XmlWriter to Write to a String Instead of to a File

You need to create a StringWriter, and pass that to the XmlWriter.

The string overload of the XmlWriter.Create is for a filename.

E.g.

using (var sw = new StringWriter()) {
  using (var xw = XmlWriter.Create(sw)) {
    // Build Xml with xw.


  }
  return sw.ToString();
}

As Richard said, StringWriter is the way forward. There's one snag, however: by default, StringWriter will advertise itself as being in UTF-16. Usually XML is in UTF-8. You can fix this by subclassing StringWriter;

public class Utf8StringWriter : StringWriter
{
    public override Encoding Encoding
    {
         get { return Encoding.UTF8; }
    }
}

This will affect the declaration written by XmlWriter. Of course, if you then write the string out elsewhere in binary form, make sure you use an encoding which matches whichever encoding you fix for the StringWriter. (The above code always assumes UTF-8; it's trivial to make a more general version which accepts an encoding in the constructor.)

You'd then use:

using (TextWriter writer = new Utf8StringWriter())
{
    using (XmlWriter xmlWriter = XmlWriter.Create(writer))
    {
        ...
    }
    return writer.ToString();
}