This sample illustrates the use of application state to read a dataset in Application_Start.

VB Application2.aspx [Run Sample] | [View Source]

Because an application and all the objects it stores can be concurrently accessed by different threads, it is better to store only infrequently modified data with application scope. Ideally an object is initialized in the Application_Start event and further access is read-only.

In the following sample a file is read in Application_Start (defined in the Global.asax file) and the content is stored in a DataView object in the application state.

Sub Application_Start()
    Dim ds As New DataSet()

    Dim fs As New FileStream(Server.MapPath("schemadata.xml"),FileMode.Open,FileAccess.Read)
    Dim reader As New StreamReader(fs)
    ds.ReadXml(reader)
    fs.Close()

    Dim view As New DataView (ds.Tables(0))
    Application("Source") = view
End Sub

In the Page_Load method, the DataView is then retrieved and used to populate a DataGrid object:

Sub Page_Load(sender As Object, e As EventArgs)
    Dim Source As New DataView  = CType(Application("Source"), DataView)
    ...
    MyDataGrid.DataSource = Source
    ...
End Sub

The advantage of this solution is that only the first request pays the price of retrieving the data. All subsequent requests use the already existing DataView object. As the data is never modified after initialization, you do not have to make any provisions for serializing access.

In the following customization page, values for user preferences are modified in the Submit_Click event handler according to user input.

Protected Sub Submit_Click(sender As Object, e As EventArgs)
    Session("BackColor") = BackColor.Value
    ...

    Response.Redirect(State("Referer").ToString())
End Sub

The individual values are retrieved using the GetStyle method:

Protected GetStyle(key As String) As String
    Return(Session(key).ToString())
End Sub

The GetStyle method is used to construct session-specific styles:

<style>
    body
    {
      font: <%=GetStyle("FontSize")%> <%=GetStyle("FontName")%>;
      background-color: <%=GetStyle("BackColor")%>;
    }
    a
    {
        color: <%=GetStyle("LinkColor")%>
    }
</style>

To verify that the values are really stored with session scope, open the sample page twice, then change one value in the first browser window and refresh the second one. The second window picks up the changes because both browser instances share a common Session object.

Configuring session state: Session state features can be configured via the <sessionState> section in a web.config file. To double the default timeout of 20 minutes, you can add the following to the web.config file of an application:

If cookies are not available, a session can be tracked by adding a session identifier to the URL. This can be enabled by setting the following:

By default, ASP.NET will store the session state in the same process that processes the request, just as ASP does. Additionally, ASP.NET can store session data in an external process, which can even reside on another machine. To enable this feature:

• Start the ASP.NET state service, either using the Services snap-in or by executing "net start aspnet_state" on the command line. The state service will by default listen on port 42424. To change the port, modify the registry key for the service: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\aspnet_state\Parameters\Port
• Set the mode attribute of the <sessionState> section to "StateServer".
• Configure the stateConnectionString attribute with the values of the machine on which you started aspnet_state.

<sessionState
  mode="StateServer"
  stateConnectionString="tcpip=localhost:42424"
/>

On the same page, a GetStyle method is used again to provide the individual values stored in the cookie:

Protected Function GetStyle(key As String) As String
  Dim cookie As HttpCookie = Request.Cookies("preferences1")
  If cookie <> Null Then
    Select Case key
      Case "ForeColor"
        Return(cookie.Values("ForeColor"))
      Case ...
    End Select
  End If
  Return("")
End Function

Verify that the sample works by opening the cookies1.aspx page and modifying the preferences. Open the page in another window, it should pick up the new preferences. Close all browser windows and open the cookies1.aspx page again. This should delete the temporary cookie and restore the default preference values.

VB Cookies2.aspx [Run Sample] | [View Source]

To make a cookie persistent between sessions, the Expires property on the HttpCookie class has to be set to a date in the future. The following code on the customization.aspx page is identical to the previous sample, with the exception of the assignment to Cookie.Expires:

Protected Sub Submit_Click(sender As Object, e As EventArgs)
    Dim cookie As New HttpCookie("preferences2")
    cookie.Values.Add("ForeColor",ForeColor.Value)
    ...
    cookie.Expires = DateTime.MaxValue ' Never Expires

    Response.Cookies.Add(cookie)

    Response.Redirect(State("Referer").ToString())
End Sub

Verify that the sample is working by modifying a value, closing all browser windows, and opening cookies2.aspx again. The window should still show the customized value.

Note that if you open the page in several browser windows, each browser window will initially show the name panel. Each window can independently navigate between the panels.