prova a vedere se la logica è questa
codice:
<%@ Page Language="VB" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<script runat="server">
Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
'qui faccio quello che devo quando invio il form al server
End Sub
Protected Sub DropDownList1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs)
Dim ddl As DropDownList = DirectCast(sender, DropDownList)
Dim index As Integer = ddl.SelectedIndex
Me.DropDownList2.Visible = (index > 0)
End Sub
</script>
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Pagina senza titolo</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
<asp:ListItem Value="0" Text="fai la scelta"></asp:ListItem>
<asp:ListItem Value="1" Text="scelta1"></asp:ListItem>
<asp:ListItem Value="2" Text="scelta2"></asp:ListItem>
</asp:DropDownList>
</div>
<div>
<asp:DropDownList ID="DropDownList2" runat="server" Visible="false">
<asp:ListItem Value="0" Text="fai la scelta"></asp:ListItem>
<asp:ListItem Value="1" Text="scelta1"></asp:ListItem>
<asp:ListItem Value="2" Text="scelta2"></asp:ListItem>
</asp:DropDownList>
</div>
<asp:TextBox ID="TextBox1" runat="server" ></asp:TextBox>
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ErrorMessage="RequiredFieldValidator" ControlToValidate="TextBox1"></asp:RequiredFieldValidator>
<asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />
</form>
</body>
</html>