Object Error with HtmlGenericControl RSS

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  webguy07

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  Object Error with HtmlGenericControl

  Apr 16, 2012 09:32 PM|LINK

  I've got a dynamic navigation, when they user clicks on a different page, that tab needs to become the active tab. Currently I'm getting NullException error and an Object error. FYI: That data is coming back fine, it's only when I try to change the tab to active it's not finding the ID.

  ******* Navigation Control *******

  <asp:Literal ID="ltl_Navigation" runat="server"></asp:Literal>

  
  
  

  ****** Navigation Code ********

   List<Navigation> theNavigation = DataHelper.GetNavigation();
          theNavigation = theNavigation.OrderBy(u => u.OrderBy).ToList();
  
          foreach (Navigation item in theNavigation)
          {
              ltl_Navigation.Text += "<li ID=" + '"' + item.TabName  + '"' + " style=" + '"' + "display:" + item.Display + '"' + " class=" + '"' +              item.Class  + '"' + " runat=" + '"' + "server" + '"' + "><span><span><a href=" + '"' + item.Location + '"' + ">" + item.Title + "</a></span></span></li>";
          }

  
  
  

  ******* Default Page ***** "setting the active tab"

     HtmlGenericControl li_active = (HtmlGenericControl)Page.Master.FindControl("tab_homeactive");
          HtmlGenericControl li_inactive = (HtmlGenericControl)Page.Master.FindControl("tab_homeinactive");
          li_active.Style["display"] = string.Empty; "This is where it's erroring out!!"
          li_inactive.Style["display"] = "none";

  
  
  

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  mm10

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  Re: Object Error with HtmlGenericControl

  Apr 17, 2012 06:35 AM|LINK

  You cannot add a server control by setting the Text property of a Literal control, even if you specify the runat=server attribute on the elements you are adding. The FindControl method searches for a control by its server side id, but the li element you are adding is not a server control and because of this the method cannot find it.

  You should define your li element as a HtmlGenericControl and add it to a UL, or any other control defined in the markup, with the runat=server attribute. Then you will be able to search for it in your server side code.

  C#:
  HtmlGenericControl li = new HtmlGenericControl("li");
  li.ID="IdOfLi";
  li.InnerText="text";
  ....
   myul.Controls.Add(li);

  
  HtmlGenericControl c = (HtmlGenericControl)Page.Master.FindControl("IdOfLi");

  ASPX:
  <ul id="myul" runat="server></ul>

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  webguy07

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  Re: Object Error with HtmlGenericControl

  Apr 17, 2012 07:18 PM|LINK

  Thanks. But I do have a runat server in the tag. 

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  Catherine Sh...

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  Re: Object Error with HtmlGenericControl

  Apr 18, 2012 07:09 AM|LINK

  Hi webguy07,

  webguy07

  Thanks. But I do have a runat server in the tag.

  Could you show the corresponding code ?

  Best wishes,

  Please mark the replies as answers if they help or unmark if not.
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  mm10

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  Re: Object Error with HtmlGenericControl

  Apr 18, 2012 08:18 AM|LINK

  webguy07

  Thanks. But I do have a runat server in the tag. 

  Yes, but it doesn't make any difference as you cannot add a server control to a Literal. It is used to display text/HTML on a page. Instead, you should add a HtmlGenericControl to a server side (with runat=server) UL if you want to able to find it with the FindControl method.

  Please 'Mark as Answer' if this post helped you.
  • Edited by mm10 on Apr 18, 2012 08:18 AM
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  webguy07

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  Re: Object Error with HtmlGenericControl

  Apr 18, 2012 01:43 PM|LINK

  If your referring to the html mark up, I can't show you because it's dynamic, but below is what it looks like in the view source when it's render. Basically based on the what page the user is on, I'm change the display value, which is why I've got an active and inactive li.

  <ul id="top-navigation">
                  <li id="tab_homeinactive" runat="server"><span><span><a href="Default.aspx">Home</a></span></span></li>
                  <li id="tab_homeactive" style="display: none;" class="active" runat="server"><span><span>Home</span></span></li>
                  <li id="tab_eventinactive" runat="server"><span><span><a href="Default.aspx">Events</a></span></span></li>
                  <li id="tab_eventactive" style="display: none;" class="active" runat="server"><span><span>Events</span></span></li>
                  <li id="tab_usersinactive" runat="server"><span><span><a href="Users.aspx">Users</a></span></span></li>
                  <li id="tab_usersactive" style="display: none;" class="active" runat="server"><span><span>Users</span></span></li>
                  <li id="tab_reportinginactive" runat="server"><span><span><a href="Reporting.aspx">Reporting</a></span></span></li>
                  <li id="tab_reportingactive" style="display: none;" class="active" runat="server"><span><span>Reporting</span></span></li>
                  <li id="tab_helpinactive" runat="server"><span><span><a href="Help.aspx">Help</a></span></span></li>
                  <li id="tab_helpactive" style="display: none;" class="active" runat="server"><span><span>Help</span></span></li>
  		</ul>

  
  
  

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  mm10

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  Re: Object Error with HtmlGenericControl

  Apr 18, 2012 02:03 PM|LINK

  You won't be able to find the li:s with id tab_* in server side code as they are NOT server controls, it's just simple text which happens to have a textual runat=server attribute which means nothing in this context.

  You never see the runat=server attribute in the rendered HTML for a server control.

  
  For example, if you write this in the markup for an ASPX page:
  <ul runat="server">
      <li>llll</li>
      </ul>

  ...it will render without the runat="server" in the browser:
  <ul>
      <li>llll</li>
      </ul>

   

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  • Edited by mm10 on Apr 18, 2012 02:04 PM
  • Edited by mm10 on Apr 18, 2012 02:04 PM
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  webguy07

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  Re: Object Error with HtmlGenericControl

  Apr 18, 2012 02:13 PM|LINK

  Ok. Then how do I get the id of the li to change the display value at run time?

  -Thanks

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  mm10

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  Re: Object Error with HtmlGenericControl

  Apr 18, 2012 02:38 PM|LINK

  Change your navigation code by creating HtmlGenericControls instead of adding text to a Literal:

   List<Navigation> theNavigation = DataHelper.GetNavigation();
          theNavigation = theNavigation.OrderBy(u => u.OrderBy).ToList();
   
          foreach (Navigation item in theNavigation)
          {
    HtmlGenericControl li = new HtmlGenericControl("li");
  li.ID = item.TabName;
  li.Style.Add("display", item.Display);
  li.InnerText="><span><a href.......";
  ......
  yourUl.Controls.Add(li);
          }

  ..and define a UL in your markup to hold the HtmlGenericControls you are creating in code:

  <ul runat="server" id="yourUl">
  </ul>

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  webguy07

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  Re: Object Error with HtmlGenericControl

  Apr 18, 2012 03:23 PM|LINK

  Ok. I'm not getting an error anymore, but now nothing is showing. Do I need to add the ul top-navigation to a panel or something after the foreach?

  -Thanks

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