In this article I will explain with an example, how to implement Required validation for RadioButtonList control using RequiredFieldValidator in ASP.Net.
Validating RadioButtonList using RequiredFieldValidator
The following HTML Markup consists of an ASP.Net RadioButtonList control, a RequiredFieldValidator and a Button.
The ID of the RadioButtonList control is set into the ControlToValidate property of the RequiredFieldValidator.
Once ControlToValidate property is set, the RequiredFieldValidator will raise error if no option is selected in the RadioButtonList control.
Select Fruit:
<asp:RadioButtonList ID="rblFruits" runat="server">
    <asp:ListItem Text="Apple" Value="1" />
    <asp:ListItem Text="Mango" Value="2" />
    <asp:ListItem Text="Papaya" Value="3" />
    <asp:ListItem Text="Banana" Value="4" />
    <asp:ListItem Text="Orange" Value="5" />
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" ErrorMessage="Please select a Fruit.<br />"
    ControlToValidate="rblFruits" runat="server" ForeColor="Red" Display="Dynamic" />
<asp:Button ID="btnSubmit" Text="Submit" runat="server"/>
RadioButtonList Required validation using RequiredFieldValidator in ASP.Net