Dropdownlist validation in Asp.net Using Required field validator

Introduction:
Here I will explain how to Dropdownlist validation in Asp.net Using Required field validator.
In previous articles I explained validate email address using  code for validating checkbox in asp.net  In ASP.Net And Many Article Relating to Validation Now I Will   Explained  Dropdownlist validation in Asp.net Using RequiredField validator  In Asp.net.

To implement this one we need to write code as shown below in your aspx page

<asp:DropDownList  CssClass="dropdown" ValidationGroup="g1" 
    ID="dropdown" runat="server" OnSelectedIndexChanged="dropdown_SelectedIndexChanged"></asp:DropDownList>
<asp:RequiredFieldValidator ControlToValidate="dropdown" ID="RequiredFieldValidator1"
ValidationGroup="g1" CssClass="dropdown" ErrorMessage="Select Dropdown"InitialValue="--Select Asprockers--" runat="server"  Display="Dynamic">
</asp:RequiredFieldValidator>


dropdown.Items.Insert(0, "--Select Asprockers--"));