[asp.net] Dropdownlist validation in Asp.net Using Required field validator

I have Dropdownlist whose value field and text field are bind at runtime. it has --select-- as first item with a value of 0 and the rest of the values are bind at runtime.

I have given validaton group for both the control and the validator as "g1" and Intialvalue=0

But still the page is posting back even if I select --select-- option.

<asp:DropDownList AutoPostBack="true" CssClass="dropdown" ValidationGroup="g1" 
    ID="ddlReportType" runat="server" 
    OnSelectedIndexChanged="ddlReportType_SelectedIndexChanged"></asp:DropDownList>

<asp:RequiredFieldValidator ControlToValidate="ddlReportType" ID="RequiredFieldValidator1"
ValidationGroup="g1" CssClass="errormesg" ErrorMessage="Please select a type"
InitialValue="0" runat="server"  Display="Dynamic">
</asp:RequiredFieldValidator>

And code Behind to Bind the Dropdown

ddlReportType.Items.Clear();
ddlReportType.DataSource = dt.Tables[0];
ddlReportType.DataTextField = "ReportType";
ddlReportType.DataValueField = "ReportTypeID";
ddlReportType.DataBind();
ddlReportType.Items.Insert(0, new ListItem("--Select--", "0"));
//ddlReportType.Items[0].Value = "0";
ddlReportType.SelectedIndex = 0;

Here use asp:CompareValidator, and compare the value to "select" option.

Use Operator="NotEqual" ValueToCompare="0" to prevent the user from submitting the "select".

<asp:CompareValidator ControlToValidate="ddlReportType" ID="CompareValidator1"
    ValidationGroup="g1" CssClass="errormesg" ErrorMessage="Please select a type"
    runat="server" Display="Dynamic" 
    Operator="NotEqual" ValueToCompare="0" Type="Integer" />

When you do above, if you select the "select " option from dropdown it will show the ErrorMessage.

I was struggling with this for a few days until I chanced on the issue when I had to build a new Dropdown. I had several DropDownList controls and attempted to get validation working with no luck. One was databound and the other was filled from the aspx page. I needed to drop the databound one and add a second manual list. In my case Validators failed if you built a dropdown like this and looked at any value (0 or -1) for either a required or compare validator:

<asp:DropDownList ID="DDL_Reason" CssClass="inputDropDown" runat="server">
<asp:ListItem>--Select--</asp:ListItem>                                                                                                
<asp:ListItem>Expired</asp:ListItem>                                                                                                
<asp:ListItem>Lost/Stolen</asp:ListItem>                                                                                                
<asp:ListItem>Location Change</asp:ListItem>                                                                                            
</asp:DropDownList>

However adding the InitialValue like this worked instantly for a compare Validator.

<asp:ListItem Text="-- Select --" Value="-1"></asp:ListItem>

Add InitialValue="0" in Required field validator tag

 <asp:RequiredFieldValidator InitialValue="-1" ID="Req_ID"
      Display="Dynamic" ValidationGroup="g1" runat="server"
      ControlToValidate="ControlID"
      InitialValue="0" ErrorMessage="ErrorMessage">
 </asp:RequiredFieldValidator>