Arpan: I tried the "Required Field Validator" but I guess it doesn't recognize listboxes. Anyway, you could probably stick strickly with Java by attaching an attribute to the listbox, etc etc.. However, if you can tolerate a post back you could accomplish this by using a PostBack from radiobutton No. 2, e.g.,
In the code behind:
Sub chkB_CheckedChanged(Sender As Object, e As EventArgs)
If lstA.SelectedIndex = -1 Then 'no selection
lblreq.Text="You must make a listbox selection!"
Exit Sub
Else
Response.Redirect("mypage.aspx"
Exit Sub
End If
End Sub
and in the HTML you'd have...
<HTML>
<body>
<form id="Form1" runat="server">
<asp:Listbox id="lstA" runat="server" ..vis/invis Java here/><br>
<asp:radiobutton id="A" runat="server" GroupName="mygrp" Text="First Choice" Checked="True"/>
<asp:radiobutton id="B" runat="server" GroupName="mygrp" Text="Second Choice" AutoPostBack="True" OnCheckedChanged="chkB_CheckedChanged"/>
<br><asp:Label id="lblreq" runat="server" Font-Bold="True" ForeColor="Red"/>
</form>
</body>
...and you could toggle the visibility of the label in the code behind if necc.
