Perform Your action in this way
<radiobuttonlist id="rblcat" runat="server">
<asp:listitem text="Circle" value="0" xmlns:asp="#unknown">
<asp:listitem text="Polygon" value="1">
now on .cs file jst do
if(convert.tostring(rblcat.selectedvalue)=="0")
{
return Circle
}
else if(convert.tostring(rblcat.selectedvalue)=="1")
{
return Polygon
}
try and let me know