How do I retrieve the content of a read XML file in a textbox on a second form?

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C#

XML

Hi Someone,
I don't have "System.Xml.XmlDocument " or System.Xml.XmlReader" in my using system possibilities. Is there something I should do to add them please? And How should I do this? I've downloaded the free version of Visual Studio free Community 2015. The only thing that are in it are "using System.Xml" and "using System.Linq". But I can't work with Linq. It's new to me. Also, it's been 5 years that I haven't code anymore so I'm very rusted.
Nevertheless, I wanted to add the content of a selected XML file in a textbox on a second form so the user can edit this file and save it afterwards.
Is there somebody that could help me please? Because the only thing that happens is the user can navigate to a folder on his harddrive, select the folder with all the XML files. He then has an overview of all files in the first textbox and can select one file to open where a second form's opening but without the textbox!? So actually I'm missing my second textbox and the xml-content.

Thanks in advance.

What I have tried:

public partial class Form1 : Form
    {
        //get files from directory
        string[] path = Directory.GetFiles(@"C:\Users\decraiec\Documents\Atrias_Automated", "*.XML");


        public Form1()
        {
            InitializeComponent();
            listBox1.SelectedIndexChanged += listBox1_SelectedIndexChanged;
        }

        private void btnGetFiles_Click(object sender, EventArgs e)
        {
            //read all files in path
            FolderBrowserDialog fbd = new FolderBrowserDialog();
            if (fbd.ShowDialog() == DialogResult.OK)
            {
                //if there is text in the listbox clear it first
                listBox1.Items.Clear();
                //create an array for the files and the directory.
                string[] files = Directory.GetFiles(fbd.SelectedPath);
                string[] dirs = Directory.GetDirectories(fbd.SelectedPath);
                //show every file in the listbox
                foreach (string file in files)
                {
                    listBox1.Items.Add(file);
                }
                foreach (string dir in dirs)
                {
                    listBox1.Items.Add(dir);
                }
            }
        }

        public void listBox1_SelectedIndexChanged(object sender, EventArgs e)
        {

            string currentItem = listBox1.SelectedItem.ToString();
            if (listBox1.SelectedIndex == -1)
            {
                MessageBox.Show("Item is not available in Listbox");
            }
            else
            {

                //read the file and give it back in the second form in the listbox
                var xmlString = File.ReadAllText(currentItem.ToString());
                var stringReader = new StringReader(xmlString);
                var dsSet = new DataSet();
                dsSet.ReadXml(stringReader);

                //show second form with listbox on it and the clicked xml file content
                frmEditXML frm2 = new frmEditXML();
                TextBox txb2 = new TextBox();
                txb2.Text = dsSet.GetXml();
                txb2.Text = dsSet.GetXmlSchema();
                frm2.ShowDialog();
                txb2.Show();

            }
        }



        private void Form1_Load(object sender, EventArgs e)
        {
            Application.Exit();
        }
    }

public partial class frmEditXML : Form
   {


       private void frmEditXML_Load(object sender, EventArgs e)
       {
              //open textfile and show content that was selected
       }



       private void btnSaveChanges_Click(object sender, EventArgs e)
       {
           //Text from the richtextbox
           string strsave = tbxSaveChanges.Text;
           //Create a new SaveFileDialog object
           using (SaveFileDialog sfdSave = new SaveFileDialog())
               try
               {
                   //Available file extension
                   sfdSave.Filter = "XML (*.xml*)|*.xml*";
                   //Show SaveFileDialog
                   if (sfdSave.ShowDialog() == DialogResult.OK && sfdSave.FileName.Length > 0)
                   {
                       //save file as xml
                       UTF8Encoding utf8 = new UTF8Encoding();
                       StreamWriter sw = new System.IO.StreamWriter(sfdSave.FileName, false, utf8);
                       sw.Write(strsave + " - NEG");
                       sw.Close();
                   }
               }
               catch (Exception errorMsg)
               {
                   MessageBox.Show(errorMsg.Message);
               }
           Application.Exit();
       }

       private void frmEditXML_FormClosed(object sender, FormClosedEventArgs e)
       {
           Application.Exit();
       }

   }