In this article I will explain with an example, how to use RequiredFieldValidator for DropDownList in ASP.Net.
RequiredFieldValidator is used to validate required inputs, for TextBoxes it is plug and play but for DropDownList an additional property InitialValue needs to be set in ASP.Net.
In the InitialValue property, the value of the Default DropDownList Item needs to be set.
HTML Markup
The HTML Markup consists of an ASP.Net DropDownList, a RequiredFieldValidator and a Button.
The first item of the DropDownList is a Default Item and its value has been set as 0.
Note: You can set any value as a Default Item value.
The RequiredFieldValidator has been assigned following properties:
1. ControlToValidate – ID of the Control to be validated.
2. InitialValue – Value which if selected then the RequiredFieldValidator will fail validation and raise error.
<asp:DropDownList ID="ddlFruits" runat="server">
    <asp:ListItem Text="---SELECT FRUIT---" Value="0"></asp:ListItem>
    <asp:ListItem Text="Mango" Value="1"></asp:ListItem>
    <asp:ListItem Text="Apple" Value="2"></asp:ListItem>
    <asp:ListItem Text="Banana" Value="3"></asp:ListItem>
    <asp:ListItem Text="Orange" Value="4"></asp:ListItem>
    <asp:ListItem Text="Lemon" Value="5"></asp:ListItem>
<asp:RequiredFieldValidator ErrorMessage="Required" ControlToValidate="ddlFruits"
    InitialValue="0" runat="server" ForeColor="Red" />
<hr />
<asp:Button ID="btnSubmit" Text="Submit" runat="server" />
Using RequiredFieldValidator for DropDownList in ASP.Net