Web Forms :: Textbox In The Form But Text_changed Not Working Correctly?

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"
ontextchanged="CountryTxt_TextChanged" AutoPostBack="True"> </asp:TextBox>