Dynamic Controls - How to access postback data?
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From: DylanSmith on 3 Jul 2008 11:40 I have a WebForm where I'm dynamically creating some controls and I'm having difficulty understanding how the state is being persisted and how to work with it. I've created a simplified example to demonstrate my issues. Lets say I have a WebForm with a DropDownList where the user selects a number from 1 to 10 (the DropDownList is not dynamically created). I also have a button on there that I use to trigger a PostBack. Based on the user's selection in the drop-down I want to create that many textboxes programatically. I got this part working easily enough by putting some code in the Page_Load event. The new textboxes even maintain their values correctly across post-backs. But what if I want to put a LiteralControl on my page that will display the sum of the values entered in all the dynamic textboxes. The problem is where do I put this code so that it can access the data in the dynamic textboxes after their values have been restored from the postback data? Based on my understanding of the page lifecycle (from this article: http://msdn.microsoft.com/en-us/library/ms972976.aspx) I was assuming that when I added the TextBoxes to the Panel it was triggering some processing behind the scenes that caused the "LoadPostbackData" logic to be applied to the textboxes. This would explain how the textbox values were being maintained across postbacks. If this were true I should be able to put my adding code immediately after adding the textboxes to the panels and their properties should be set to the appropriate values. This turns out not to work, and the Sum always displays as 0. If you're going to suggest I use the Init event to create the dynamic textboxes then I have the problem of retrieving the value from the DropDownList that tells me how many I need to create, because the DropDownList's SelectedItem hasn't been updated yet in the Init event because the PostBack data hasn't been applied (that's why I was using the Load event). Here is the code I have: AddingMachine.aspx ================== <body> <form id="form1" runat="server"> <div> <asp:DropDownList ID="DropDownList1" runat="server"> <asp:ListItem>1</asp:ListItem> <asp:ListItem>2</asp:ListItem> <asp:ListItem>3</asp:ListItem> <asp:ListItem>4</asp:ListItem> <asp:ListItem>5</asp:ListItem> <asp:ListItem>6</asp:ListItem> <asp:ListItem>7</asp:ListItem> <asp:ListItem>8</asp:ListItem> <asp:ListItem>9</asp:ListItem> </asp:DropDownList> <asp:Button ID="Button1" runat="server" Text="Button" /> <asp:Literal ID="Literal1" runat="server"></asp:Literal> <asp:Panel ID="Panel1" runat="server"></asp:Panel> </div> </form> </body> AddingMachine.aspx.cs ===================== public partial class AddingMachine : System.Web.UI.Page { protected void Page_Load(object sender, EventArgs e) { for (int i = 0; i < Int32.Parse(DropDownList1.SelectedItem.Value); i++) { TextBox NewTextBox = new TextBox(); NewTextBox.Text = "0"; Panel1.Controls.Add(NewTextBox); } int Sum = 0; foreach(Control CurControl in Panel1.Controls) { if(CurControl.GetType() == typeof(TextBox)) { TextBox TargetTextBox = (TextBox)CurControl; Sum += Int32.Parse(TargetTextBox.Text); } } Literal1.Text = "Sum = " + Sum.ToString(); } }
From: Hans Kesting on 3 Jul 2008 11:54 DylanSmith presented the following explanation : > I have a WebForm where I'm dynamically creating some controls and I'm having > difficulty understanding how the state is being persisted and how to work > with it. > > I've created a simplified example to demonstrate my issues. Lets say I have > a WebForm with a DropDownList where the user selects a number from 1 to 10 > (the DropDownList is not dynamically created). I also have a button on there > that I use to trigger a PostBack. > > Based on the user's selection in the drop-down I want to create that many > textboxes programatically. I got this part working easily enough by putting > some code in the Page_Load event. The new textboxes even maintain their > values correctly across post-backs. > > But what if I want to put a LiteralControl on my page that will display the > sum of the values entered in all the dynamic textboxes. The problem is where > do I put this code so that it can access the data in the dynamic textboxes > after their values have been restored from the postback data? > > Based on my understanding of the page lifecycle (from this article: > http://msdn.microsoft.com/en-us/library/ms972976.aspx) I was assuming that > when I added the TextBoxes to the Panel it was triggering some processing > behind the scenes that caused the "LoadPostbackData" logic to be applied to > the textboxes. This would explain how the textbox values were being > maintained across postbacks. If this were true I should be able to put my > adding code immediately after adding the textboxes to the panels and their > properties should be set to the appropriate values. > > This turns out not to work, and the Sum always displays as 0. > > If you're going to suggest I use the Init event to create the dynamic > textboxes then I have the problem of retrieving the value from the > DropDownList that tells me how many I need to create, because the > DropDownList's SelectedItem hasn't been updated yet in the Init event because > the PostBack data hasn't been applied (that's why I was using the Load > event). > > Here is the code I have: > > AddingMachine.aspx > ================== > <body> > <form id="form1" runat="server"> > <div> > > <asp:DropDownList ID="DropDownList1" runat="server"> > <asp:ListItem>1</asp:ListItem> > <asp:ListItem>2</asp:ListItem> > <asp:ListItem>3</asp:ListItem> > <asp:ListItem>4</asp:ListItem> > <asp:ListItem>5</asp:ListItem> > <asp:ListItem>6</asp:ListItem> > <asp:ListItem>7</asp:ListItem> > <asp:ListItem>8</asp:ListItem> > <asp:ListItem>9</asp:ListItem> > </asp:DropDownList> > <asp:Button ID="Button1" runat="server" Text="Button" /> > <asp:Literal ID="Literal1" runat="server"></asp:Literal> > > <asp:Panel ID="Panel1" runat="server"></asp:Panel> > </div> > </form> > </body> > > > AddingMachine.aspx.cs > ===================== > public partial class AddingMachine : System.Web.UI.Page { > protected void Page_Load(object sender, EventArgs e) { > for (int i = 0; i < Int32.Parse(DropDownList1.SelectedItem.Value); > i++) { > TextBox NewTextBox = new TextBox(); > NewTextBox.Text = "0"; > Panel1.Controls.Add(NewTextBox); > } > > int Sum = 0; > > foreach(Control CurControl in Panel1.Controls) { > if(CurControl.GetType() == typeof(TextBox)) { > TextBox TargetTextBox = (TextBox)CurControl; > Sum += Int32.Parse(TargetTextBox.Text); > } > } > Literal1.Text = "Sum = " + Sum.ToString(); > } > } Try moving the sum-code to the PreRender event. I think that "LoadPostbackData" is executed immediately *after* the "Load", so too late for your code but in time for the PreRender. Hans Kesting
From: DylanSmith on 3 Jul 2008 12:11
"Hans Kesting" wrote: > Try moving the sum-code to the PreRender event. > I think that "LoadPostbackData" is executed immediately *after* the > "Load", so too late for your code but in time for the PreRender. > > Hans Kesting Thanks Hans, that works great and should solve both my simplifiied scenario and my actual real-world scenario. But just for curiosities sake, what if I had a more involved scenario where I needed to read the value of my drop-down and use that to create some dynamic controls, then read the values in my dynamic controls and use that to create some more and then read the values of those and perform some logic, and so on and so on. Is there a way to explicitly force the "LoadPostbackData" processing to occur without having to wait for the PreRender event? |