how to pass XmlDocument Object to XmlTextWriter

Hi Experts,
 I have a method which takes XmlDocument object as an argument. Now This XmlDocument Object contains the File Name in which I m interested to write my XML. Now How can I use this object to write into my XML file???

I m using XmlTextWriter, but producing Error.....
For clarity I m writing the Code 4 u..

XmlDocument document = new XmlDocument();
            string filename="Peter.xml";
            document.Load(filename);
                                     // or
            document.Save(filename);  // Not Sure

myclass.mymethod(document);  // call 2 mthod
     ................
   ..................

public void mymethod(XmlDocument doc)    // method definition
{
  XmlTextWriter tw = XmlTextWriter(doc,null);
 tw.WriteStartDocument();
 tw.WriteStartElement(c.GetType().ToString());
 ............................
}

It is giving me error
C:\Serializer\Class1.cs(51): 'System.Xml.XmlTextWriter' denotes a 'class' which is not valid in the given context....

if I try to Change XmlDocument object to String variable, It also produces an error....  Type Error

Note 1: I have to make such a method in which I have to pass an XmlDocument Object and write into the file( plz Don't say Y i want to pass XmlDocument object)

Note2: I m not sure how to use AppendChild, CreateElement .... Tell me ..preferably with any example

Plz anyone who can give me any clue how to do it ?
All I NEED to write the XmlDocument object to use for writing XML file....

Looking forward for a quick Response..

Thanks!
Ahit...