I'm trying to create a singleton object in my ASP.NET web app. The definition is like this:
Public Module Providers
Public AppConnectionStringProvider As IConnectionStringProvider
End Module
And I'm setting this in Global.asax like this:
Sub Application_Start(ByVal sender As Object, ByVal e As EventArgs)
...
'create a default connection string provider
AppConnectionStringProvider = New MyConnectionStringProvider
....
End Sub
This works fine, but sometimes I see errors in my log file that are caused, ultimately, by using AppConnectionStringProvider when it is Nothing/null. I thought that by initializing it in Application_Start I would guarantee that this variable is always non-null, but there seem to be some circumstances where this is not so. What could cause this? I'm thinking of threading issues, but I can't see what they would be.
Edit: Below is the stack trace. Roughly, it's a simple request for the site home page:
Exception of type 'System.Web.HttpUnhandledException' was thrown.
System.Web.HttpApplication.ExecuteStep
System.Web.HttpApplication.CallHandlerExecutionStep
System.Web.HttpApplication.IExecutionStep.Execute
ASP.default_aspx.ProcessRequest
System.Web.UI.Page.ProcessRequest
System.Web.UI.Page.ProcessRequest
System.Web.UI.Page.ProcessRequest
System.Web.UI.Page.ProcessRequestMain
System.Web.UI.Page.HandleError
Object reference not set to an instance of an object.
System.Web.UI.Page.ProcessRequestMain
System.Web.UI.Control.LoadRecursive
MyApp.Default.Page_Load.21[Default.aspx.vb]
...
MyApp.SomeModule..ctor.6[Utilities.vb]
MyApp.get_ConnectionString.70[Connections.vb]
And in that last method, at that line of code, it's trying to use the object that was initialized in Application_Start, but it seems to be null, when it should have a value.
If your singleton object should dispose at sessions end then add your object to a session variable. Session[MyConnectionStringProvider] = New MyConnectionStringProvider(). alternatively if you'd like your singleton object to be alive for the application and share it with all users then add it to an application variable. Application[MyConnectionStringProvider] = New MyConnectionStringProvider. This way your object will not be null and available for all requests. I hope this helps.
The problem turned out to be a very subtle sequencing issue. I had a class that referenced the AppConnectionStringProvider object, but the code that set the provider object had a constructor that implicitly created an object that created another object that created another object that depended on the provider. So even though it looked like the object could not be null, the code that set it depended on it not being null. I had to refactor a bit to disentangle these dependencies, but now it's working fine. Thanks for everyone's comments.
Related
I have some Ajax on a web page that feeds some data to a server-side VB.Net method. Once that data is in the server-side method, I need to call another server-side method to use the data I just collected. Here is a really simplified example:
' This method gets the input from the Ajax code on the web page.
<System.Web.Services.WebMethod> _
Public Shared Sub GetAwesome(VBInputText As String)
Dim strTest As String = VBInputText
' Now that we have collected input from the user,
' we need to run a method that does a ton of other stuff.
DisplayAwesome(VBInputText)
End Sub
Protected Sub DisplayAwesome(AwesomeIn As String)
' The real app does a lot more than this. For this example, it
' just sets the text of a literal.
litAwesomeResult.Text = AwesomeIn
End Sub
Of course, in the above example DisplayAwesome(VBInputText) gives me the 'Cannot refer to an instance member...' error. So, is it possible now to call Protected Sub DisplayAwesome from Public Shared Sub GetAwesome? I'm hoping to stay close to this sort of solution because it would play very well with the app as it is already written by another coworker.
unfortunately you cannot do this, Since the page method DisplayAwesome is defined as Protected and you requires an instance of the class to access the Protected method. But changes in another instance will not reflect in the current UI. another thing you can do is Make DisplayAwesome as Shared, but this time you cannot access the UI elements inside the shared function.
The thing you can do in this situation is, return data to the called method(in front end) and handle the litAwesomeResult.Text there
Call sub with name of Form Class like this:
FormName.DisplayAwesome(VBInputText)
In VB.Net, you can call the method not shared from a shared method with Name of Form Class by default instance, because The default instance is an object Form type that the VB application framework create and manage it, when the form is added to the project.
For more info see this :
VB.NET Default Form Instances
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
our ASP.NET application is using COM+ to connect to Database
we have this structure:
A Base Class :
Imports System.EnterpriseServices
Public Class Base Inherits ServicedComponent
A Child Class:
Public Class Member Inherits Base
'Propreties
.
.
.
'Methods
Public Sub SetMember(ByVal SelectedQueue As String)
...
End Sub
In a Aspx page, we search for a member and set details:
Dim newMember As Member = New Member
newMember.SetMember(MemberNumber)
Session("SelectedMember") = newMember
We then dispose newMember:
If Not newMember Is Nothing Then
newMember.Dispose()
End If
but whenver we access the session we got an exception:
If Not Session("SelectedMember") Is Nothing Then
'Something
Else
'Something else
End If
the exception is :
Cannot access a disposed object. Object name: 'ServicedComponent'.
How can I dispose the object but keep my session valid?
I can see what you're doing wrong, but can't be clear on what would be right. Your logic as stated is:
Obtain object.
Store object.
Clean-up object, rendering it useless.
Retrieve object.
Use object.
Having 3 before 5 makes no sense.
If the object is quick to obtain, you should just do so on every page. (Often people over-estimate the cost of this).
If the object is slow to obtain, and it makes sense to store for a long term, then it shouldn't need to be cleaned-up. What is Dispose() actually doing here? With it obtaining and releasing resources used by members as needed.
I suspect that the first is the one to go for here, but that's mostly a guess.
I'd also be concerned when you talk about the database, does your object hold a database connection? If so, and pooling is available, then you should be releasing those connections as fast as possible, rather than holding onto them.
I have a VB.NET site. At the top of many of my pages I have code such as this:
Partial Class _Default
Inherits System.Web.UI.Page
Dim fns As New Functions
Dim bOnOff As Boolean
Dim LNBs As New LimsNetBusiness.SiteUI.SiteUI
Dim LNBu As New LimsNetBusiness.User.user
Dim LNBp As New LimsNetBusiness.PasswordFunctions.Password
When I publish the site. I get "Object reference not set to an instance of an object." on line:
Dim LNBs As New LimsNetBusiness.SiteUI.SiteUI
Why? And how do I fix?
You would need to show us the constructor of LimsNetBusiness.SiteUI.SiteUI I would assume.
Given that this problem only happens remotely, I'm thinking the constructor accesses an asset, connection or config file that isn't available on the server.
My recommendation is to open the DLL with Reflector and see what resources it accesses/other potentials for a null dereference.
Oddly you are saying there is no Sub New(), but I'm curious how you can create a variable of that type without having a constructor.
You mention that SiteUI passes through to your data layer - are you confident the data layer access is working fine remotely?
Not enough info. Unfortunately LimsNetBusiness is not a .net namespace. I would suggest looking into the SiteUI constructor and see if you fail inside there.
Is LimsNetBusiness a seperate DLL? Did you publish that too?
Does LimsNetBusiness.SiteUI.SiteUI reference a table, or perhaps web.config file? Maybe a table row was deleted or something similar.
This is a NullReferenceException Some where along the way a NullReferenceException is occurring.
Now you didn't provide enough information about what LimsNetBusiness is but if I had to guess:
Since I can't see you stack trace, you should be aware of the fact that the exception might be contained in the code that is instantiated in the constructor of LimsNetBusiness.SiteUI.SiteUI
If LimsNetBusiness.SiteUI is a static Property, you will need to make sure that you instantiate the returned object.
It's possible that the error is occuring in the initialization of LNBs. Since that happens in the dim statement, you probably won't see the location of the error if this is true. You can try moving the initialization of LNBs to an assignment statement:
Dim LNBs As LimsNetBusiness.SiteUI.SiteUI
LNBs = new LimsNetBusiness.SiteUI.SiteUI
Also, check in the initialization of LimsNetBusiness.SiteUI.SiteUI and make sure there is a "new" everywhere that there should be.
I've been given the thrilling task of re-writing our exception handling system. Whilst I will state that handling exceptions from an application-wide point of view isn't something we want, typically it's unavoidable when our team are understaffed for the sheer amount of work we need to push out the door, so please, no flaming the globalised solution to exception handling here :)
I've had a good hunt to see what common solutions exist. At the moment we use Global.asax with the Application_Error event to do Server.GetLastError() which is placed in Session state then a redirect is called to another page where the session data is then retrieved and output in a human readable format. The redirect also calls a sproc which will carefully audit the error information which is a) e-mailed to the developers and b) viewed from a web page only viewable by developers.
The new way I've seen of doing things is using the IHttpModule interface using a class in App_Code to do something along these lines (this is my quick implementation)
Imports Microsoft.VisualBasic
Public Class ErrorModule : Implements IHttpModule
Public Sub Dispose() Implements System.Web.IHttpModule.Dispose
' Not used
End Sub
Public Sub Init(ByVal context As System.Web.HttpApplication) Implements System.Web.IHttpModule.Init
AddHandler context.Error, AddressOf context_Error
End Sub
Public Sub context_Error(ByVal sender As Object, ByVal e As EventArgs)
Dim ex As Exception = HttpContext.Current.Server.GetLastError
' do something with the error
' call the stored procedure
' redirect the user to the error page
HttpContext.Current.Server.ClearError()
HttpContext.Current.Response.Redirect("index.htm")
End Sub
End Class
My question is, what is the benefit of this solution over using Global.asax events? Additionally, what is the best way to hand the data to an error page?
EDIT: The code above does work by the way ;)
EDIT: Also, how does the HttpModule work behind the scenes? Does it just register the Error event to that particular function on application start?
UPDATE:
Upon much further investigation it seems grabbing session data is really, really messy when it comes to using IHttpModule interface. I don't think MS have matured HttpModule enough for it to be used in our particular scenario - until there are events specific to session data it's too dangerous for us to use.
Using a module has the advantage of being easily removable, all you need to do to disable it is to remove it from <httpModules> in your config.
As far as your data goes, try going with Server.Transfer or Server.RewritePath - that will keep all the current data (including the last server error).
If for some reason it clears the last error, you can save the error to HttpContext.Items before the transfer/rewrite and then retrieve it afterwards.
Edit: In response to your edit, an IHttpModule attaches to any appropriate events in it's IHttpModule.Init implementation.
HttpModule basically does the same thing as Global.asax. It's designed as a more reusable and self-contained module for event handling.