FAQ: Why do dynamic controls disappear on postback and not raise events? RSS

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  mbanavige

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  Re: FAQ: Why do dynamic controls disappear on postback and not raise events?

  Mar 20, 2009 01:15 PM|LINK

  You would need to use FindControl to get a reference to the dynamic control.  Once you have a reference to it, you can then access its properties.

  Mike Banavige
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  blbirajdar02

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  Re: FAQ: Why do dynamic controls disappear on postback and not raise events?

  Jun 23, 2009 02:22 PM|LINK

   Thanks for the post. I have implemented the AJAX tabs to be added dynamically based on your post.
  

  Balaji Birajdar
  Pune, India
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  miteshsura

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  Re: FAQ: Why do dynamic controls disappear on postback and not raise events?

  Jul 07, 2009 01:58 AM|LINK

  thanks for posting this. 

  I have tweaked this to addi usercontrol dynamically. I was able to add on buton click event 1 time, but I wanted to add usercontrol N number time. N->> number of button hits by the end user. 

  Now I need to fig out how to retain the value using viewstate in my usercontrol.

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  ranjit.verma

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  Re: FAQ: Why do dynamic controls disappear on postback and not raise events?

  Jul 10, 2009 11:16 AM|LINK

  If the controls are created in Page_Load() event, it will disappear on postback and will not raise events. this happens because the control could not be initialized in Page_Load() event.
  

  To retain the controls, it can be created within Page_Init() event.

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  mbanavige

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  Re: FAQ: Why do dynamic controls disappear on postback and not raise events?

  Jul 10, 2009 02:22 PM|LINK

  miteshsura

  thanks for posting this. 

  I have tweaked this to addi usercontrol dynamically. I was able to add on buton click event 1 time, but I wanted to add usercontrol N number time. N->> number of button hits by the end user. 

  Now I need to fig out how to retain the value using viewstate in my usercontrol.

   

  You could add your counter to viewstate and simply keep track of the number of controls you need to dynamically create.  Then on each postback during the Page_Load event, you check your counter that was stored in viewstate and recreate the number of dynamic controls indicated by that counter.  In order to insure that the dynamic controls behave properly, you need to also insure that they get consistent ID's.  So i would use the counter to help create those controls ids.  so your controls would end up getting ids like; dynamicControl1, dynamicControl2, dynamicControl3 ...

  To simplify accessing a counter in viewstate, i like to wrap that up in a page property.  Theres an example of that in this wiki article: http://wiki.asp.net/page.aspx/58/viewstate/

   

  Mike Banavige
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  mbanavige

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  Re: FAQ: Why do dynamic controls disappear on postback and not raise events?

  Jul 10, 2009 02:47 PM|LINK

  Here's a quick example of adding N number of dynamic controls to a page. 

  First the markup:

  <form id="form1" runat="server">
      <div>
          <asp:Button ID="cmdProcessControls" runat="server" Text="Process" />
          <asp:Button ID="cmdAddTextbox" runat="server" Text="Add Textbox" />
          <asp:PlaceHolder ID="PlaceHolder1" runat="server" />
      </div>
      </form>

  
  Now the code-behind:

      'this property will be used to keep track of 
      'how many dynamic controls were created.
      Private Property TextboxCounter() As Integer
          Get
              Dim o As Object = ViewState("TextboxCounter")
              If o Is Nothing Then Return 0
              Return DirectCast(o, Integer)
          End Get
          Set(ByVal value As Integer)
              ViewState("TextboxCounter") = value
          End Set
      End Property
  
      Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
          'recreate dynamic controls on all postback by page_load at the latest
          'we use page_load instead of init so that we can access viewstate.
          If Page.IsPostBack Then
              For i As Integer = 1 To TextboxCounter
                  Dim ctrl As New TextBox
                  'It's very important to specify the controls ID
                  ctrl.ID = "dynamicTextBox" & i
                  Me.PlaceHolder1.Controls.Add(ctrl)
              Next
          End If
      End Sub
  
      Protected Sub cmdAddTextbox_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles cmdAddTextbox.Click
          'increment counter
          TextboxCounter += 1
  
          'add new dynamic control
          Dim ctrl As New TextBox
          ctrl.ID = "dynamicTextBox" & TextboxCounter
          Me.PlaceHolder1.Controls.Add(ctrl)
  
      End Sub
  
      Protected Sub cmdProcessControls_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles cmdProcessControls.Click
  
          'loop through any textbox controls and see what they contain
          For Each ctrl As TextBox In Me.PlaceHolder1.Controls
              Response.Write(String.Format("Control ID: {0}, Text: {1} <br/>", ctrl.ID, ctrl.Text))
          Next
  
      End Sub

  
   

   

  Mike Banavige
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  miteshsura

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  Re: FAQ: Why do dynamic controls disappear on postback and not raise events?

  Jul 10, 2009 08:03 PM|LINK

  this is exactly what I was looking for,  I was trying to achieve that via page init event and using __EVENTTARGET, but for some reasons for button __EVENTTARGET was null, than I read somewhere button and image button do not support that.  I replaced that with link button, but FindContorl would't find the link button id, because I was using master page.. your solution worked like butter on hot pan!! thanks

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  NisGau

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  Re: FAQ: Why do dynamic controls disappear on postback and not raise events?

  Jul 16, 2009 04:01 PM|LINK

  If Viewstate is used to store the value of dynamic contrlols then Page_Load is the event to be used. ViewState values are available in Page_Load - but they are not available during the Page_Init event.

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  Rimbik

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  Re: FAQ: Why do dynamic controls disappear on postback and not raise events?

  Jul 17, 2009 04:59 AM|LINK

  If you look @ the PageLoad life cycle then you will see LoadViewState() executes after the Page_Init() so you can not have ViewState values in Page_Init().
  
  
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  Bal budhi Vidya dehu mohe,Harahu Kalesa Vikar
  [My Site: WCF, My Site
  and Silverlight site @ Silverlight version].
  
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  rgenco1

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  Re: FAQ: Why do dynamic controls disappear on postback and not raise events?

  Sep 17, 2009 04:05 PM|LINK

   Hi all,

  i use the example to adding N number of dynamic controls to a page . I want to create dropdownlist populate by Dataset and in cmdProcessControls_Click I wanto to show the item choise by user in dropdownlist but I don't get the same behaviour. When Click in cmdProcessControls button i view dropdownlist without item and label control empty. If I use dropdownlist populate by listitems It works. 

  private int StepCounter
      {
          get
          {
              object o1 = ViewState["StepCounter"];
              if (o1 == null) return 0;
              return (int)o1;
          }
          set { ViewState["StepCounter"] = value; }
      }
  
  
      void Page_PreRender()
      {
          cmdProcessControls.Enabled = StepCounter > 0;
      }
  
  
      protected void Page_Load(object sender, EventArgs e)
      {
          //recreate dynamic controls on all postback by page_load at the latest
          //we use page_load instead of init so that we can access viewstate.
          if (Page.IsPostBack)
          {
              for (int j = 1; j <= StepCounter; j++)
              {
                  DropDownList ddltest = new DropDownList();
                  ddltest.ID = "ddltest" + j;
                  //ddltest.AutoPostBack = false;
                  Label lblDomanda = new Label();
                  lblDomanda.ID = "lblD" + j;
                  Label lblTest = new Label();
                  lblTest.ID = "lbl" + j;
                  this.PlaceHolder1.Controls.Add(lblDomanda);
                  this.PlaceHolder1.Controls.Add(lblTest);
                  this.PlaceHolder1.Controls.Add(ddltest);
              }
  
  
          }
          else
          {
              Label lblTest = new Label();
              lblTest.Text = "Inizio Procedura Guidata. Clicca su Add Domanda per Proseguire";
              this.PlaceHolder1.Controls.Add(lblTest);
          }
  
  
      }
  
  
  
  
      protected void cmdProcessControls_Click(object sender, EventArgs e)
      {
          
  
  
  
          string tipoControl = string.Empty;
          foreach (Control ddl in this.PlaceHolder1.Controls)
          {
              tipoControl = ddl.GetType().ToString();
              if ( tipoControl == "System.Web.UI.WebControls.DropDownList")
                  Response.Write(string.Format("Control ID: {0}, Text: {1} <br/>", ((DropDownList)ddl).ID, ((DropDownList)ddl).SelectedValue));
              else if (tipoControl == "System.Web.UI.WebControls.Label")
                  Response.Write(string.Format("LABEL ID: {0}, Text: {1} <br/>", ((Label)ddl).ID, ((Label)ddl).Text));
          }
  
  
      }
  
  
      protected void cmdAddTextbox_Click(object sender, EventArgs e)
      {
          string prova = string.Empty;
          SqlDataAdapter sdaDomande;
          SqlDataAdapter sdaRisposte;
          string constr = ConfigurationManager.ConnectionStrings["ProteoConnectionString"].ConnectionString;
          SqlConnection connection = new SqlConnection(constr);
          if (StepCounter == 0)
          {
              sdaDomande = new SqlDataAdapter("SELECT ID_Domanda, Testo, ID_Domanda_Padre, Valore FROM Domande WHERE ID_Domanda = ID_Domanda_Padre ORDER BY ID_Domanda", connection);
          }
          else
          {
              sdaDomande = new SqlDataAdapter("SELECT ID_Domanda, Testo, ID_Domanda_Padre, Valore FROM Domande WHERE ID_Domanda = @ID_Domanda_Padre ORDER BY ID_Domanda", connection);
          }
          DataSet ds = new DataSet();
          sdaDomande.Fill(ds, "Domande");
          //prova = "D" + ds.Tables["Domande"].Rows[0]["ID_Domanda"].ToString() + "-" + ds.Tables["Domande"].Rows[0]["Testo"].ToString();
          sdaRisposte = new SqlDataAdapter("SELECT * FROM Risposte WHERE ID_Domanda = @ID_Domanda ORDER BY ID_Risposta, ID_Domanda", connection);
          DataSet dsRisp = new DataSet();
          
  
          //increment counter
          StepCounter += 1;
          for (int i = 0; i <= ds.Tables["Domande"].Rows.Count - 1; i++)
          {
              prova = ds.Tables["Domande"].Rows[i]["ID_Domanda"].ToString();
              sdaRisposte.SelectCommand.Parameters.Add("@ID_Domanda", SqlDbType.Int).Value = prova;
              sdaRisposte.Fill(dsRisp, "Risposte");
  
  
              //add new dynamic control
  
  
              DropDownList ddltest = new DropDownList();
              ddltest.DataSource = dsRisp;
              ddltest.ID = "ddltest" + StepCounter;
              ddltest.AutoPostBack = false;
              ddltest.DataTextField = "Testo";
              ddltest.DataValueField = "ID_Risposta";
              ddltest.DataBind();
              Label lblDomanda = new Label();
              lblDomanda.Text = "D" + ds.Tables["Domande"].Rows[i]["ID_Domanda"].ToString() + "-" + ds.Tables["Domande"].Rows[i]["Testo"].ToString();
              lblDomanda.ID = "lblD" + StepCounter;
              Label lblTest = new Label();
              lblTest.Text = "Seleziona Risposta";
              lblTest.ID = "lbl" + StepCounter;
              this.PlaceHolder1.Controls.Add(lblDomanda);
              this.PlaceHolder1.Controls.Add(lblTest);
              this.PlaceHolder1.Controls.Add(ddltest);
          }
      }

  
   

  <head id="Head1" runat="server">
      <style type="text/css">
          html
          {
              font:14px Georgia,Serif;
          }
          fieldset
          {
              display:block;
              width:600px;
              padding:20px;
              margin:10px;
          }
      </style>
      <title>Wizard</title>
  </head>
  <body>
      <form id="form1" runat="server">  
      <div>  
         <%-- <asp:Button ID="cmdProcessControls" runat="server" Text="Fine" 
              onclick="cmdProcessControls_Click" /> --%> 
          <%--<asp:Button ID="cmdAddTextbox" runat="server" Text="Add domande" onclick="cmdAddTextbox_Click" 
               /> --%> 
          
       
              <asp:Label id="lblStepNumber" Runat="server" />
  
              <fieldset>
              <legend><asp:Literal ID="ltlStep" runat="server" /></legend>
              <asp:PlaceHolder ID="PlaceHolder1" runat="server" />
              </fieldset>
  
      <asp:Button 
          ID="cmdProcessControls" runat="server" onclick="cmdProcessControls_Click"
          Text="< Fine"
          CausesValidation="false"/>
      
      <asp:Button
              ID="cmdAddTextbox" runat="server" onclick="cmdAddTextbox_Click" 
          Text="Add domande >" />
  
      </div>
  </form> 
  </body>

  I try to reply the idcontrols how explain in your post but my code don't work like I want.

  Any ideas to help me?

  Thanks,

  Roberto