ASP.NET

void btnTest_Click(object sender, EventArgs e)
       {
           Response.Write("<Script>alert('messge') </Script>");
           Response.Redirect("page2");
       }

<script language="javascript">
       function onButtonClick()
        {
            alert("message");
            document.location = "http://www.google.fr";
        }
    </script>


<asp:Button ID="btnTest" runat="server" OnClientClick="javascript:onButtonClick()" />

void btnTest_Click(object sender, EventArgs e)
{
    Response.Write(@"<Script> alert('message');
                     document.location = 'http://www.google.fr'; </Script>");
}