How Do I write and read data of xml file for save the data of windows form in c#

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C#

i have a windows form contains datagridview (having student information) and a menu(having items(save,open)
i want that

after filling all student information

--> if user click save button
save as window will open
-->then user entered the name of file and file will save (extension of file could be anything ..for example .sgc)
---> then all the data of datagridview that entered by user will save on that file
same as a want to open that file..

please help..!!!
Here is code..

private void saveAsToolStripMenuItem_Click(object sender, EventArgs e)
 {
     string folderPath = string.Empty;

     SaveFileDialog savefiledialog1 = new SaveFileDialog();
     savefiledialog1.Filter = "Xml files (*.xml)|*.xml|All files (*.*)|*.*";
     savefiledialog1.RestoreDirectory = true;

     if (savefiledialog1.ShowDialog() == DialogResult.OK)
     {
         folderPath = savefiledialog1.FileName;


                 if (!System.IO.File.Exists(folderPath))
                 {
                     XmlDeclaration declaration = doc.CreateXmlDeclaration("1.0", "UTF-8", "yes");
                     XmlComment comment = doc.CreateComment("This is an XML Generated File");
                     XmlElement root = doc.CreateElement("calcs");
                     XmlElement calc = doc.CreateElement("calc");
                     XmlAttribute Name = doc.CreateAttribute("Name");
                     //Add the values for each nodes
                     Name.Value = textBox5.Text;


                     //Construct the document
                     doc.AppendChild(declaration);
                     doc.AppendChild(comment);
                     doc.AppendChild(root);
                     root.AppendChild(calc);
                     calc.Attributes.Append(Name);

                     doc.Save(savefiledialog1.FileName);

                 }


                 //Show confirmation message
                 MessageBox.Show("Details have been added to the XML File.");

                 //Reset text fields for new input
                 //textBox5.Text = String.Empty;
             }
         }

 private void openSignalChainToolStripMenuItem_Click(object sender, EventArgs e)
 {
     XmlDataDocument xmldoc = new XmlDataDocument();
     OpenFileDialog openfiledialog1 = new OpenFileDialog();
   //  openfiledialog1.InitialDirectory = "c:\\";
     openfiledialog1.FileName = "Document";
     openfiledialog1.DefaultExt = ".xml";
     openfiledialog1.Filter = "xml file (.xml)|*.xml";
    openfiledialog1.FilterIndex = 2;
     Nullable<bool> result = Convert.ToBoolean(openfiledialog1.ShowDialog());
     openfiledialog1.RestoreDirectory = true;
     if (result==true)
     {
             string filename = openfiledialog1.FileName;
             try
             {
                 XmlNodeList xmlnode;
                 int i = 0;
                 string str = null;
                 FileStream fs = new FileStream(filename, FileMode.Open, FileAccess.Read);
                 xmldoc.Load(fs);
                 xmlnode = xmldoc.GetElementsByTagName("Product");
                 for (i = 0; i <= xmlnode.Count - 1; i++)
                 {
                     xmlnode[i].ChildNodes.Item(0).InnerText.Trim();
                     str = xmlnode[i].ChildNodes.Item(0).InnerText.Trim() + " | " + xmlnode[i].ChildNodes.Item(1).InnerText.Trim() + " | " + xmlnode[i].ChildNodes.Item(2).InnerText.Trim();
                     MessageBox.Show(str);
                     textBox5.Text = str;
                 }
             }
             catch (Exception ex)
             {
                 MessageBox.Show("File can not found!!!!");
             }


     }
 }