如何使下拉列表中的必填字段验证器？

Question

I have a dropdownlist where I added an extra value to (Choose channel...). If this value is selected I want a message displayed that is required, what I tried is:

<td>
    <asp:DropDownList ID="ServiceChannelDropDownList" AppendDataBoundItems="true"
    runat="server" DataTextField="Description" DataValueField="ID">
   <asp:ListItem Value="-1" Text="Choose channel..." />
   </asp:DropDownList>
   <asp:RequiredFieldValidator ID="DropDownListRequiredFieldValidator" runat="server"
                                ControlToValidate="ServiceChannelDropDownList" 
                                InitialValue="-1"
                                ErrorMessage="*"                                 
    </asp:RequiredFieldValidator>
</td>

Unfortunately this just let me select the initial value (which naturally does not exist in the database) without saying it is not allowed. How to solve this?

Answer 1

You need to add the default value in your code behind(.cs) where you bind Dropdownlist data, instead of doing it in .aspx page

ServiceChannelDropDownList.DataBind();
ServiceChannelDropDownList.Items.Insert(0, new ListItem("Choose channel...","-1"));

Answer 2

<td>
    <asp:DropDownList ID="ServiceChannelDropDownList" AppendDataBoundItems="true"
    runat="server" DataTextField="Description" DataValueField="ID">
   <asp:ListItem Value="0" Text="Choose channel..." />
   </asp:DropDownList>
   <asp:RequiredFieldValidator ID="DropDownListRequiredFieldValidator" runat="server"
                                ControlToValidate="ServiceChannelDropDownList" 
                                InitialValue="0"
                                ErrorMessage="*"                                 
    </asp:RequiredFieldValidator>
</td>

Answer 3

I needed to add a ValidationGroup to the RequiredFieldValidator, eg,

<asp:RequiredFieldValidator ID="DropDownListRequiredFieldValidator" runat="server"
                            ControlToValidate="ServiceChannelDropDownList"
                            InitialValue="0"
                            ErrorMessage="Channel is required."
                            ValidationGroup="WorkflowValidation" 
                            CssClass="ValidationCss" 
                            EnableTheming="false">*
                        </asp:RequiredFieldValidator>

Thanks all for the help!