Global Variable won't increment/decrement

[langka]

Hi all,

I have a problem with incrementing and decrementing 'G_Index' global variable. Here is my 'default.aspx.vb' page code:

Imports System
Imports System.IO
Imports System.Data

Partial Class _Default
Inherits System.Web.UI.Page

Dim G_Index As Integer
'Public Shared G_Index As Integer

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
G_Index = 1
End Sub

Protected Sub btnPrevious_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnPrevious.Click
labelJobNo.Text = (Val(labelJobNo.Text) - 1).ToString
G_Index -= 1
MsgBox("G_Index: " + G_Index)
End Sub

Protected Sub btnNext_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnNext.Click
labelJobNo.Text = (Val(labelJobNo.Text) + 1).ToString
G_Index += 1
MsgBox("G_Index: " & G_Index.ToString)
End Sub
End Class

When I ran the page and viewed the value of 'G_Index' using MsgBox, the value only incremented once and the value stayed at 2 after a few clicking on the buttons. Why the value of global variable 'G_Index' NOT change?? Is my MS Web Developer 2008 setting has a problem?? Please pin point what is wrong with my code?

At the moment I am using labelJobNo.Text value to act as a global value to be accessed by several buttons and subs, but I would like to be able to use G_Index as a global value.
Any help really appreciated...

Thank you,

[Harsh Gupta]

Because everytime your page refreshes, the value of G_Index is set to 1 in your page_load event.

You would need to wrap it in the check condition whether page is postbacking or loading for the first time.

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
 
If Not Page.IsPostBack
     G_Index = 1
End If
 
End Sub

[langka]

Thanks for your respond, unfortunately I followed your suggested code and it still did not change the G_Index value, I don't know why it won't increment nor decrement?

Thanks,

[space_monkey]

Wouldn't this be a result of the stateless nature of the web? Once the server serves the page it forgets all about it, so you would have to either put the G_Index in the viewstate or store it as a hidden field on the page itself.

[space_monkey]

Here's an example of maintaining a value across postbacks. Its not the only way but....

Private G_INDEX_NAME As String = "G_INDEX"
 
    Private Property G_INDEX() As Integer
        Get
            Return CType(ViewState.Item(G_INDEX_NAME), Integer)
        End Get
        Set(ByVal value As Integer)
            ViewState.Item(G_INDEX_NAME) = value.ToString
        End Set
    End Property
 
    Protected Sub btnAdd_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnAdd.Click
        G_INDEX += 1
        txtResult.Text = G_INDEX.ToString
    End Sub
 
    Protected Sub btnSub_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnSub.Click
        G_INDEX -= 1
        txtResult.Text = G_INDEX.ToString
    End Sub
 
    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If Not Page.IsPostBack Then
            G_INDEX = 1
            txtResult.Text = G_INDEX.ToString
        End If
    End Sub

wow that code highlighting can be hard on the eyes.

[nemaroller]

You can define your G_Index in your global.asax Application_Start event, add it to the Application bucket, and all instances of your application will share that integer value.

'this is Global.asax.vb
Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
Application.Add("G_Index", 1)
End Sub


'this is your page
'something like below
Dim r as Integer = Convert.ToInt32(Application("G_Index"))

You can then use that variable or increment anywhere.

[langka]

Hi, Thank you for your solutions Harsh, space_monkey and Nemaroller.

First, I tried Space_monkey suggestion and it still did not change the value.

Then, I tried Nemaroller suggestion that introduces an error as following:
" Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:


Line 8: 'Public Shared G_INDEX As Integer
Line 9: 'Dim G_INDEX As Integer
Line 10: Dim r As Integer = Convert.ToInt32(Application("G_INDEX"))
Line 11: 'Private G_INDEX_NAME As String = "G_INDEX"
Line 12: "

It seems cause of the error was from "Dim r As Integer = Convert.ToInt32(Application("G_INDEX"))". I have added a 'Global.asax' to the project:

<%@ Application Language="VB" %>

<script runat="server">

    Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
        ' Code that runs on application startup
        Application.Add("G_INDEX", 1)
    End Sub
    
    Sub Application_End(ByVal sender As Object, ByVal e As EventArgs)
        ' Code that runs on application shutdown
    End Sub
        
    Sub Application_Error(ByVal sender As Object, ByVal e As EventArgs)
        ' Code that runs when an unhandled error occurs
    End Sub

    Sub Session_Start(ByVal sender As Object, ByVal e As EventArgs)
        ' Code that runs when a new session is started
    End Sub

    Sub Session_End(ByVal sender As Object, ByVal e As EventArgs)
        ' Code that runs when a session ends. 
        ' Note: The Session_End event is raised only when the sessionstate mode
        ' is set to InProc in the Web.config file. If session mode is set to StateServer 
        ' or SQLServer, the event is not raised.
    End Sub
       
</script>

By the way, why didn't I get a 'Global.asax.vb' file when I add 'Global.asax' to the project? I am using MS Visual Web Developer 2008 Express Edition. At the moment I am using a Val(labelJobNo.text) to store a value of a global variable.
Thank you all...

[mendhak]

Global.asax is not a web page to be viewed. It's simply a class, so all your code goes into global.asax.

Place a breakpoint at

Application.Add("G_INDEX", 1)

And when it hits this line, press F10, then do a quick watch and evaluate Application("G_INDEX"). Anything?

Let me further add to your confusion, though. Because it's an ASP.NET application, I recommend using static variables as they are considered more efficient. So what you can do is create a class named AppGlobals (for example) and define a static variable in it. Give it a default value, then access it from your pages using AppGlobals.VarName.

[nemaroller]

Since you want to actually change that value multiple times, a static member would be inappropriate.

I think Mendhak was stating you could define a static variable name like the following so it could be used like so:

public class AppGlobals
{
public static string GINDEX = "GIndex";
}

r = Convert.ToInt32(Application(AppGlobals.GINDEX));

Of course, doing that you sacrifice memory for having a static string that hangs around for the lifetime of the application pool and therefore can't be reclaimed until the application is torn down, whereas using

Application("GIndex")

uses an instance string to do a lookup, which will be reclaimed when the method falls out of scope (or at least sometime when necessary).

What mendhak was also NOT saying was to declare a public variable in your Global class and expect it to be inherently thread-safe. You would have to wrap accessors around it which would need to lock access to the variable from other threads while it was being manipulated. The Application cache provides that for you, so you might as well use it, unless you want further type safety.

[mendhak]

Yes, that too. :P