convert this in xml string

Too late: you already convert something into XML. Manually.

Do be serious. I hate expression "reinvent the wheel" because reinventing the wheel can be very fruitful, but what you are doing is reinventing a wooden wheel created by cutting out a piece of the tree trunk while others use rubber air-pumped tires with leak sensors, etc. This is too naive, low-quality, unsupportable, etc.

Instead, consider using the following:
http://msdn.microsoft.com/en-us/library/ms733127.aspx[^] (preferred approach),
http://msdn.microsoft.com/en-us/library/system.xml.xmldocument.aspx[^] (simplest, best for not to big volumes of data),
http://msdn.microsoft.com/en-us/library/system.xml.xmlreader.aspx[^] and http://msdn.microsoft.com/en-us/library/system.xml.xmlwriter.aspx[^] (fastest, lower-level),
http://msdn.microsoft.com/en-us/library/system.xml.linq.xdocument.aspx[^] (somewhat similar to XmlDocument, not DOM-complient but LINQ-oriented).

More then enough to choose from.

—SA

You try the following code,

public Response processRequest(Routing routing, Message msg)
        {
            StringBuilder payload = new StringBuilder();
 
            payload.Append("<message>");
            payload.Append("<routing>");
            payload.Append("<subscriptionid>");
            payload.Append(routing.subscriptionID);
            payload.Append("</subscriptionid>");
            payload.Append("<touserid>");
            payload.Append(routing.ToUserID);
            payload.Append("</touserid>");
            payload.Append("<todomain>");
            payload.Append(routing.ToDomain);
            payload.Append("</todomain>");
            payload.Append("<tochannel>");
            payload.Append(routing.ToChannel);
            payload.Append("</tochannel>");
            payload.Append("<type>");
            payload.Append(msg.type.ToString());
            payload.Append("</type>");
            payload.Append("</routing>");
            payload.Append(msg.getXmlPayload());
            payload.Append("</message>");
 
            
String xmlString =
        @"
        <!-- This is a sample XML document -->
        <items>
          <item>test with a child element <more /> stuff</item>
        </items>";
// Create an XmlReader
using (XmlReader reader = XmlReader.Create(new StringReader(xmlString)))
{
    XmlWriterSettings ws = new XmlWriterSettings();
    ws.Indent = true;
    using (XmlWriter writer = XmlWriter.Create(output, ws))
    {

        // Parse the file and display each of the nodes.
        while (reader.Read())
        {
            switch (reader.NodeType)
            {
                case XmlNodeType.Element:
                    writer.WriteStartElement(reader.Name);
                    break;
                case XmlNodeType.Text:
                    writer.WriteString(reader.Value);
                    break;
                case XmlNodeType.XmlDeclaration:
                case XmlNodeType.ProcessingInstruction:
                    writer.WriteProcessingInstruction(reader.Name, reader.Value);
                    break;
                case XmlNodeType.Comment:
                    writer.WriteComment(reader.Value);
                    break;
                case XmlNodeType.EndElement:
                    writer.WriteFullEndElement();
                    break;
            }
        }

    }
}
return this.GetResponse(this.InvokeWebMethod(payload.ToString()));
        }

Hope be helpful.
Theingi win

check the following code

 XmlWriter xmlWriter = XmlWriter.Create("test.xml");

xmlWriter.WriteStartDocument();

xmlWriter.WriteStartElement("message");

xmlWriter.WriteStartElement("routing");

xmlWriter.WriteStartElement("subscriptionid");
xmlWriter.WriteString(routing.subscriptionID);
xmlWriter.WriteEndElement();

xmlWriter.WriteStartElement("touserid");
xmlWriter.WriteString(routing.ToUserID);
xmlWriter.WriteEndElement();

xmlWriter.WriteStartElement("todomain");
xmlWriter.WriteString(routing.ToDomain);
xmlWriter.WriteEndElement();


xmlWriter.WriteStartElement("tochannel");
xmlWriter.WriteString(routing.ToChannel);
xmlWriter.WriteEndElement();

xmlWriter.WriteStartElement("type");
xmlWriter.WriteString(routing.type.ToString());
xmlWriter.WriteEndElement();

xmlWriter.WriteEndElement();

xmlWriter.WriteEndElement();
xmlWriter.Close();