As an extension method:
public static class Extensions
{
public static string AsString(this XmlDocument xmlDoc)
{
using (StringWriter sw = new StringWriter())
{
using (XmlTextWriter tx = new XmlTextWriter(sw))
{
xmlDoc.WriteTo(tx);
string strXmlText = sw.ToString();
return strXmlText;
}
}
}
}
Now to use simply:
yourXmlDoc.AsString()