Partial Class Preferences_MyPreferences
Inherits System.Web.UI.Page
Dim userID As String = Session("UserID")
This is just a page in asp.net. I want to be able to grab the Session("UserID") but every time I try, I get this error:
Session state can only be used when enableSessionState is set to true, either in a configuration file or in the Page directive. Please also make sure that System.Web.SessionStateModule or a custom session state module is included in the \\ section in the application configuration.
If I put that Dim userID inside say the Page_Load Event, then it works fine. Why does it have to be inside an event? I want to dim it once, and use it throughout the page.
Consider wrapping your call to the Session in a property in your code behind?
Public ReadOnly Property UserID() As String
Get
Return Session("UserID")
End Get
End Property
If you declare it as you have there, the variable is initialized and the session variable is expected to be ready for usage, but it is too early in the page life cycle to allow that. The preferred method would be as #p.campbell has suggested and wrap it in a Property or similar method. To answer the question though, the exception is generated because you are attempting to use session before it is available.
You need to read up on ASP.NET Page Lifecycles. The Session object doesn't become available until a certain point in the page lifecycle; if you want to grab the UserID once, you need to do it after the session becomes available.
The reason it doesn't work in your example is that the constructor for your Preferences_MyPreferences page is executed before the request object is available to the page. You should instead load it during the Page_Init or Page_Load event.
Related
Maybe someone can help me out. I have created a simple web user control, a date and time picker, to be dropped onto my webform. This all works very nicely, I can set properties, usee the control satisfactorily for ui etc.
When it comes time to "use" the controls selected_date_time property, I just cant get it to persist. Nothing.. I have researched and tried endlessly using viewsate, context and session. Onbiouosly session works, but its dirty, and I am using two copies of this control (start and end time), so session vars really needs to be hacked to maket his work.
Am I missing something? The control gets initialized every time something happens, and obviuosly loses its state information. The ui keeps its state tho, as I can select a date, write that date to a label, and that persists. But when I try to access my properties of the control to retreive the selected combined date and time, (that is already persisting visually), its nothing. I debuggeed and it gets initialized everytime I do any form of post on the page.
Can someone please shed some light on this for me? Its really starting to be an issue now.
Thanks in advance.
Example: (simple components)
UC _ save_method
ViewState("var_time") = "My veiwstate text"
form _read_method
dim str as string = ViewState("var_time")
form sees nothing in the viewstate var.
I have also tried it with normal properties and values, which wasn't working, which is why I moved on to viewsate var's for my properties. Right now, I am just trying to get the viewstate to work even without properties.
Seems my form and the control must have two seperate viewstates? I am a bit of a n00b concerning viewstates.
Thanks
[solution]
You have to explicitly reset your properties on the prerender method of the control. My misunderstanding was that the page and the control share the same single viewstate. Turns out the control and the page that its on, has its own independant viewstates.
So absically, on your property set function in the control, set your value in the viewstate, and upon prerender, take that viewstate value, and set the property get variable =- the value in viewstate.
You can now access the property from your page as if everything was normal and the world is not ending... pheww
Thanks to cnay for directing me.
'Usr control xyz
Protected Sub Page_PreRender(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.PreRender
my_time = ViewState("var_time")
'my_time is the get variable for property date_time
End Sub
'page use
xyz.date_time
Perhaps, you should post the complete control code to get the better answer but speaking in general terms - view-state (or control state) is a typically correct approach to persists control state such as properties. Each control instance get the different view-state bag and hence collisions are easily avoided.
Now, coming to your problem, a typical editable control first restores its state from view-state and then uses the request data to modify the state as needed. For example, a simple text-box control would persists its text value in view-state. On post-back, the text value will be restored from the view-state and then gets overwritten by the value present in the request (looked up by UniqueID property).
Now, for user controls, typically you can use child controls values (or properties) to derive the control value/properties - so these may not use view-state. However, if you add properties/state that is not backup by child controls then you may have to back them up in the view-state. So let's say your user control has two child controls - one for Date and other for time then can combine their values to get the selected date-time value for your control.
I have login page, once logged in I create a session variable to store the UserName.
I've used this variable to retrieve info for this User from Account table with AccountID, name etc and I return the AccountID on the page using a label (lblAccountID).
I have a button to "Add Funds" to this account, which redirects to the AddFunds.aspx page
How can I pass the AccountID into the AddFunds.aspx page which will be used to insert details into Funds table which has the AccountID.
I don't want the AccountID to be visible on the AddFunds.aspx page.
there are multiple ways to achieve this. i can explain you in brief about the 4 types which we use in our daily programming life cycle.
Please go through the below points.
1 Query String.
FirstForm.aspx.cs
Response.Redirect(“SecondForm.aspx?Parameter=” + TextBox1.Text);
SecondForm.aspx.cs
TextBox1.Text = Request. QueryString["Parameter"].ToString();
This is the most reliable way when you are passing integer kind of value or other short parameters.More advance in this method if you are using any special characters in the value while passing it through query string, you must encode the value before passing it to next page. So our code snippet of will be something like this:
FirstForm.aspx.cs
Response.Redirect(“SecondForm.aspx?Parameter=” + Server.UrlEncode(TextBox1.Text));
SecondForm.aspx.cs
TextBox1.Text = Server.UrlDecode(Request.QueryString["Parameter"].ToString());
2. Passing value through context object
Passing value through context object is another widely used method.
FirstForm.aspx.cs
TextBox1.Text = this.Context.Items["Parameter"].ToString();
SecondForm.aspx.cs
this.Context.Items["Parameter"] = TextBox1.Text;
Server.Transfer(“SecondForm.aspx”, true);
Note that we are navigating to another page using Server.Transfer instead of Response.Redirect.Some of us also use Session object to pass values. In that method, value is store in Session object and then later pulled out from Session object in Second page.
3. Posting form to another page instead of PostBack
Third method of passing value by posting page to another form. Here is the example of that:
FirstForm.aspx.cs
private void Page_Load(object sender, System.EventArgs e)
{
buttonSubmit.Attributes.Add(“onclick”, “return PostPage();”);
}
And we create a javascript function to post the form.
SecondForm.aspx.cs
function PostPage()
{
document.Form1.action = “SecondForm.aspx”;
document.Form1.method = “POST”;
document.Form1.submit();
}
TextBox1.Text = Request.Form["TextBox1"].ToString();
Here we are posting the form to another page instead of itself. You might get viewstate invalid or error in second page using this method. To handle this error is to put EnableViewStateMac=false
4. Another method is by adding PostBackURL property of control for cross page post back
In ASP.NET 2.0, Microsoft has solved this problem by adding PostBackURL property of control for cross page post back. Implementation is a matter of setting one property of control and you are done.
FirstForm.aspx.cs
<asp:Button id=buttonPassValue style=”Z-INDEX: 102″ runat=”server” Text=”Button” PostBackUrl=”~/SecondForm.aspx”></asp:Button>
SecondForm.aspx.cs
TextBox1.Text = Request.Form["TextBox1"].ToString();
In above example, we are assigning PostBackUrl property of the button we can determine the page to which it will post instead of itself. In next page, we can access all controls of the previous page using Request object.
You can also use PreviousPage class to access controls of previous page instead of using classic Request object.
SecondForm.aspx
TextBox textBoxTemp = (TextBox) PreviousPage.FindControl(“TextBox1″);
TextBox1.Text = textBoxTemp.Text;
As you have noticed, this is also a simple and clean implementation of passing value between pages.
Reference: "How to: Pass Values Between ASP.NET Web Pages"
You need to store it in a session variable:
int AccountIdVar;
Session["AccountID"] = AccountIdVar;
then you can retrieve later by
int AccountIdVar = (int)Session["AccountID"];
You can either use the session variable you stored in the previous page as it should still be accessible or another way is to pass the id over via a querystring such as www.foofoofoo.com?Id=23456.
As the others said, you can use Session or Querystring values. You can also just POST to the new page
The How To Pass Values Between Pages page is worth looking at too.
The MSDN article about Pass Values Between ASP.NET Web Pages is the best place to look for.
For this we can also use Global variable, create a module class in that declare all variables with public data type, then assign the values. Once the value is assigned it can be accessed from anywhere.
This has been bugging me for a while but when I databind a control using with a Session variable as the parameter which has not been initialized there is an exception thrown which I can't seem to catch anywhere.
Ideally if the session varaible is not set I would just like to redirect but I can't seem to figure out where I need to check for this instance.
You must check the session object on page_init event.
Check it in your page load.
Sub Page_Load()
if Not Page.ispostback()
if session("Value") <>"" then
me.hiddenfield.value = Session("ValueName")
Else
Response.redirect("PAge.aspx")
End if
End if
End Sub
I tend to add some hidden fields as sessions eventually time out
then make the datasource use the hidden control for its reference
What's the best way to view the data that LoadPostData event is loading to the controls in ASP.NET?
It's actually really simple. The NameValueCollection that get's passed to this method of EVERY control that implements the IPostbackDataHandler interface is the contents of Page.Request.Form. So you can access it at any time by getting a Watch on HttpContext.Current.Request.Form.
Ugh... I would suggest setting your IDE environment up to debug the .net framework, and set a breakpoint on the LoadPostData() method of Control. That's a bit heavy-handed, but if you're willing to wade through the recursive calls to the Control class (perhaps set a conditional breakpoint on the method?), you will be able to get to the data that way.
Good luck!
If you want to be sure you're looking at the data going into a particular control, you can subclass its control type and break during a custom implementation of IPostBackDataHandler.LoadPostData.
For example, you have a programmatically added control to collect the user's city. Change:
Public City As Textbox
to
Public City As BreakableLoadPostDataTextBox
Public Class BreakableLoadPostDataTextBox
Inherits TextBox
Protected Overrides Function LoadPostData( _
ByVal postDataKey As String, _
ByVal postCollection As System.Collections.Specialized.NameValueCollection) _
As Boolean
Return MyBase.LoadPostData(postDataKey, postCollection) ' Break here
End Function
End Class
Set a breakpoint on the Return call. When execution breaks, you should be able to see the postDataKey that's being used to read the control's new value out of the postCollection. You can of course augment this method to your heart's content with Trace calls and whatnot.
I have object A which in turn has a property of type Object B
Class A
property x as Object B
End Class
On my ASP.NET page when I select a gridview item which maps to an object of type A I serialize the object onto the QueryString and pass it to the next page.
However I run into problems if property x actually has some value as it looks like I exceed the QueryString capacity length of 4k (although I didn't think the objects were that large)
I have already considered the following approaches to do this
Session Variables
Approach not used as I have read that this is bad practice.
Using a unique key for the object and retrieving it on the next page.
Approach not used as the objects do not map to a single instance in a table, they arte composed of data from different databases.
So I guess my question is two fold
Is it worth using GKZip to compress the querystring further (is this possible??)
What other methods would people suggest to do this?
If displaying the url of the next page in the browser does not matter, you could use the context.items collection.
context.items.add("keyA", objectA)
server.transfer("nextPage.aspx")
Then on the next page:
public sub page_load(...)
dim objectA as A = ctype(context.items("keyA"), objectA)
dim objectB as B = objectA.B
end sub
One reason to use this is if you want the users to believe that the next page is really a part of the first page. To them, it only appears as if a PostBack has occurred.
Also, you don't really need a unique key using this approach if the only way to use "next page" is if you first came from "first page". The scope for the context items collections is specific to just this particular request.
I agree with the other posters who mentioned that serialized objects on the querystring is a much worse evil than using session state. If you do use session state, just remember to clear the key you use immediately after using it.
I don't understand why you wouldn't use session state but...
Option 1: Viewstate
Option 2: Form parameters instead of querystring
But also be aware that you do not get the same object back when you serialize/deserialize. You get a new object initialized with the values of the original that were serialized out. You're going to end up with two of the object.
EDIT: You can store values in viewstate using the same syntax as Session state
ViewState["key"] = val;
The value has to be serializeable though.
While storing objects in session might be considered bad practice, it's lightyears better than passing them via serialized querystrings.
Back in classic asp, storing objects in session was considered bad practice because you created thread-affinity, and you also limited your ability to scale the site by adding other web servers. This is no longer a problem with asp.net (as long as you use an external stateserver).
There are other reasons to avoid session variables, but in your case I think that's the way to go.
Another option is to combine the 2 pages that need access to this object into one page, using panels to hide and display the needed "sub-pages" and use viewstate to store the object.
I don't think passing it in the query string, or storing it in the session, is a good idea.
You need one of the following:
a) A caching layer. Something like Microsoft Velocity would work, but I doubt you need something on that scale.
b) Put the keys to each object in the databases that you need in the query string and retrieve them the next time around. (E.g. myurl.com/mypage.aspx?db1objectkey=123&db2objectkey=345&db3objectkey=456)
Using session state seems like the most practical way to do this, its exactly what its designed for.
Cache is probably not the answer here either. As Telos mentioned, I'm not sure why you're not considering session.
If you have a page that depends on this data being available, then you just throw a guard clause in the page load...
public void Page_Load()
{
if(!IsPostBack)
{
const string key = "FunkyObject";
if(Session[key] == null)
Response.Redirect("firstStep.aspx");
var obj = (FunkyObject)Session[key];
DoSomething(obj);
}
}
If session is absolutely out of the quesiton, then you'll have to re-materialize this object on the other page. Just send the unique identifier in the querystring so you can pull it back again.
Session isn't always available. For instance when XSS (cross-site-scripting) security settings on IE prevent the storage of third-party cookies. If your site is being called within an IFrame from a site that's not your DNS domain, your cookies are going to be blocked by default. No cookies = no session.
Another example is where you have to pass control to another website that will make the callback to your site as a pure URL, not a post. In this case you have to store your session parameters in a querystring parameter, something that's tough to do given the 4k size constraint and URL encoding, not to mention encryption, etc.
The issue is that most of the built-in serialisation methods are pretty verbose, thus one has to resort to a roll-your-own method, probably using reflection.
Another reason for not using sessions is simply to give a better user experience; sessions get cleared after N minutes and when the server restarts. OK, in this case a viewstate is preferable, but sometimes it's not possible to use a form. OK, one could rely on JavaScript to do a postback, but again, that's not always possible.
These are the problems I'm currently coding around.
Here is what I do:
Page1.aspx - Add a public property of an instance of my object. Add a button (Button1) with the PostBackURL property set to ~/Page2.aspx
Private _RP as ReportParameters
Public ReadOnly Property ReportParams() as ReportParameters
Get
Return _RP
End Get
End Property
Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click
_RP = New ReportParameters
_RP.Name = "Report 1"
_RP.Param = "42"
End Sub
Now, on the second page, Page2.aspx add the following to the Markup at the top of the page under the first directive:
<%# PreviousPageType VirtualPath="~/Default.aspx" %>
Then for the Page_Load in the code behind for Page2.aspx, add the following
If Not Page.PreviousPage is Nothing Then
Response.write (PreviousPage.ReportParams.Name & " " & PreviousPage.ReportParams.Param)
End If
Faced with a similar situation what I did, is to XML serialize the object and pass it around as query string parameter. The difficulty with this approach was that despite encoding, the receiving form throws exception saying "potentially dangerous request...". The way I got around was to encrypt the serialized object and then encode to pass it around as query string parameter. Which in turn made the query string tamper proof (bonus wandering into the HMAC territory)!
FormA XML serializes an object > encrypts the serialized string > encode > pass as query string to FormB FormB decrypts the query parameter value (as request.querystring decodes also) > deserialize the resulting XML string to object using XmlSerializer.
I can share my VB.NET code upon request to howIdidit-at-applecart-dot-net