Unable to type in AJAX ComboBox DropDownStyle DropDownList in ASP.Net

hello, I work with C # .

 I specified DropDownStyle = "DropDownList", so that users are not allowed to enter text that does not correspond to an item in the list. but my problem is that I can not typed text in the combobox even if I type the text that corresponds to an element of the list of combobox!!

<asp:ComboBox ID="Cmb_unitte_filtre" runat="server"  
                              AutoCompleteMode="SuggestAppend"  
                              DropDownStyle="DropDownList" MaxLength="0" AutoPostBack="False" ></asp:ComboBox>