boh! basta fare una qualunque prova: a me funziona :master:
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)
        Dim dt As DataTable = libreria.GetDataTable(StringaConnessioneTest, "select * from tabella13 where [id]=3")
        If dt.Rows.Count = 1 Then
            Dim r As DataRow = dt.Rows(0)
            For i As Integer = 1 To 5
                Dim s As String = libreria.NullToString(r(i))
                If s <> "0" Then
                    Me.DropDownList1.Items.Add(s)
                End If
                
            Next
        End If

    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">
        <asp:Button ID="Button1" runat="server" Text="Carica lista" OnClick="Button1_Click" />
        <asp:DropDownList ID="DropDownList1" runat="server"></asp:DropDownList>
        <asp:LinkButton ID="LinkButton1" runat="server">Refresh</asp:LinkButton>
    </form>
</body>
</html>