XmlTextWriter xmlWriter = new XmlTextWriter(filename_users, _
  System.Text.Encoding.UTF8);
xmlWriter.Formatting = Formatting.Indented;
xmlWriter.WriteProcessingInstruction("xml", "version='1.0' encoding='UTF-8'");
xmlWriter.WriteStartElement("Client");
xmlWriter.Formatting = Formatting.Indented;
xmlWriter.WriteProcessingInstruction
xmlWriter.Close();
xmlDoc.Load(filename_users);
 
XmlNode root = xmlDoc.DocumentElement;
XmlElement childNode = xmlDoc.CreateElement("allow");
XmlElement childNode2 = xmlDoc.CreateElement("id");
XmlElement childNode3 = xmlDoc.CreateElement("ip");
XmlElement childNode4 = xmlDoc.CreateElement("authorizations");
 
XmlText textNodeID = xmlDoc.CreateTextNode("0");
textNodeID.Value = "0";
 
XmlText textNodeIP = xmlDoc.CreateTextNode("127.0.0.1");
textNodeIP.Value = "127.0.0.1";
 
XmlText textNodeAuthorizations = xmlDoc.CreateTextNode("false");
textNodeAuthorizations.Value = "false";				
 
root.AppendChild(childNode);
 
childNode.AppendChild(childNode2);
childNode2.AppendChild(textNodeID);
 
childNode.AppendChild(childNode3);
childNode3.AppendChild(textNodeIP);
 
childNode.AppendChild(childNode4);
childNode4.AppendChild(textNodeAuthorizations);
 
xmlDoc.Save(filename_users);

  <xs:schema id="Client" xmlns="" xmlns:xs="http://www.w3.org/2001/XMLSchema" _
    xmlns:msdata="urn:schemas-microsoft-com:xml-msdata">
    <xs:element name="Client" msdata:IsDataSet="true" msdata:Locale="en-US">
      <xs:complexType>
        <xs:choice minOccurs="0" maxOccurs="unbounded">
          <xs:element name="allow">
            <xs:complexType>
              <xs:sequence>
                <xs:element name="id" type="xs:string" default="2" _
                  minOccurs="0" />
                <xs:element name="ip" type="xs:string" minOccurs="0" />
                <xs:element name="authorizations" type="xs:boolean" _
                  minOccurs="0" />
              </xs:sequence>
            </xs:complexType>
          </xs:element>
        </xs:choice>
      </xs:complexType>
    </xs:element>
  </xs:schema>

ein bug im Programm wird immer als v1.0 bezeichnet