C# Shiznit - Adding controls dynamically to a page in ASP.NET

Adding controls dynamically to a page in ASP.NET

posted on Friday, September 30, 2005 by bobby @ 1:13 pm

26 comments

ASP.NET added viewstate in an attempt to persist state for controls across postbacks. Viewstate does what it's supposed to, but also presents some challenges when adding controls dynamically. If you've ever tried to modify the control hierarchy before viewstate has fully loaded, you've prolly seen this error:

"

Failed to load viewstate.
The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request.

"

For example, if I dynamically add a textbox to a page, that textbox MUST exist before the viewstate for the page is loaded after a postback. This is because the when the page attempts to load viewstate, the control hierarchy must match the control hierarchy that existed when viewstate was originally created.

So when you add a control dynamically, it must exist during the postback - at least long enough for viewstate to be loaded properly. This usually involves re-adding any dynamic controls that were present at last page load. What this means is that you can dynamically add, update and remove controls, but only at certain times in the page's lifecycle. Here is a breakdown of a typical page lifecycle:

Note: I'm omitting events and methods invoked by IPostBackDataHandler and IPostBackEventHandler for the purpose of this article.

Instantiate - by constructor.
Initialize - This is when the markup code is initialized into the control hierarchy. The Init event is fired and OnInit is called.
Load Viewstate - The LoadViewState method is called.
Load - The Load event is fired and the OnLoad method is called.
Pre Render - The PreRender event is fired and the OnPreRender method is called.
Save Viewstate - The SaveViewState method is called
Render - The Render method is called.
Unload - The Unload event is fired and the OnUnload method is called.

Given the above series of events, dynamically added controls should be added before the LoadViewState method is called and they must be added in the exact same order they were added on the previous page load. If they are out of order, or a control is missing, you'll get that stellar exception above.

So, when do you do it? Well, to me, Initialization seems to be the best place. Add the controls back in during the OnInit method. Let viewstate completely load itself back up, then do whatever you want... add more controls, remove existing controls, etc. Just make sure you add/remove controls before SaveViewState is called - this shouldn't be a problem to most since the only way to eff this up would be during the Render method - and if you're messing with the control hierarchy here, then you prolly shouldn't be reading this article.

Knowing the page's lifecycle of events and the timing of critical methods is paramount to getting dynamic controls to work. If you don't learn it well, you'll spend ten fold the amount of time fixing related bugs - and to me, these are the most frustrating to debug.

CommentsComments

Hi,

You should look at the Page.LoadControl method in .NET 2.0.

You can add the control in the onLoad event, because the dynamically added control will go through the LoadViewstate events even if the LoadViewState event is alreday finished for the page.

Regards

There is one important issue that is not addressed in your article and is a cause for the same error. When controls dynamically added as result of a postback event, the container (page, placeholder etc) will invoke recursive method for each new control (Control.LoadViewStateRecursive) to load viewstate for newly added controls. Unfortunately, those controls did not exist when page persisted viewstate and page will crash.

Hi

The comment of Igor is about .NET 1.1.

You can add controls in a postback event without any problem with the LoadControl in .Net 2.0.

Regards

First of all, thank you for the article. I have a page that loads dynamically a series of user controls, always in the same order and from the onInit event. All these user controls are the same except one, which loads a different *.ascx file depending on the postback. From the five choices available to the user (five different *.ascx files) there is ONE , only ONE, that provokes this error. And this error has appeared after adding a new control (a datagrid) to that *.ascx file. If I delete the datagrid, the page works well. Any suggestion? Thanks in advance

The datagrid has the EnableViewState property set to true by default, the others are probably controls like a literal or label which have a default value of false. All of the problems mentioned above are irrelevant if you do not need the ViewState for the control being added, which is true for most dynamically added controls.

I little precisation, a "parent" control can also add dynamic children controls in the LoadViewState. This is useful when you save some information of the children controls in the viewstate.

For example, if your "parent" control has x DropDownList in his children controls, and you save the x in the viewstate, you can use the following code:

Copy code to clipboard in IE or select code for Firefox

Copy code to clipboard in IE or select code for Firefox

protected override void LoadViewState(object savedState)
{
  base.LoadViewState(savedState);
  for (int num1=0;num1&lt;this.NumberDropDownList;num1++) this.Controls.Add(new DropDownList());
}

That because the ViewState of the children controls is loaded only after leaving the parent LoadViewState method]

So what is the solution to having a DataGrid (with ViewState enabled) in a dynamically added UserControl? I have a page with about 20 UserControls that I want to load individually from a TreeView vontrol (a roundabout way to replace the use of frames)

I have a hidden textbox that then keeps track of what UserControl is currently loaded.

OnLoad of the parent page I say:



 if (currentControl.Text != "")

        {

            UserControl uc = (UserControl)Page.LoadControl(currentControl.Text);

            PlaceHolder1.Controls.Add(uc);

        }

Then for my TreeView selectedNodeChanged



    protected void TreeView1_SelectedNodeChanged(object sender, EventArgs e)

    {

        TreeView t = (TreeView)sender;

        UserControl uc = (UserControl)Page.LoadControl(t.SelectedValue);

        for (int i = 0;i< PlaceHolder1.Controls.Count;i++ )

        {

            PlaceHolder1.Controls.RemoveAt(i);

            

        }

        

        PlaceHolder1.Controls.Add(uc);

        currentControl.Text = t.SelectedValue;

    }

But I get the following error

The SqlDataSource control 'DSPartyList' does not have a naming container. Ensure that the control is added to the page before calling DataBind.

Where DSPartyList is a DataGrid on the previously loaded control.

Any assistance would be great!

C.

Good article, for a better understanding of page life cycle and the events n methods called during the page ...

As I know you can add controls without any problems till (including) the OnLoad phase. Even though LoadPostbackData event gets called before this phase, controls, that were added on OnLoad phase, are participating in LoadPostbackData event and are able to load their view state as page scrolls all phases for such controls till OnLoad phase.
This applies both for .Net 1.1 and .Net 2.0

hahahahah

I recognize myself in this part of the article

"nd if you're messing with the control hierarchy here, then you prolly shouldn't be reading this article."

That's me !!!!

Ok then I'll continue to search why my code is still giving me the ViewState error.

By the Way, thanks for the article

Louis

hi hi hi

Chris Katsuo thank you! your LoadControl comment just saved my life (or maybe just my job :))

Sweet article!

Now I have a dynamically created user control that has dynamically created checkboxes in it :)

ytutyyu

Hi,
I love to work in ASP.Net.
Thanks for the info and thanks for the sharing your link

I HATE to work in ASP.Net. I absolutely loathe it. Tasks that are so simple in classic ASP are way too complex in .NET.

Maybe it's time to change careers again.

I think it's a Incomplete article. i appreciate senthil for writing an article and publish in asp.net website. If you add button we should need to handle it Click event at least. We need to preserve the dynamic controls if the page post back. We can't say this an article, but a small tip for how to create asp.net controls at run time.

ASP.NET has gone in unmanaged and unstable state where we have 100s of project coded in ASP.NET 2.0 and when we open them in 2008 nothing opens, upgrading to 3.5 are mere nightmares. And finally so many new technologies just to boost the marketing headlines. But come on guys, its easy for one blogger to just blog about able to mix technologies with writing 100s of lines of code which one geek can understand. Have you even worked on any project involving more then 5 developers? Practically we are spending more monty to train new things to our staff, more money to buy new tools, more money to manage projects and more and more money to sit and search world of forums to find why one thing doesnt work then actually doing simple development.

I've yet to have a client request a simple application.... Hard to do SIMPLE development if none of the applications we are asked to write are simple in nature. I'm guessing if you have unstable asp.net code then someone was using an approach that wasn't well structured. Most likely they either didn't know enough to accomplish a good structure OR they weren't given the time to develope a good stable structure (usually the case). General rule of thumb is the more rushed the development the sooner you get to throw it away and do it again.

First, import the "System.Data.OleDb" namespace. We need this namespace to work with Microsoft Access and other OLE DB database providers. We will create the connection to the database in the Page_Load subroutine. We create a dbconn variable as a new OleDbConnection class with a connection string which identifies the OLE DB provider and the location of the database. Then we open the database connection:

Nice!

How does visual studio run in Parallels? I've got a macbook pro, and develop / work on .net applications (so i need windows) at the moment im using bootcamp. But im in love with Mac OSX, and would love to use it for things like email, documents, msn messenger and the web. But then need to be able to use Windows for Visual Studio 2005! In parallels does intellisense work? How do things compile (fast enough) ? Ever used Mono?

I have never been impressed by posts developed on the web but this one makes me change my mind.

Can someone explain how to get jQuery validation working with this release? It seems like lots has changed/moved since previous releases - making it very hard to get a straight answer from Google!

Thank you very much for the information. I think it will add a great effect to my article, I am almost sure. Anyway the feedback is truly positive! wish you good luck in your work and will be waiting for novelties!

Administrator Login
	Invalid login attempts are logged.
Username:
Password:

Contact Me

Name
Email
Message Ctrl + ↓/↑ to shorten/lengthen