textbox in the form but text_changed not working correctly

Hiya,


I got a textbox in my form and I want to when a user start typing in the textbox the listbox below the textbox will select the item that starts with that string


for example, it i type I
than in myserver side code will call that SearchString method which will go through that listbox and do the rest...

protected void CountryTxt_TextChanged(object sender, EventArgs e)
    {
        string s = CountryTxt.Text;
        SearchString(s);
    }


the problem is it only stops in the textchanged after I press the enter key; it doesn't seem to work just by typing the characters in the textbox...
<asp:TextBox ID="MachineTxt"
         runat="server" Width="42px"
     &nbs ...