Ho risolto così:
codice:
<script type="text/javascript" language="javascript">
function miadiv(select)
{
if (select.selectedIndex == 0)
{
document.getElementById('Div1').style.display = 'none';
ValidatorEnable(document.getElementById('R2'), false);
}
else
{
document.getElementById('Div1').style.display = 'block';
ValidatorEnable(document.getElementById('R2'), true);
}
}
</script>
<select name="rag" onchange="miaDiv(this)">
<option value="0">No, I don't</option>
<option value="1">Yes, I do</option>
</select>
Vat:
<asp:TextBox ID="Vat" Runat="server" CssClass="casella"></asp:TextBox>
<asp:RequiredFieldValidator ID="R1" Enabled ="false" Runat="server" ErrorMessage="*" Display="Dynamic" ControlToValidate="Vat" Font-Size="18px" Width="250px" ForeColor="#FF0000">
<div id="Div1" style="float:left; margin-left:670px; width: 320px; top:541px; display: none; position:absolute;">
Company Name:
<asp:TextBox ID="Company" Runat="server" CssClass="casella"></asp:TextBox>
<asp:RequiredFieldValidator ID="R2" Enabled ="false" Runat="server" ErrorMessage="*" Display="Dynamic" ControlToValidate="Company" Font-Size="18px" Width="250px" ForeColor="#FF0000">
</div>