String result = null;
    WebResponse resp = (WebResponse)req.GetResponse();
    Stream readstream = resp.GetResponseStream();
    StreamReader read = new StreamReader(readstream);

    result = read.ReadToEnd();

The problem is:

req.ContentType = "application/x-www-form-urlencoded";

This tells the server your data is URL-encoded, and '<' is a URL metacharacter. Either URLEncode your data, or don't tell the server the data is URLEncoded when it isn't.