Hi,

I have a web site with an UpdatePanel, a Label within that panel and a 
Button outside the panel. All controls are created programatically (see 
code below).
When I click the button to update the label's text, I get a 
NullReferenceException in the page's Render method. My own code runs 
without any problems.

Any one with some ideas? I already did some Google reseach but came out 
with nothing that would help be :(

<code>
public partial class test : System.Web.UI.Page {

   UpdatePanel up;
   Label l1;
   Panel p;
   UpdateProgress upp;
   Button b;
   UpdateProgressOverlayExtender upoe;

   protected override void OnPreInit(EventArgs e) {
     base.OnPreInit(e);
     // create dynamic controls
     up = new UpdatePanel();

     p = new Panel();
     up.ContentTemplateContainer.Controls.Add(p);

     l1 = new Label();
     p.Controls.Add(l1);

     this.form1.Controls.Add(up);
   	
     b = new Button();
     this.form1.Controls.Add(b);
   }

   protected override void OnLoad(EventArgs e) {
     base.OnLoad(e);

     // control properties
     l1.ID = "l1";

     // some backgroud
     p.Style.Value =
       "width:100%; height:100px; text-align:center; " +
       "background:#3366a4; border:solid 1px black;";

     up.ID = "up";
     up.ChildrenAsTriggers = false;
     up.UpdateMode = UpdatePanelUpdateMode.Conditional;

     b.Text = "Update Panel";
     b.ID = "b";
     b.Click += new EventHandler(this.ButtonClick);

     // add button as update trigger
     AsyncPostBackTrigger apt = new AsyncPostBackTrigger();
     apt.ControlID = b.ClientID;
     up.Triggers.Add(apt);

     this.ScriptManager1.RegisterAsyncPostBackControl(b);
   }

   protected void ButtonClick(object sender, EventArgs e) {
     // wait a litte
     Thread.Sleep(2000);
     // set label text
     l1.Text = "Panel refreshed at " + DateTime.Now.ToString();
   }

   protected override void Render(HtmlTextWriter writer) {
     try {
       // here the error occurs
       base.Render(writer);
     } catch (Exception ex) {
       writer.Write(ex.ToString());
     }
   }
}
</code>

Thanks,
Tobi

Hi,

just in case someone is interested:

I think I have solved the problem. The AsyncPostBackTrigger did not 
re-initialize on postback. Therefore, the associaed control was not set 
properly, which caused the exception.

As a solution, I derived a class MyUpdatePanel from the original 
UpdatePanel class and overrode the OnLoad method. Here I ensured the 
initialization on postbacks explicitly. In the base implementation this 
is not done. I wonder if its either a bug or by intention. Any 
suggestions here? So far I had no drawbacks.

<code>
public class MyUpdatePanel : UpdatePanel {
   protected override void OnLoad(EventArgs e) {
     // explicit initialize on postback
     if (this.Page.IsPostBack) {
       this.Initialize();
     }
     base.OnLoad(e);
   }
}
</code>

Tobias Schrer schrieb:

> Hi,
> 
> I have a web site with an UpdatePanel, a Label within that panel and a 
> Button outside the panel. All controls are created programatically (see 
> code below).
> When I click the button to update the label's text, I get a 
> NullReferenceException in the page's Render method. My own code runs 
> without any problems.
> 
> Any one with some ideas? I already did some Google reseach but came out 
> with nothing that would help be :(
> 
> <code>
> public partial class test : System.Web.UI.Page {
> 
>   UpdatePanel up;
>   Label l1;
>   Panel p;
>   UpdateProgress upp;
>   Button b;
>   UpdateProgressOverlayExtender upoe;
> 
>   protected override void OnPreInit(EventArgs e) {
>     base.OnPreInit(e);
>     // create dynamic controls
>     up = new UpdatePanel();
> 
>     p = new Panel();
>     up.ContentTemplateContainer.Controls.Add(p);
> 
>     l1 = new Label();
>     p.Controls.Add(l1);
> 
>     this.form1.Controls.Add(up);
>      
>     b = new Button();
>     this.form1.Controls.Add(b);
>   }
> 
>   protected override void OnLoad(EventArgs e) {
>     base.OnLoad(e);
> 
>     // control properties
>     l1.ID = "l1";
> 
>     // some backgroud
>     p.Style.Value =
>       "width:100%; height:100px; text-align:center; " +
>       "background:#3366a4; border:solid 1px black;";
> 
>     up.ID = "up";
>     up.ChildrenAsTriggers = false;
>     up.UpdateMode = UpdatePanelUpdateMode.Conditional;
> 
>     b.Text = "Update Panel";
>     b.ID = "b";
>     b.Click += new EventHandler(this.ButtonClick);
> 
>     // add button as update trigger
>     AsyncPostBackTrigger apt = new AsyncPostBackTrigger();
>     apt.ControlID = b.ClientID;
>     up.Triggers.Add(apt);
> 
>     this.ScriptManager1.RegisterAsyncPostBackControl(b);
>   }
> 
>   protected void ButtonClick(object sender, EventArgs e) {
>     // wait a litte
>     Thread.Sleep(2000);
>     // set label text
>     l1.Text = "Panel refreshed at " + DateTime.Now.ToString();
>   }
> 
>   protected override void Render(HtmlTextWriter writer) {
>     try {
>       // here the error occurs
>       base.Render(writer);
>     } catch (Exception ex) {
>       writer.Write(ex.ToString());
>     }
>   }
> }
> </code>
> 
> Thanks,
> Tobi