How To Insert New Line In txt File using codebehind

See more:

C#

ASP.NET

C#4.0

hello Friends,
in my web page i am using two textboxes to add record in a text file. but the problem is that every time when i insert new record,previous record in the text file automatically delete,and the text file contains only the current record.
i want previous record present in the text file and in every entry textfile should update with previous record and new record.
kindly help me fix this issue.
Thanks in advance

my code is

<asp:TextBox ID="txt1" runat="server"></asp:TextBox><br />
<asp:TextBox ID="txt2" runat="server"></asp:TextBox><br />
<asp:Button ID="btn" runat="server" Text="save" OnClick="btn_Click" />

and my code behind is like this

protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void btn_Click(object sender, EventArgs e)
        {
            TextWriter tw = new StreamWriter(Server.MapPath("date.txt"));
            tw.WriteLine(txt1.Text + txt2.Text + System.DateTime.Now);
            tw.Close();
        }