Friday, 13 March 2009

An Improved MsgBox for Asp.Net

Often times a simple messagebox is required at the client under ASP.Net. This is easy to do by sending back a JavaScript Alert(). However, this does not work so well if the user navigates around using the back and forward buttons, as they will see the message multiple times. Also, we would like our msgbox to be usable from AJAX enabled apps, so that an asynchronous postback can also trigger an alert.

The below 3.5 code meets these criteria:

Module PageExtensions
    <extension()> _
    Public Sub MsgBox(ByVal aPage As Page, ByVal aMessage As String)

        Dim timeStr As String = "msgbox" & Date.Now().Ticks.ToString

        ' Set a cookie after showing alert so as will not re-show if back button or refresh is used.

        Dim alertScript As String = "if (document.cookie.indexOf('" & timeStr & "') == -1) {"
        alertScript &= "alert(""" & aMessage & """);"
        alertScript &= "document.cookie = '" & timeStr & "=deleted';}"

        System.Web.UI.ScriptManager.RegisterStartupScript(aPage, GetType(String), aMessage, alertScript, True)

    End Sub

End Module

To use within a page, you can now simply type:
MsgBox("Hello World"), just like in a Windows App.

A temporary cookie is set for each msgbox invoked so that the msgbox will only be shown once. The method is an extension method of the Page class, so needs to be contained within a Module, or used without the attribute.

Note also you will need a reference to System.Web.Extensions in your project.