XMLTextWriter

理屈よりソース！要素だけ・要素と属性・要素と文字列・まっ色々ある。
(@ITの川俣さんのコードを姑息に変更せず、そのまま載せます)

var stringWriter = new StringWriter();

var xmlWriter = new XmlTextWriter( stringWriter );

try {

	xmlWriter.WriteStartDocument();
	xmlWriter.WriteStartElement( "a" );

	xmlWriter.WriteAttributeString( "attr2", "", "sample" );
	xmlWriter.WriteStartElement( "b" );

	xmlWriter.WriteEndElement();
	xmlWriter.WriteString( "\r\n" );
	xmlWriter.WriteStartElement( "c" );

	xmlWriter.WriteString( "sample3" );

	xmlWriter.WriteEndElement();
	xmlWriter.WriteElementString( "d", "sample4" );

	xmlWriter.WriteEndElement();
	xmlWriter.WriteWhitespace( "\r\n" );
	xmlWriter.WriteComment( "end of document" );
	xmlWriter.WriteEndDocument();

} finally {

	xmlWriter.Close();

}

Trace.Write( stringWriter.ToString() );