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.
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
What I am trying to do is simple in principle, but the lifecycle of ASP.NET pages is throwing a bucket of cold water into my day.
Here is the problem
We have implemented URL Redirection and I've inherited code that reads like this in Global.ASAX, on Sub Application_BeginRequest:
dv.Table = CommonFunctions.ConvertXmlFileToDataSet("/XmlData/WebAppFolders.xml").Tables("Application")
dv.RowFilter = "'" + fullOrigionalPath + "' LIKE '%'+ folder + '%'"
If dv.Count > 0 Then 'match on key to redirect
If fullOrigionalPath.EndsWith(dv(0)("folder") + "/") Then
Context.RewritePath(dv(0)("basePage"), True)
Else 'missing page in directory --> redirect
HttpContext.Current.Items("Raise404") = "true"
Response.Redirect("/" + dv(0)("folder"))
End If
Return
End If
Basically we are reading a large XML file that contains the URL redirects. That's working fine. The problem happens in the line...
HttpContext.Current.Items("Raise404") = "true"
In the context of Application_BeginRequest, the session object is not yet available, so I could not use it to store a flag which I named Raise404. Instead I had to resort to using the Items collection.
The problem occurs when the redirect takes place. The new page lifecycle destroys the Items array and overwrites it with a new empty one.
By the time I try to use my flag Raise404, it no longer exists on my page PreRender event.
To complicate matters we use master pages, and so I was asked to place the code that we want to execute in the master page
Ideally, if the Items array wasn't being destroyed, this code would work:
Private Sub Page_PreRender(sender As Object, e As System.EventArgs) Handles Me.PreRender
If HttpContext.Current.Items("Raise404") IsNot Nothing AndAlso HttpContext.Current.Items("Raise404").Equals("true") Then
Response.StatusCode = 404
HttpContext.Current.Items("Raise404") = Nothing
End If
End Sub
I am not sure what kind of variable I could use to store my flag and allow it to survive the redirect.
Any ideas?
Update: The problem is that the HTTP Handler servicing my request is System.Web.DefaultHTTPHandler, which does not implement IRequiresSessionState, and so when my request is being handled inside Global ASAX there is no session created. So, it seems like the solution will be to write a custom HTTPHandler that implements IRequiresSessionState, and use that for all my .aspx files. Even then, a session state is not created in Global.ASAX until PreRequestHandlerExecute is raised. So, putting it all together I think I need to write a custom HTTP Handler that implements IRequiresSessionState, and delay the redirection of the page until PreRequestHandlerExecute is raised where I will be able to store my flag into the Session state, and only after that, redirect my page.
Not very elegant, and I wonder if there will be any performance implications.
Have you considered using the query string for that?
Response.Redirect("/" & dv(0)("folder") & "?Raise404=true")
Then, in your Master Page, simply check QueryString("Raise404") and act accordingly.
The only drawback I can see is that a malicious client could deliberately add Raise404=true to the query string, which is not possible with your current solution. However, I do not see how that could do any harm.
As I suspected, my problem was directly tied to the fact that IIS 8 will avoid using an HTTPHandler that implements IRequiresSessionState if for performance reasons it detects that a session object is not needed, and so because the redirect code was happening during the Application_BeginRequest event, IIS was handling up to that point my requests using System.Web.DefaultHTTPHandler.
So, upon further research, I found this which was tremendous help:
IRequiresSessionState - how do I use it?
And I also documented myself on this article:
http://forums.iis.net/t/1094546.aspx
What solved my problem was to write a dummy HTTPHandler class. I added a new class to my app_code folder:
Imports System.Web
Public Class HTTPRequestHandler : Implements IHttpHandler, IRequiresSessionState
Private OriginalHandler As IHttpHandler
Public Sub New(handler As IHttpHandler)
Me.OriginalHandler = handler
End Sub
Public Sub ProcessRequest(context As HttpContext) Implements IHttpHandler.ProcessRequest
Throw New InvalidOperationException("HTTPRequestHandler cannot process requests.")
End Sub
Public ReadOnly Property IsReusable() As Boolean Implements IHttpHandler.IsReusable
Get
Return True
End Get
End Property
Public ReadOnly Property Handler() As IHttpHandler
Get
If Me.OriginalHandler IsNot Nothing Then
Return Me.OriginalHandler
End If
Return HttpContext.Current.Handler
End Get
End Property
End Class
Now, during Application_BeginRequest, session is not available but HttpContext.Current.Items is available. So, as the second article suggests, during PostMapRequestHandler I put a condition:
If HttpContext.Current.Items("Raise404") IsNot Nothing Then
Context.Handler = New MyNameSpace.HTTPRequestHandler(Context.Handler)
End If
And finally, that created the session object that I could finally use during PreRequestHandlerExecute:
HttpContext.Current.Session("Raise404") = "true"
Response.Redirect(HttpContext.Current.Items("Redirect"))
I will definitely not argue that this is not elegant, but it works and I can raise my 404 error during Page_PreRender on my master page.
I hope this may help others.
Cheers.
I have a WebForm with many controls, including a large number of grids. I added some callback functions on these grids to refresh their DataSource and to update some global variables in a public static class after each refresh.
Normally I would add some method calls in the PageLoadComplete Event Handler, but the callbacks get raised after that. I also tried using the OnPrerender, OnPrerenderComplete and OnSaveStateComplete method overrides, but these don't seem to work with callbacks. (I can't use the OnUnload override since I need to use the Request object.)
And here is the Actual Question:
Is there any event handler I can use? Or can I create an event and raise it after everything else has finished?
I want it to execute as the last step in every callback/post-back of any kind (but before page unload so i can still use the Response and Request objects).
The grids are DevExpress's ASPxGridViews.
UPDATE
After searching for a while, I found out that the PreRender event is fired during async Postbacks of asp:UpdatePanels but not during DevExpress's Callbacks (it is normal behavior).
So, what would really help is finding/creating an event which I can use. I'm also thinking of grabbing the data I want from the Request Object during the PageLoad execution and using them later at Unload.
What are the pros and cons of each approach? Is there anything important that I need to know before making a decision, or is there anything I might be overlooking? I'm concerned about going with the second option since I'd have to declare some class variables and I feel like there are already too many of them.
Given the fact that the PreRender,PreRenderComplete and SaveStateComplete method overrides aren't executed during DevExpress's CallBacks, I found an alternative way to use the OnUnload Method.
First on the Page_Load Method, After some checks using the Request Object, and if the conditions are met, a Handler is attached to the Unload Event, and using a Lambda expression arguments are passed to this sub.
Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load
'//Some Code//
If Request.Params("__CALLBACKPARAM").Contains("REFRESH") Then
AddHandler Me.Unload, Sub() Page_Unload(sender,e,Request)
End If
End Sub
This way the Page_Unload sub will be executed only when needed, with all the arguments it needs, while successfully avoiding the addition of more global variables.
Protected Sub Page_Unload(sender As Object, e as System.EventArgs, req As HttpRequest)
'//Some Code//
End Sub
The only problem is that i cant remove this handler, but it is no big deal, since on every request to the server the objects are recreated.
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
I need to get a value from an API I made with ASHX and normally it is called from javascript but I need to call it right in ASP.NET I figured this shouldn't be a problem but I'm not sure the syntax.
Well you have a couple options
You can refactor the code in your ASHX to be in a shared library so you can access the methods directly and so can the handler.
You can instantiate the handler and invoke the members if they aren't private.
You can create a webrequest to the handler and handle the response.
These are just a few of the easy ways.
I personally like the first method because it promotes code reuse, but depending on scenario you can do what you like.
Edit to provide answers for question in comment.
Essentially Yes... Instead of having a bunch of code in your handler you make a class called something meaningful to you contextually. Inside that class you place the logic that was in your handler. Then from your handler you can create an instance or call a static version of the class (depending on how you implemented it) passing it the HttpContext object or whatever is required for that logic to run correctly. Do the same thing in your ASPX page. You can now call into an object that contains the logic from anywhere in your app instead of having it reside in the handler alone.
EX:
Public Class MyCommonLogic
Public Shared Function ReturnSomethingCommon(context As HttpContext) As String
Return "Hello World!"
End Function
End Class
Then from the handler or the aspx page..
Dim something As String = MyCommonLogic.ReturnSomethingCommon(...)
I made the function static, but that is just an example of course I would implement it however would make more sense in your scenario.
Changed code to VB sorry about that.
If the ASHX is on the same server especially if its within the same web app, you should refactor your logic out of the ashx into a common class that both the aspx and ashx can call.
Otherwise you can look at using: System.Net.WebClient