Un esempio di passaggio parametri con showModalDialog:

codice:
<!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 runat="server">
    <title>Pagina Chiamante</title>
    
    <script type="text/javascript">
        function Parametri()
        {
            this.Sender = null;
            this.Parametro1 = "";
            this.Parametro2 = ""
        }
        
        function Caratteristiche(){
           var feat = "dialogHeight:400px;resizable:yes;";
           return feat;
        }

        function apriPop() 
        {
            var winFeatures = Caratteristiche();
            dialogArgs = new Parametri();
            dialogArgs.Parametro1 = "Ciao";
            dialogArgs.Parametro2 = "Kahm";
            dialogArgs.Sender = window;
            var retVal = window.showModalDialog( 'pop.aspx', dialogArgs, winFeatures );
        }
    </script>
</head>

<body>

<form name="oForm" runat=server>
    <asp:Button ID="btApri" runat="server" Text="Apri popup" OnClientClick="apriPop();" />
</form>
</body>
</html>
pop.aspx:
codice:
<!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 runat="server">
    <title>Popup</title>
    <script>
    function leggiParametri() 
    {
       alert('Parametri: ' + window.dialogArguments.Parametro1 + ' -- ' + window.dialogArguments.Parametro2); 
    }
    </script>
</head>

<body onload="leggiParametri();">
    <form id="form1" runat="server">
    <div>
         

         Salve, sono stata aperta con showModalDialog !
         

    </div>
    </form>
</body>
</html>