Confirmation Message before closing the page (asp.net, vb.net, webapp)

I know message box is only for window apps, but what about for web apps. I would like to ask help on to do this on web apps, confirm first from the user when he/she clicks [Sign out] button, if he/she really wants to sign out with a [yes] or [no] button. Thank you!

You (sort of) can't do this in code-behind, because the client interacts with the user, not the server. That said, you can give instructions for the client to do what you want. You just have to combine it with Javascript.

    Protected Sub ConfirmBox(msg As String, callback As String, title As String)
        Dim confirmScript As String = String.Format("Confirmation('{0}','{1}');", msg, callback)
        ScriptManager.RegisterStartupScript(Me, Me.GetType, "WindowsScript", confirmScript, True)
    End Sub

Then you need the Javascript:

    <script type="text/javascript">
        function Confirmation(message, target) {
            if (confirm(message)) {
                __doPostBack("Confirmation", target);
            }
        }
    </script>

Finally, in Page_Load, check if your confirmation is confirmed:

    Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load
        If IsPostBack Then
            If Request.Params.Get("__EVENTTARGET") = "Confirmation" Then
                Select Case Request.Params.Get("__EVENTARGUMENT")
                    Case "ConfirmSignOut"
                        DoConfirmationFunctionality()
                End Select
            End If

The DoConfirmationFunctionality should have the code you want to perform.