Use XmlTextWriter to generate XML

using (MemoryStream ms = new MemoryStream())
{
  XmlTextWriter Xwriter = new XmlTextWriter(ms, Encoding.UTF8);

  Xwriter.WriteStartDocument();  //Put the XML declaration
  Xwriter.Formatting = Formatting.Indented;  //Format the XML document

  Xwriter.WriteComment("XML sample file for ch2.2"); //Put a comment
  Xwriter.WriteStartElement("po");  //This is the root element
  Xwriter.WriteAttributeString("id", "PO1456");

  Xwriter.WriteStartElement("address");
  Xwriter.WriteAttributeString("type", "shipping");
  Xwriter.WriteElementString("name", "Frits Mendels");
  Xwriter.WriteElementString("street", "152 Cherry St");
  Xwriter.WriteElementString("city", "San Francisco");
  Xwriter.WriteElementString("state", "CA");
  Xwriter.WriteElementString("zip", "94045");
  Xwriter.WriteEndElement();

  Xwriter.WriteStartElement("items");
  Xwriter.WriteStartElement("item");
  Xwriter.WriteAttributeString("quantity", "1");
  Xwriter.WriteAttributeString("productCode", "R-273");
  Xwriter.WriteAttributeString("description", "14.4 Volt Cordless Drill");
  Xwriter.WriteAttributeString("unitCost", "189.95");
  Xwriter.WriteEndElement();

  Xwriter.WriteStartElement("item");
  Xwriter.WriteAttributeString("quantity", "1");
  Xwriter.WriteAttributeString("productCode", "1632S");
  Xwriter.WriteAttributeString("description", "12 Piece Drill Bit Set");
  Xwriter.WriteAttributeString("unitCost", "14.95");
  Xwriter.WriteEndElement();
  Xwriter.WriteEndElement();

  Xwriter.WriteEndElement();
  Xwriter.WriteEndDocument();
  Xwriter.Flush();  //flush everything from the buffer
  ms.Position = 0;
  StreamReader sr = new StreamReader(ms);
  return sr.ReadToEnd();
}

The output will be

Frits Mendels
152 Cherry St
San Francisco
CA
94045

We can also use StreamWriter class instead of MemoryStream, just replace this line of code

XmlTextWriter Xwriter = new XmlTextWriter(ms, Encoding.UTF8);

with

StreamWriter sw = new StreamWriter();
XmlTextWriter Xwriter = new XmlTextWriter(sw);

and remove the using of MemoryStream. Also replace this part

Xwriter.Flush();
ms.Position = 0;
StreamReader sr = new StreamReader(ms);
return sr.ReadToEnd();

with

Xwriter.Flush();
Xwriter.Close();
return sw;

Here is an example from Microsoft Technet.