I've taken over the maintenance of the website (ASP.NET VB) and on one particular page I noticed the below code
Inherits System.Web.UI.Page
Public Shared UserNumber As String
Private Sub Page_Init(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Init
UserNumber = Session("UserNumber")
...
End Sub
My question is whether the variable UserNumber can be accessed or changed by any other user than the current one?
Many thanks
A Shared variable is the same as a static variable in C#.
These can be shared across all instances, therefore different users would share the same variable.
Looking at the variable name, I would assume you would need to remove this Shared
You are applying a Session variable value to the Shared String on Page_Init.
Therefore each time the user loads the page, their session variable will override the current value.
If you are not using this variable outside of this class, then I would recommend changing it to:
Protected UserNumber As String
EDIT My initial answer was incorrect. Trying again...
Technically, as #Curt indicated, a Shared variable is shared across instances of the class.
However, with the code as is, it is less likely that the value will be shared amongst users as it is set to each user's local copy of the value in their Session in Page_Init.
There is a possible "race condition" where after the shared variable UserNumber has been initialised in Page_Init, and another user submits a request which updates the value of that variable from their session, the first user will then see the second user's value. i.e. users can see other user's values for concurrent requests.
Instead, I recommend using a ReadOnly non-shared property to get the value from the Session once:
Private mUserNumber As String
Public ReadOnly Property UserNumber As String
Get
If String.IsNullOrEmpty(mUserNumber) Then
mUserNumber = Session("UserNumber")
End If
Return mUserNumber
End Get
End Property
This uses a pattern called "Lazy-loading", which I use in read-only properties a lot on pages to improve performance and readability of code.
Related
I am trying to set a variable on every page load automatically from the Global.asax file.
I thought I could do this from Application_BeginRequest but it doesn't seem to be working.
The variable is set from a database, essentially I am doing a very quick call to the DB to check if I need to do something else. I can't go into the detail now but can't use a cache for this.
So, I have tried many ideas, but not getting anywhere, here are two most logical to my mind:
1: Setting the variable in Application_BeginRequest
Public Class Global_asax
Inherits System.Web.HttpApplication
Shared Property MyVar as String
Sub Application_BeginRequest(ByVal sender As Object, ByVal e As EventArgs)
MyVar = CallToDatabase()
End Sub
End Class
2: Setting the variable at the start of the Global.asax file:
Public Class Global_asax
Inherits System.Web.HttpApplication
Shared Property MyVar as String = CallToDatabase()
End Class
Of these two, these are the problems:
Version (1) doesn't seem to set the variable at all, when trying to access I am getting an empty value.
Version (2) does work, but the value is static across page loads, meaning that if I refresh or move to another page, the value remains the same even when I know it should have changed.
You can use inheritance. Create a class (BasePage) and every page can inherit this base page. In the Page_Load (or Pre_Init) of the base page, put your DB check. This will allow you to check on some pages, but not all (if that is ever necessary).
I have a session variable with a datatable assigned to it. For some reason the results from the datatable (display to user in a GridView) are being shared accross multiple users who are logged in. I thought each session was independent? So when one user makes changes then the other users see those results added to their results. Not sure why. I am not using Application variables.
I initialize the Session variable in Global_asax, then populate it on a button command after the user has filled out the required entries.
Imports System.Web.SessionState
Public Class Global_asax
Inherits System.Web.HttpApplication
Sub Session_Start(ByVal sender As Object, ByVal e As EventArgs)
Session("RDDT") = New DataTable
End Sub
End Class
As it stands, I did overlook something. There was a usercontrol that someone else had created that had Public variables. The application was treating them as static. All I did was remove them and changed where in use to Session varibles. The Application works as intended now.
I am using this example I found to learn how to load class files and access variables through them. This is in a file called Class1.vb in the App_Code folder (this is not an app project):
Imports Microsoft.VisualBasic
Public Class my_class
Public Shared Sub my_sub()
Dim vartest As String
vartest = 10
HttpContext.Current.Session("myvar") = vartest
End Sub
End Class
This is the codebehind on the aspx file:
Imports my_class
Partial Public Class test
Inherits System.Web.UI.Page
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
my_class.my_sub()
Label1.Text = HttpContext.Current.Session("myvar")
End Sub
End Class
How could I access the vartest variable without using a session, since if this is accessed by multiple functions at the same time the variable can be overwritten I assume. Is it possible to go the other way, where a variable is sent to a class file?
It sounds like you need a quick overview of some basic ASP.Net Webforms concepts. Up first I'll counter a common newbie misconception:
Your Page class does not hang around on the web server for very long
I think many new ASP.Net developers have this idea of the web server keeping a single instance of their page class for every user session that hits their site, and each postback or event uses this same page class instance. That's just not how it works. ASP.Net page class instances are nearly always created and destroyed again in well under a second, and most experienced developers see it as a big problem if it takes longer.
ASP.NET relies on the HTTP protocol
The thing to remember here is ASP.Net still relies on the HTTP protocol, and http boils down to requests and responses. When you view a web page, your browser first sends a request to a server. The server responds, usually with an html document. The browser will then parse the html; based on what it sees in the html the browser may send more requests to the server for additional resources, such as javascript, images, or css files. Each request results in a separate response, and the browser uses all these resources to render the page to the screen. However, the ASP.Net runtime normally does not have to process the additional requests (that would make things slower) — ony the initial html needs ASP.Net support; you want the other resources to be basic files that can be cached.
The ASP.Net runtime creates a new instance of your class for every request.
When the ASP.net runtime processes a request for a page, it will create a new instance of your page class. The runtime will follow the ASP.Net Page lifecycle (this should really be named the "ASP.Net Page Request Lifecycle"), and call certain methods or raise certain events in this class instance, in a specific order defined by the lifecycle.
This means every postback or event runs in a different instance of your class.
It also means every postback or event is rebuilding and transmitting all of the html the goes into your page, and not just the portions you want to change. For your server code, the consequence is the only thing class-level variables are really good for in ASP.Net is things that will be used within a single http request. For the browser, the consequence is you're working with a brand new DOM after every event.
To understand all of that, it's important here to also have a good understanding of the difference between a class and an instance of a class. A couple items in your question make me unsure whether you have this understanding yet.
The ASP.Net runtime shares one application instance among all users of your site
The web server typically only has one instance of your application for the entire web site and all it's users. Therefore, anything with a Shared/static scope is common to every user. It's rarely appropriate in ASP.Net for anything to be Shared/static.
So how do you handle data that should live with a single user or visit to your site?
This is exactly what the Session is for. A session will always be unique to an individual request at any given time. You're worried about multiple functions accessing the session at the same time, but this does not happen. The ASP.Net Page Lifecycle ensures that unless you manually spawn additional threads, only one function at a time is running for a given HttpContext and Session. If a user somehow sends two requests at about the same time that should have the same Session/HttpContext, one will be held by the ASP.Net runtime until the other is completed. If you don't want to reference the session all the time, you can build properties in your class that wrap session variables. See #Pankaj's answer for an example.
First, a Session has user-scope, so it will not be overwritten by another Request.
Is it safe to access asp.net session variables through static properties of a static object?
You could encapsulate the access into a property:
Public Shared Property MyVar() As String
Get
If HttpContext.Current.Session("MyVar") Is Nothing Then
HttpContext.Current.Session("MyVar") = ""
End If
Return DirectCast(HttpContext.Current.Session("MyVar"), String)
End Get
Set(value As String)
HttpContext.Current.Session("MyVar") = value
End Set
End Property
Then you can get the variable by:
Label1.Text = my_class.MyVar
In addition to the "Tim Schmelter" reply....
You can create a BaseClass which will inherit from
System.Web.UI.Page
Place the property as suggested by "Tim". The only change you need to do is to change the access modifier to Protected and you should remove Public and Shared
You can also keep other common functions, properties that can we reused in other classes also... Similarly you can create BaseControls as well for your User controls
Finally, inherit this class in the web form....
Hope this will help you...
Base Class code
Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Web
Public Class BaseClass
Inherits System.Web.UI.Page
Protected Property MyVar() As String
Get
If HttpContext.Current.Session("MyVar") Is Nothing Then
HttpContext.Current.Session("MyVar") = ""
End If
Return Convert.ToString(HttpContext.Current.Session("MyVar"))
End Get
Set
HttpContext.Current.Session("MyVar") = value
End Set
End Property
End Class
Sample Code "Behind Code" - Showing the usage of Protected member Data from Base Class
Imports System
Imports System.Collections.Generic
Imports System.Linq
Imports System.Web
Imports System.Web.UI
Imports System.Web.UI.WebControls
Public Partial Class Default5
Inherits BaseClass
Protected Sub Page_Load(sender As Object, e As EventArgs)
If Not Page.IsPostBack Then
Dim str As String = Me.MyVar
End If
End Sub
End Class
Generally you can use different places to store application state: Application (application wide, saves state into application domain), Session (there can be saved everything what will be accessed by current browser session), ViewState (variables stored in hidden input field and will be posted on every postback). Of course you can also save state to database or file. I'm not sure what you want to achieve, but looks like you looking for something like ViewState.
Read ASP.NET State Management
In an ASP.NET 3.5 VB web app, I successfully manage to cache an object containing several personal details such as name, address, etc. One of the items is CreditNum which I'd like to change in the cache on the fly. Is there a way to access this directly in the cache or do I have to destroy and rebuild the whole object just to change the value of objMemberDetails.CreditNum?
The cache is set using:
Public Shared Sub CacheSet(ByVal key As String, ByVal value As Object)
Dim userID As String = HttpContext.Current.User.Identity.Name
HttpContext.Current.Cache(key & "_" & userID) = value
End Sub
Besides that this answer might help; Cache is really there to help you add, read, and drop the objects that your app requires frequently.
I have a sub routine where I pull in information from a database. I want to use variables from this sub routine in another sub routine. I tried making everything public but it doesn't want to be friendly and share.
Dim strEmail as String
Public Sub readDB()
strEmail = "whatever#doohikcy.com"
End Sub
Public Sub submit_btn(ByVal sender As Object, ByVal e As EventArgs)
Response.Write(strEmail)
End Sub
The readDB would read the database and set the variables and do whatever it needs to do. Then when they submit the form it would e-mail the form to whatever the email address was.
Every time an asp.net page loads, the global variables are wiped out. You can avoid this by using Session variables.
Public Sub readDB()
Session("strEmail") = "whatever#doohikcy.com"
End Sub
Then:
Public Sub submit_btn(ByVal sender As Object, ByVal e As EventArgs)
Response.Write(CStr(Session("strEmail")))
End Sub
Passing them as arguments is great, if the sequence of calls to the subs in question makes that feasible. If you literally need a variable that is visible to different subs in the same class (code-behind page in ASP.Net), what you are looking for is probably a private member variable. Declare it outside of any sub or function with the access modifier Private, and all the subs in the class will be able to access it.
Private _foo As String
The underscore is a convention that some people love, some hate. It comes in handy in VB if you want to define a property to expose the variable, where you can't use Foo as distinctly different from foo, but that's another story.
This is not the same as what would generally be understood by the term Global variable in the ASP.Net sense, where the variable would be visible throughout the application context, which lends itself to unintended consequences in the least. A private member variable is only visible to the class that owns it.
EDIT: Your example code was added after my initial answer. My VB is a little rusty but as you've written it, strEmail looks like it should have class-level visibility, including inside of submitbtn (someone correct me if I'm wrong). One possibility, since you mentioned that you are calling readDB in Page_Load is if you're checking for postback in page load, and only calling readDB on the initial load, not on postback, which would be the case when the button is clicked. You may have seen examples that include a check for Postback out of hand and not realized what it does (I only suggest that because you mentioned that you are new to ASP.Net and it's not intuitive if you're new to it--no offense intended).
Protected Sub Page_Load (sender as object, e as EventArgs)
If Not IsPostback
// this doesn't get called when the button is clicked so
// strEmail would not be populated when submitbtn is invoked
readDB
End If
End Sub
That's a bit of a guess out of nowhere though, so it might be way off base. Have you set break-points in Page_Load, readDB and submitbtn to see the state of strEmail in each?