binding combo box text to text box

Question

 <asp:ComboBox ID="cmbBatchno"  OnSelectedIndexChanged="cmbBatchno_SelectedIndexChanged"
                                                                                    ItemInsertLocation="Prepend"  runat="server" AutoPostBack="True" DropDownStyle="Simple"
                                                                                    AutoCompleteMode="Append">
                                                                                </asp:ComboBox>
                                                                                <asp:TextBox ID="txtbatchno" runat="server" Text='<%#Bind("cmbBatchno") %>'   OnTextChanged="cmbBatchno_TextChanged" Visible="true"></asp:TextBox>