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Populating Mulitiselect Listbox

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Chris Leuty

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Apr 26, 2004, 3:50:55 PM4/26/04
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I am populating a multiselect Listbox from a dataset, with the content of
the listbox filled by one table, and the selections determined from another
table. So far, I have been keeping the dataset a denormalized mirror of the
database, but I'm not having much luck getting the selection logic down (I
haven't found a 'hook' where I can access the listbox object as an object to
set the listitem's selected property before it gets rendered)..

Before denormalizing the data in hopes of simplifying the binding, I thought
I would ask if I was missing an obviously simple approach to doing what I am
after (and leaving the dataset denormalized).

Thanks


Scott G.

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Apr 27, 2004, 9:48:41 AM4/27/04
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 I think you should be able to do this with a single table; have you tried something like:
 
ListBox lb = new ListBox();
lb.SelectionMode = ListSelectionMode.Multiple;
lb.DataSource = whatever;
lb.DataBind();
// now set from the list box
foreach (ListItem item in lb.Items)
{
 if (IsItemSelected(item.Value)) // IsItemSelected is your method
 {
  item.Selected = true;
 }
}
 
// or do it from the data side
foreach (DataRow row in whatever)
{
 // blah, whatever you have to determine what's selected.
 if (row["IsSelected"] == "True")
 {
  lb.Items.FindByValue(row["Value"].ToString()).Selected = true;
 }
}
 
Scott
 
 

Chris Leuty

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Apr 27, 2004, 12:15:03 PM4/27/04
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The listbox is currently in the aspx, bound like this:
        <TD vAlign="top" align="right">
            <asp:label id="Label13" runat="server" CssClass="form-label">Trades</asp:label>
        </TD>
        <TD>
            <asp:listbox id="lstTrades" runat="server" CssClass="form-text" Width="154px" SelectionMode="Multiple" DataSource='<%# ContactInfo1 %>' DataMember="Trades" DataTextField="TradeName" DataValueField="TradeID">
            </asp:listbox>
        </TD>
I have code similar to your examples in the codebehind, but I'm having a hard time finding a place where the Listbox is available as an object (or perhaps my code is fubar).  I'm trying to get the listbox reference for the asp:listbox item above by doing the following (in this case, I was doing it during the event that bound the data. e is a DataListCommandEventArgs):
lstTrades = CType(e.Item.FindControl("lstTrades"), ListBox)

I think once I get to that point, the actual selection based on the data will be easy. So far, I haven't found a good place to hook this line (I keep getting back Nothing).

 

Scott G.

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Apr 27, 2004, 12:34:38 PM4/27/04
to
 
 A good spot is probably Page_Load; if you are using the default code that VS.NET generates then this method should be there.... again, if you are using VS.NET there should already be a member field call "lstTrades" as a protected member of the code behind for the ASPX page.
 
 If you want the ListBox during a ListBox DataBinding event; then just CType the sender (the first parent) to the event handler; In C# that might look like (for the ListBox DataBindingEvent):
 
private void lstTrades_DataBinding(object sender, EventArgs e)
{
    ListBox lb = (ListBox)sender;
}
 
 I'm a bit confused if you if you are trying to bind the data to the ListBox in an event for the ListBox; if that's what you are asking, then I think you'd be better off binding the data to the ListBox in the containing page.
 
Scott

Chris Leuty

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Apr 27, 2004, 1:23:06 PM4/27/04
to
I did bind the list data in the asp:listbox.  What I'm trying to do now is mark certain items as selected, which is found in a different table.  To be specific, the listbox contains trades (Plumbing, Electrical, Masonry, etc), and a second table contains the trades that apply to the displayed form (i.e. this vendor does this sort of work).  This listbox is part of an Edit Template on a DataList (so that the selections can be changed). 
 
I chose DataBinding() only as an example of where I have tried to get a reference to the listbox (it didn't work).  I had to define the listbox in the code (VS.NET did not create a reference to it; I figured it was because it was part of the Edit Template, which is late bound), so I felt that I needed to find a place in the process where the listbox was created, but not yet sent back to the browser.  The point where the data is bound seemed to make the most sense to me, but I can't grab a reference to it.
 
Thanks for your help so far.

Scott G.

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Apr 27, 2004, 2:15:32 PM4/27/04
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 Got it; here's an example in C# that gets you to the control; in the DG_EditCommand handler, you'll see that I grab the index of the item and then grab the item to get the ListBox1; now this seems to be the same as the code you posted in a message or two ago. If you run this page as a test you should see the "found control" message.
 
Scott
 
 
<%@ Page language="c#" AutoEventWireup="false" Trace="true" %> <%@ Page language="c#" AutoEventWireup="false" Trace="true" %>
<%@ Import Namespace="System.Data" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >
<html>
 <head>
  <title>DGBind</title>
  <script language="C#" runat="server">
 protected override void OnLoad(EventArgs e)
 {
  this.DG.DataSource = MyFakeDataSet;
  this.DG.DataMember = "Table1";
  this.DG.EditCommand += new DataGridCommandEventHandler(DG_EditCommand);
  this.DG.CancelCommand += new DataGridCommandEventHandler(DG_CancelCommand);
  if (!this.IsPostBack)
  {
   this.DG.DataBind();
  }
 }
 
 private void DG_EditCommand(object source, DataGridCommandEventArgs e)
 {
  this.DG.EditItemIndex = e.Item.ItemIndex;
  this.DG.DataBind();
  ListBox lb = (ListBox)this.DG.Items[e.Item.ItemIndex].FindControl("ListBox1");
  if (lb != null)
  {
   this.Trace.Warn("found control");
  }
 }
 
 private void DG_CancelCommand(object source, DataGridCommandEventArgs e)
 {
  this.DG.EditItemIndex = -1;
  this.DG.DataBind();
 }
 
 private DataSet m_ds;
 private DataSet MyFakeDataSet {
  get {
   if (m_ds != null) {
    return m_ds;
   }
   m_ds = new DataSet();
   DataTable t = new DataTable("Table1");
   t.Columns.Add("Column1");
   m_ds.Tables.Add(t);
   
   for (int i = 0; i < 10; i++) {
    DataRow r = m_ds.Tables[0].NewRow();
    r["Column1"] = i.ToString();
    m_ds.Tables[0].Rows.Add(r);
   }
   return m_ds;
  }
 }
  </script>
 </head>
 <body>
  <form id="Form1" method="post" runat="server">
   <asp:datagrid id="DG" runat="server" autogeneratecolumns="False">
    <columns>
     <asp:editcommandcolumn buttontype="LinkButton" updatetext="Update" headertext="Column1" canceltext="Cancel"
      edittext="Edit"></asp:editcommandcolumn>
     <asp:templatecolumn headertext="Column1">
      <itemtemplate>
       <asp:Label id="Label1" runat="server" Text='<%# DataBinder.Eval(Container, "DataItem.Column1") %>'>
       </asp:label>
      </itemtemplate>
      <edititemtemplate>
       <asp:listbox id="ListBox1" runat="server"></asp:listbox>
      </edititemtemplate>
     </asp:templatecolumn>
    </columns>
   </asp:datagrid>
  </form>
 </body>
</html>
<%@ Import Namespace="System.Data" %>

Chris Leuty

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Apr 27, 2004, 3:39:54 PM4/27/04
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Thanks, that worked.  I converted the C# into VB and it looked like this:

lstTrades =

CType(dlContactList.Items(e.Item.ItemIndex).FindControl("lstTrades"), ListBox)

VB casting is nasty looking to me (I much prefer C#, but I inherited this project....you know how that goes...). Anyway, you can see that the old line (lstTrades = CType(e.Item.FindControl("lstTrades"), ListBox)) is simply looking in the wrong Item collection.  Would help to look in the DataList hierarchy. Duh.

Thanks for the help

Chris

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