Mentre attendo come consigliato da Andrea, qualche passo avanti...

La convalida secondo condizioni previste adesso funziona, quello che non riesco a fare è nella funzione javascript far visualizzare una alert quando i campi del form sono lasciati vuoti...


codice:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="button.aspx.cs" Inherits="SqlServer_button" %>
 
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title></title>
</head>
<body onload="InitValidators()">
    <form id="form1" runat="server">
        ...
        <asp:TextBox ID="DateStart" runat="server"></asp:TextBox>
        <asp:TextBox ID="DateEnd" runat="server"></asp:TextBox>
        ...
        <asp:ImageButton ID="Button1" ImageUrl="images/cerca_icon.gif" runat="server" OnClick="Button1_Click"
            OnClientClick="javascript:return InitValidators();return false;" />
        ...
        

        

        <asp:TextBox ID="DateStart1" runat="server"></asp:TextBox>
        <asp:TextBox ID="DateEnd1" runat="server"></asp:TextBox>
        ...
        <asp:ImageButton ID="Button2" ImageUrl="images/cerca_icon.gif" runat="server" OnClick="Button2_Click"
            OnClientClick="javascript:return InitValidators();return false;" />
        ...

    </form>
    <script type="text/javascript" language="javascript">
        function InitValidators() {
 
            var DateStart = document.getElementById('<%=DateStart.ClientID%>');
            var DateEnd = document.getElementById('<%=DateEnd.ClientID%>')
            var DateStart1 = document.getElementById('<%=DateStart1.ClientID%>');
            var DateEnd1 = document.getElementById('<%=DateEnd1.ClientID%>')
 
            if (DateStart.value == '' || DateEnd.value == '') {
                ValidatorEnable(document.getElementById('<%= RequiredFieldValidator1.ClientID%>'), true);
                ValidatorEnable(document.getElementById('<%= RequiredFieldValidator2.ClientID%>'), true);
            }
            else {
                ValidatorEnable(document.getElementById('<%= RequiredFieldValidator1.ClientID%>'), false);
                ValidatorEnable(document.getElementById('<%= RequiredFieldValidator2.ClientID%>'), false);
            }
 
            if (DateStart1.value == '' || DateEnd1.value == '') {
                ValidatorEnable(document.getElementById('<%= RequiredFieldValidator3.ClientID%>'), true);
                ValidatorEnable(document.getElementById('<%= RequiredFieldValidator4.ClientID%>'), true);
            }
            else {
                ValidatorEnable(document.getElementById('<%= RequiredFieldValidator3.ClientID%>'), false);
                ValidatorEnable(document.getElementById('<%= RequiredFieldValidator4.ClientID%>'), false);
            }
 
 
        }
    </script>
</body>
</html>