Combobox text editor

See more:

C#

ASP.NET

Can anyone help me in solving the problem?

I have a combobox with the items:

<asp:ComboBox ID="ComboBox2" runat="server" AutoCompleteMode="SuggestAppend" CaseSensitive="false">
   <Items>
      <asp:ListItem>--select--</asp:ListItem>
      <asp:ListItem>apple</asp:ListItem>
      <asp:ListItem>bat</asp:ListItem>
      <asp:ListItem>cat</asp:ListItem>
      <asp:ListItem>eat</asp:ListItem>
      <asp:ListItem>mat</asp:ListItem>
   </Items>
</asp:ComboBox>

When I am typing 'apple' it is hilighting apple but
When I am typing 'zyx' it is not hilighting because it is not there in items list.

But, when I AM PRESSING SAVE BUTTON it allowing 'zyx' into database.
I need if entered text is not matched, text has to get clear or I should not allow wrong text