RoundedCorners Properties:

·         TargetControlID - The ID of the button or link for this extender to operate on

·         Radius - The radius of the corners (and height of the added area).

·         Corners - The corners of the target control that will be rounded (can be None, TopLeft, TopRight, BottomRight, BottomLeft, Top, Right, Bottom, Left, or All)

Code:

 <%-- ScriptManager control manages client script for AJAX enabled ASP.NET pages.This enables partial-page rendering and Web-service calls.You have to used this if you want to use ajax control toolkit--%>
<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
 <%--Add the RoundedCorners Extender and set the  --%>
  
<cc1:RoundedCornersExtender ID="RoundedCornersExtender1" runat="server" TargetControlID="RadioButtonList1" Radius="20" Corners="Bottom">
</cc1:RoundedCornersExtender>
  
<br/><br/>
<asp:RadioButtonList ID="RadioButtonList1" runat="server"BackColor="#3399FF">
 <asp:ListItem>c#</asp:ListItem>
 <asp:ListItem>j#</asp:ListItem>
 <asp:ListItem>vb</asp:ListItem>
</asp:RadioButtonList>

Run the project

Here Corners="Bottom" so bottom area will be rounded