I use GridView & Repeater (and other like DropDownList) controls extensively in my application along with ObjectDataSource components and classes that serve data for ObjectDataSource (TypeName / SelectMethod attributes on ObjectDataSource). I recently noticed that SelectMethod is only called when
IsPostBack == false
Also, when I bind manually, I always bind when !IsPostBack. I never was curious how controls maintain their data between postbacks, until now (I have to create GridView with sorting/pagination etc and I want to do it efficently).
Could you explain / provide some links with descriptions how it's done?
Also I don't get one thing: when working with GridView and I iterate over rows, sometimes I need to access its GridViewRow.DataItem property in order to get backing object (usually to get some kind of ID). And sometimes it's null - sometimes not. I couldn't figure out why. However then it's null, I still can access (for instance) GridViewRow.DataKeys.
Thanks
EDIT: know when answers say that it thanks to ViewState, I have another question: If I data bind 100 business objects which are pretty heavy and I only use a few properties while data binding (let's say I also use OnRowDataBound event to render some additional data), does it mean that whole objects are serialized?
ASP.Net uses ViewState a hidden variable in the all HTML pages to maintain state of a page.
Reasd this for more understanding
http://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&sqi=2&ved=0CFMQFjAB&url=http%3A%2F%2Fmsdn.microsoft.com%2Fen-us%2Flibrary%2Fms972976.aspx&ei=UpE0UK3uBsOqrAe94ICwBg&usg=AFQjCNG5ErbrFH0ZYV_WW-jonIl25xEsDQ
They store it in a hidden field on the page called ViewState (or more precisely "__VIEWSTATE"). If you do a View Source on your ASPX page, you'll see it.
You too can store and retrieve state information for your page in the ViewState by accessing the ViewState property of your Page.
I'd start here http://msdn.microsoft.com/en-us/library/ms972976.aspx
So, yes, if you store a large number of large objects, there is a large amount of data being transferred to and fro to the client in the page cycle. Used carefully, Viewstate can be useful, but used carelessly, it can have negative effects.
Related
I need to pass a Infragistics ultrawebtree from a page to another. I used Session("data") for this and then I just access it from another page. But when I assign the session value after casting to a Infragistics ultrawebtree control in my second page, it doesnot build the tree structure and it is invisibile. Is I am wroung in this approach, is there any other way to pass the entire tree structure from one page and display it into another page. Any help is appreciated
If anything, I would store the datasource in session instead of the tree. Storing the entire control in session creates a lot of unnecessary overhead, and limits your options per implementation on the next page.
EDIT: You should be able to export the tree structure to XML format, and store the XML in session. According to the documentation there should be WriteXmlDoc() and WriteXmlString() functions available. There's also ReadXmlDoc() and ReadXmlString() functions you can use to repopulate the tree on the next page.
Here's the documentation for the UltraWebTree. You'll see these methods under the Public Methods section:
Only the data should be stored in session. Storing controls in session is likely to cause issues because the control will be disposed during page unload and controls shouldn't be accessed after being disposed which is what would happen when accessing the control from session. See Why Controls Shouldn't be Stored in Session for more details.
I have a composite control that has a couple of private fields that reference values in the cache and these private fields are called during the constructor method. Since a string key is used to identify the value in the cache, I must have a way of storing that string key in such a way that it is available at the time the control is instantiated, and I have to be able to reference it on postbacks without it changing.
In addition, this key is generated the first time the control is loaded, but it should not be changed again after that first time.
How can I accomplish this?
I have already tried saving it to viewstate, but that doesn't work because viewstate is not yet available at the time the control is instantiated.
I have tried using a private field and then checking against Page.IsPostback in the constructor and if it isn't postback, I assign a value to the private field, but on subsequent postbacks it looses it's value, and I can't reassign it in the Page.IsPostBack again because it is an autogenerated GUID.
This has got to be something folks have had to do before....
There isn't a lot of state info available during control construction at all, so this could be difficult. Is there some reason you can't move your code which accesses the Cache'ed info into the control's Init event?
I assume you can't use Session because the information stored is related to that specific request/postback. If it's not specific to that request, using Session could be a possibility - but I think you may encounter other problems trying to deal with control state so early in the lifetime.
After seeing your comment to the other answer; you should be able to move your code that checks for the cached datasource into the control's Init or even Load event, so the state will be available.
Also, incidentally; are you sure you really need to cache this data? That could end up taking up a lot of server memory.
Have you tried Session?
You can store anything you like in the session object for one particular user, maintaining the value / object between postbacks.
If you want to store on a global basis and not per ser basis, try Application
Although this isn't the best solution (rearranging your logic to fit the lifecycle model generally is), have you tried accessing the Request directly? I once really wanted to get the selected value off a DropDownList very early in the lifecycle so I could adjust some elements in the building, and I did it like this:
myDropDownList.SelectedValue = Page.Request.Form[myDropDownList.UniqueID];
So instead of waiting for the viewstate to load the server-side proxie's values, I just got it myself from the client-side control value that was passed in on the post. I probably would do things differently if I redesigned that page, but it seems to have worked out alright for now and it solved the problem I was having.
I'm sure this has been answered but I cannot find it...perhaps because the terms are used for so many differing questions...anyway.
how is the best way to store information about building dynamic controls on postback etc in the init event...
e.g. take the classic "questionnaire" scenario...question.aspx?id=1
get the id from the querystring
load the questions from where ever and build controls dynamically,
or dynamically add each question
user control for how ever many
questions etc
page posts back (user presses submit I guess!)
in the init event...where should you have already stored the id to
recreate the question controls?
things i've tried/thought of so far:
I initially stored the value in
viewstate as it's relevant to the
page, but the viewstate value isn't
available in page init (or is it??)
a hidden variable and then accessing the value in the form collection
session - I don't want to use session, seems problematic, e.g. what if the user views two questionnaires?
the querystring again? but that seems too easy to be changed and the dynamic controls just wouldn't make sense...
in the controls themselves? e.g. disregard the questionnaire id all together and simply go off the question id perhaps?? assuming that control ids would be in some format like controlQ# e.g. controlQ1, controlQ2. this means that each question id must be unique
any help/guidance etc much appreciated!!!
The query string will still be available during a postback, so you could take the ID from there.
If you are concerned about users manipulating the query string parameters, then maybe you should add some additional parameters which will allow you to verify the correctness of the query string during a postback. As a simple example, you could add a MD5 hash of the ID parameter and the current session's ID, and recreate/compare that checksum during a postback.
try to create/load the usercontrols in the page OnLoad.
if you want to load inside a usercontrol other usercontrols use the OnInit event.
use the viewstate for the persistence of the information you will need to re-instantiate the usercontrols during the postback. to map the postback data to the controltree you have to ensure that you have the same control-tree. after that mapping you could change your controltree (load some other controls e.g.)
contols-id: yes, you have to give them unique ids, otherwise it will not work. if you store your questions in a database, there should not be any problem to use the database-primary key for this, so name them e.g. question-1, question-2 (the controls)
alt text http://img3.imageshack.us/img3/1488/advancedsearch.png
I'm building an "Advanced Search" interface in an ASP.NET application. I don't need SO to write this thing for me, but I'm stuck on a specific problem regarding dynamic controls and ViewState. I would like some direction for how to approach this. Here's my situation:
Ingredients:
A serviceable set of API objects representing entities, fields, and searches, which handles constructing a search, generating SQL, and returning the results. So that's all taken care of.
ASP.NET 3.5
Desired Interface Functionality:
(1) On initial page load, the interface gets a preconfigured Search object with a set of SearchCriterion objects. It binds them into a set of controls (see image above.)
Some search items are simpler, like:
Field (DropDownList) | Operator (DropDownList) | Value (TextBox)
Search Criterion controls for some field types have important information stored in viewstate, like:
Field (DropDownList) | Operator (DropDownList) | Value (DropDownList) where the "Value" dropdownlist is populated by a database query.
Some fields are lookups to other Entities, which causes a chain of field selectors, like:
Field (DropDownList) Field (DropDownList) | Operator (DropDownList) | Value
(2) The user modifies the search by:
Adding and Removing search criteria by clicking respective buttons
Configuring existing criteria by changing the Field, Operator, or Value. Changes to Field or Operator will require the control to reconfigure itself by changing the available operators, changing the "Value" input control to a different type, or adding/removing DropDownLists from the "Fields" section if Lookup-type fields are selected/unselected.
(3) Finally, the user hits "Search" to see their results.
The Problem:
As you probably already know if you're answering this question, controls added dynamically to the page disappear on postback. I've created a UserControl that manipulates the control collection and neatly accomplishes step (1) above as you can see in the attached image. (I'm not concerned about style at this point, obviously.)
However on Postback, the controls are all gone, and my Search API object is gone. If I could get the dynamically generated control collection to just play nice and stick in ViewState, I could examine the controls on postback, rebuild the Search object, then handle control events neatly.
Possible Solutions
I could make the Search object serializable and store it in viewstate. Then on page load I could grab it and reconstruct the control collection at page load time. However I'm not sure if this would play nicely with controls raising events, and what happens to the viewstate of Drop-down lists that contain data from the database - could I get it back? It's highly undesirable for me to have to re-query the database on every postback.
I could develop a custom server control (see this link) for this kind of thing... but that is a new topic for me and would involve some learning, plus I'm not totally sure if a custom server control would work any more nicely with non-fixed control collections. Anybody know about that?
I was thinking that I might be able to accomplish this using databound controls - for example I could bind my criterion collection to a repeater which has a fixed control collection (maybe hide the non-used "value" controls, use an inner repeater for the "Field" drop-down lists). Then all the information would stay in ViewState... right?
Any new ideas would be greatly appreciated.
thanks for your help.
b.Fandango
I've been coding for about a day and I got this working beautifully using the third option I suggested in my question - old-school databound controls. Actually I only thought of the idea when I was forced to write out the question in detail - doesn't that just happen to you all the time?
I put my SearchCriterionControl into an asp:Repeater and bound it to my object collection. For the Field Chooser I put an asp:DropDownList inside a nested asp:Repeater and bound the Field array to that. Everything works beautifully, keeps state, actually required very little code. So I never had to dynamically add controls to the page, thank goodness.
Thanks for your suggestions, Ender, Matt and andrewWinn.
Since no one else has taken a stab at this for 2 hours, I'll throw my hat in the ring with a solution that does not rely on viewstate at all (or the ASP.NET model of postbacks).
What if you grabbed all the input values with jQuery and instead of doing a post-back did a post against the page (or a new results.aspx page)? Or, you could make the entire thing asyncrhonous and do an Ajax request against a web method, get fed the results, and populate on the client side as needed?
The unfortunate thing here is you have to reconstruct which type of controls were used to figure construct your search query since that data wont be passed with the viewstate. But I imagine you were already going to have to do some kind of translation of your input data into a query form anyway.
Read here for more information about using jQuery to hit an ASP.NET page method. Remember - page methods must be static (it's an easy oversight).
I'm not sure what you're doing server side to construct your query - but I would highly recommend LINQ. I did a similar "advanced search" function previously, and after a few different attempts found that LINQ was a wonderful tool for this problem, regardless of whether I was hitting SQL with LINQtoSQL or just hitting an in-memory collection of objects.
This worked so well because 1) LINQ is deferred execution and 2) A LINQ query returns another queryable object. The implication here is that you can chain your LINQ queries together as you construct them from your input, instead of having to do a single massive clause translation to SQL or whatever backstore you are using (one of my attempts was constructing SQL clauses with strings, but still passing input data via SQLParameters for SQL injection protection - it was messy and complicated when hand crafted LINQ was orders of magnitude easier to understand and implement).
For example:
List<string> data; // or perhaps your a DB Context for LINQtoSQL?
var query = data.Where(item => item.contains("foo"));
if( {user supplies length search option} )
query = query.Where(item => item.Length < 5);
// etc, etc.
// LINQ doesn't do anything until the query is iterated, at which point
// it will construct the SQL statement without you worrying about details or parameter binding
foreach(string value in query)
; // do something with the results
Because of deferred execution and the queryable return type, you can concatenate LINQ queries to this expression all day long and let it worry about the implementation details (such as converting to a SQL query) at execution time.
I can't provide you with the exact steps that you will need to do, but I HIGHLY suggest looking into asp.net page life cycle. I created a user control as a DLL one time. I had to capture postback data at specific steps in the lifecycle and recreate and rebind the data at other steps. Additionally thinkgs like viewstate are only available at certain points also. I know that I had to override On_init, On_prerender and some other methods.
Sorry I couldn't be more help, but I don't have the code with me (its with an old employer). I hope this helps.
If you are adding controls to the controls tree dynamically, you need to add them on postpack as well. Just call the method that builds the control on Page_Load or Page_Init and the controls should stay on the page on postback.
Ok so the obvious answer is, because the flow of a composite control demands my childcontrols to be created at a certain point in time. I got a problem i think other people must have had as well.
My control is a composite "container/collection" control. It will be fed with an object and based on that objects data it will create a number of childcontrols. So my control will render a header (always) and x-number of, say TextBox controls (based on the object it was fed with).
I'm creating my header in CreateChildControls() obviously, but i can't possibly create my TextBoxes there as well, because i don't know if the object (to base the TextBoxes on) has been fed yet? I thought of exposing a property/method to set/fed the object through, but i'm not sure when it will be called.
So what do i do? I mean i can't possibly create the TextBoxes in CreateChildControls() or can I? I mean - when is CreateChildControls() called - i know i can call EnsureChildControls() (which i already do in a property to set the innerText of the header - since i need the header to be created before setting its innerText obviously).
How about this
var c = new MyContainerControl();
c.Header = "fun";
c.TextBoxObject = myTextBoxes;
That would raise an error (or at best not create any TextBox'es) if i put the building of the TextBoxes in CreateChildControls().
Would it be more sane to instead just store the Header in a member variable and thus not having to call EnsureChildControls() in the exposed method/property setting the Header innerText. I just don't like this aproach much, since it would be complicating things by adding extra logic to store temporarely and later having to figure out when to set it (probably in PreRender).
Also i guess i could make some kind of Databound control, ensuring the data be present at the time of calling .DataBind(). I really don't like this either since last i looked at creating databound controls it got very complicated.
This really should be an easy task to solve - I know I'm missing something somewhere...
what you're describing IS a databound control. And yes, it's somewhat complicated, but it's the proper design paradigm for this type of instance.
That said, had you considered utilizing the repeater control rather than trying to roll out your own composite which behaves in the exact same manner? Rather than passing it a random object, pass it a collection or an iList with the number of text areas you're wanting.