Save data from TextBox into XML File

Try to bind XML file to your Textbox value for Saving Data.

DataSet ds = new DataSet();
ds.ReadXml(Server.MapPath("YourXMLFile"));
ds.Tables[0].Rows[0]["NodeName"] =textbox1.Text; 
ds.AcceptChanges();
ds.WriteXml(Server.MapPath("YourXMLFile"));

You can persist (store and load to/from file/stream/string) data structure of any complexity without any special XML-related effort using Data Contract. You should better prefer it to any other serialization method due to its data-driven matter, lack of intrusive programming, good support of compatibility, support of world-unique schema and ease of use.

Please see: http://msdn.microsoft.com/en-us/library/ms733127.aspx[^].

See my past answer where I advocate this great approach:
How can I utilize XML File streamwriter and reader in my form application?[^],
Creating a property files...[^].

—SA

Load and save objects to XML using serialization

Try the following simple code :

public String s = Server.mappath( "\\Titles.xml");

XmlTextWriter writer = new XmlTextWriter(s, null);
               writer.WriteStartElement("menu");
               writer.WriteStartElement("buttons");
               writer.WriteStartElement("button");
               writer.WriteElementString("label", textBox1.Text);

               writer.WriteEndElement();
               writer.WriteEndElement();
               writer.WriteEndElement();
               writer.Close();

Where s is the name of your xml file that is you have.