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i have a doubt on how to show a popup???`

   if (machineID.Count != 0)
            { 
           checkMachineGrpState(machineID);
            }

            else
           {
                FormsAuthentication.SignOut();
                Session.Abandon();
                Response.Redirect("~/Default.aspx");
            }

Ok now what im am doing in else statement is signing out the user and sending him back to the log out page.... I need to how him some pop up message that he is being signed out i cant figure out how to do that... i tried messagebox but it wont work with servver and client side.. I want to use AJAX but dont know how... any suggestions.... thanks

There are a couple of different ways you can go about this. Here's a simple example.

Your Default.aspx page will need to display the message to the user when they've logged out, so you might want a way to distinguish when you want to show the message. You could add a query string param to your redirect, like:

Response.Redirect("~/Default.aspx?ShowLogout=true");

Now on your Default.aspx page, you have a number of options. You could simply show a hidden control on the page, or write out some Javascript to show an alert box:

if (!String.IsNullOrEmpty(Request.QueryString["ShowLogout"]))
                ClientScript.RegisterStartupScript(this.GetType(), "LogoutMsg", "<script>alert('You have been logged out.');</script>");

This will simply write out a script tag that runs when the user views the page. From here, you can make it more elegant by showing the user a better dialog box. For example, you could use jQuery to create a nice looking dialog box, and call the Javascript function to show it rather than calling alert in my example.