กำหนดค่า InitialValue = "-1"
ตั้งค่า ListItem Value = "-1"
<asp:RequiredFieldValidator ID="RequiedFieldValidatior1" Display="Dynamic"
InitialValue="-1" EnableClientScript ="true"
ValidationGroup="mainGroup" runat="server" ControlToValidate="DropDownList1"
Text="*" ErrorMessage="*" SetFocusOnError="True" ForeColor="red">
</asp:RequiredFieldValidator>
<asp:DropDownList ID="deedDropDownList" CssClass="form-control" runat="server" >
<asp:ListItem Value="-1">กรุณาระบุ</asp:ListItem>
<asp:ListItem Value="1">เขียว</asp:ListItem>
<asp:ListItem Value="2">ขาว</asp:ListItem>
<asp:ListItem Value="3">เหลือง</asp:ListItem>
</asp:DropDownList>