Display GridView rows value in multiline TextBox on Button click using C# in ASP.Net

hi

in default.aspx page I put gridview ,button and text box

and bind grid view from database...

    private void ViewUsersInfo()
    {
        DataTable dt = DummyDataTable();
        using (SqlConnection conn = General.GetConnection())
        {
            using (SqlCommand _cmd = General.GetCommand("ViewphoneInfo", conn))
            {
                conn.Open();
                SqlDataReader _dr = _cmd.ExecuteReader();
             
                if (_dr.HasRows)
                {
                    GridView1.DataSource = _dr;
                    GridView1.DataBind();
                }                 
            }
        }

    }

and:

   <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
   <Columns>
        <asp:TemplateField ItemStyle-Width="200px" ItemStyle-VerticalAlign="Middle" ShowHeader="False">
             <ItemTemplate>
                  <asp:Label ID="LblName" runat="server" Text='<%#Eval("Number")%>'></asp:Label>
             </ItemTemplate>
        </asp:TemplateField>
   </Columns>
   </asp:GridView>
   <asp:Button ID="Button7" runat="server" Text="Button" OnClick="Button7_Click" />
   <asp:TextBox ID="Txttest" runat="server"></asp:TextBox>

I want when I click on button7 it will put all data from gridview into textbox...

How I can do it?

Best regards

neda