.NET: Posting Xml to a web-server?

Question

What is the proper way to post an XmlDocument to a web-server? Here is the skeleton function:

public static void PostXml(XmlDocument doc, String url)
{ 
   //TODO: write this
}

---

Right now i use:

//Warning: Do not use this PostXml implmentation
//It doesn't adjust the Xml to match the encoding used by WebClient
public static void PostXml(XmlDocument doc, String url)
{
   using (WebClient wc = new WebClient())
   {
      wc.UploadString(url, DocumentToStr(doc));
   }
}

Where DocumentToStr is a perfectly valid and correct method:

/// <summary>
/// Convert an XmlDocument to a String
/// </summary>
/// <param name="doc">The XmlDocument to be converted to a string</param>
/// <returns>The String version of the XmlDocument</returns>
private static String DocumentToStr(XmlDocument doc)
{
    using (StringWriter writer = new StringWriter())
    {
       doc.Save(writer);
       return writer.ToString();
    }
}

The problem with my implementation of PostXml is that it posts the String exactly as is. This means that (in my case) the http request is:

POST https://stackoverflow.com/upload.php HTTP/1.1
Host: stackoverflow.com
Content-Length: 557
Expect: 100-continue

<?xml version="1.0" encoding="utf-16"?>
<AccuSpeedData MACAddress="00252f21279e" Date="2010-10-07 10:49:41:768">
  <Secret SharedKey="1234567890abcdefghijklmnopqr" />
  <RegisterSet TimeStamp="2010-10-07 10:49:41:768">
    <Register Address="total:power" Type="Analog" Value="485" />
    <Register Address="total:voltage" Type="Analog" Value="121.4" />
    <Register Address="total:kVA" Type="Analog" Value="570" />
  </RegisterSet>
</AccuSpeedData>

You'll notice that the xml declaration has an incorrect encoding:

<?xml version="1.0" encoding="utf-16"?>

The WebClient is not sending the request in utf-16 unicode, that was how Strings in .NET are stored. i don't even know the encoding used by the WebClient.

---

The http post of the xml needs to be properly encoded, which normally happens during a call to:

Save(textWriter)

During a call to Save the XmlDocument object will adjust the xml-declaration based on Encoding of the TextWriter it is being asked to save to. Unfortunately WebClient doesn't expose a TextWriter that i can save the XmlDocument to.

See also

• Post XML to .net web service
• HTTP Post of XML string and save it as .xml on server (django/GAE)
• Send XML via HTTP Post to IP:Port
• MSDN: XmlDocument.Save Method (TextWriter)
• Sending gzipped data in WebRequest?
• C# web request with POST encoding question
• GetRequestStream throws Timeout exception randomly
• Writing XML with UTF-8 Encoding using XmlTextWriter and StringWriter

Answer 1

There's this solution. Subclass StringWriter:

public class StringWriterWithEncoding : StringWriter
{
  Encoding encoding;

  public StringWriterWithEncoding (StringBuilder builder, Encoding encoding) :base(builder)
  {
    this.encoding = encoding;
  }

  public override Encoding Encoding
  {
    get { return encoding; }
  }
}

Answer 2

Why not use an XmlWriter for which you've specified an encoding. XmlWriter requires an underlying stream, but you can just give it a MemoryStream.

As for which encoding to use, why not just pass the WebClient.Encoding? :

public static void PostXml(XmlDocument doc, String url)
{
    using (WebClient wc = new WebClient())
    {
        wc.UploadData(url, DocumentToData(doc, wc.Encoding));
    }
}

private static byte[] DocumentToData(XmlDocument doc, Encoding encoding)
{
    XmlWriterSettings settings = new XmlWriterSettings();
    settings.Encoding = encoding;
    settings.Indent = true;

    using (MemoryStream s = new System.IO.MemoryStream())
    {
        using (XmlWriter writer = XmlWriter.Create(s, settings))
        {
            doc.Save(writer);

            // You could make the return type string, but why make 
            // the web client round trip it back to bytes again?
            // return encoding.GetString(s.GetBuffer());

            return s.GetBuffer();
        }
    }
}

I haven't tested this in a real application, so let me know if I'm missing something here.