How to Write XML in Textbox?

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XML

TextBoxBase

I fetch the XML data in Textbox from Database.Now i want to make some changes and update in DB.when i try to make changes textbox not allowed text,but it remmove the data. MY code

private void button1_Click(object sender, EventArgs e)
   {
       connection.Open();
       string str = "select col1 from Table where Clo2='" + txt1.Text + "'";
       SqlCommand command = new SqlCommand(str, con);
       XmlReader xmlreader = command .ExecuteXmlReader();
       xmlreader.Read();
       txt2.Text = xml.ReadOuterXml();
       XmlDocument document = new XmlDocument();
       document .PreserveWhitespace = true;
       document .LoadXml(txt2.Text);

   }

how can i enter text in Textbox