try this.
it creates its own file stream and sets the access to append.
it replaces your fiest line

Hi,

If you have a specific XML schema, you can use a DataSet not bound to a database or connection and provide your own schema. This way you can use all data bound controls and table cursors and at the end you can load/save the whole dataset back as XML.

Loren Soth

First thanks all for your time & help.
I've found out another easier way to solve my problem.

About checking whether the file is already existed or not there is static method in "system.IO" called "File.Exist" ,which check whether te file is already existed or not.

About the appending Problem ,I've used "XmlDocument" class
inorder to use some methods in it .

The result'll be like that:
</USERS>
<User>
<UserName>Buggaya</UserName>

<Email>Buggaya@gmail.com</Email>
</User>
</USERS>




Here's my code :

I have four text boxes,
txtISBN.Text = string.Empty;
txtBookName.Text = string.Empty;
txtPublication.Text = string.Empty;
txtPublishYear.Text = string.Empty;
and File path

string sStartupPath = Application.StartupPath;

After filling all text boxes when you will click on Create XML Doc button, it will check if xml file is not exist than it will create an xml file and add element into it.

In this sample I'm not validating any data.

private void btnCreateXMLDoc_Click(object sender, EventArgs e)
{

AddBook(txtISBN.Text, txtBookName.Text, txtPublication.Text, txtPublishYear.Text);


}

private void AddBook(string ISBN, string Title, string Publisher, string PublishYear)
{
XmlElement booksElement = null ;
if (!File.Exists(sStartupPath + @"\Books.xml"))
{

if (booksElement == null)
{
booksElement = xmlDoc.CreateElement("Books");
xmlDoc.AppendChild(booksElement);
xmlDoc.Save(sStartupPath + @"\Books.xml");
}
}
else
{
xmlDoc.Load(sStartupPath + @"\Books.xml");
booksElement = xmlDoc.DocumentElement; //getting the root element name
}


XmlElement bookElement = xmlDoc.CreateElement("Book");

//Attributes are extra information added to a node element itself
XmlAttribute bookAttribute = xmlDoc.CreateAttribute("ISBN");
bookElement.SetAttributeNode(bookAttribute);
bookAttribute.Value = ISBN.Trim();

XmlElement titleElement = xmlDoc.CreateElement("Title");
titleElement.InnerText = Title.Trim();
bookElement.AppendChild(titleElement);

XmlElement releaseElement = xmlDoc.CreateElement("Release");
releaseElement.InnerText = PublishYear.Trim();
bookElement.AppendChild(releaseElement);

XmlElement publisherElement = xmlDoc.CreateElement("Publisher");
publisherElement.InnerText = Publisher.Trim();
bookElement.AppendChild(publisherElement);

booksElement.AppendChild(bookElement);
xmlDoc.Save(sStartupPath + @"\Books.xml");


}