Question

I am trying to create a UI that creates rows that can be dynamically populated.

I am at the point where I can add a Panel to my list that contains a DropDownList and has an associated Remove Button.

The Remove Button has an OnClick event bound that allows me to remove that specific panel.

I am having an issue when trying to bind a SelectedIndexChanged EventHandler to my DropDownList it does not.

I suspect this had something to do with how I am recreating my controls after every Postback.

I am not asking for or expecting a straightforward “Add this or Change this in the code.” I really want to have a understand where I am going wrong, sorry for asking so much! :s

protected void Page_Load(object sender, EventArgs e)
{
    //Showing this for the the poster that asked.
    if (!IsPostBack)
    {
        GetClustersFromDB(user);
        BindGrid();
        BindState();            
    }
    if (Session["persistControls"] != null)
    {
        persistControls = (List<Panel>)Session["persistControls"];
        int count = 0;

        foreach (Panel dynamicControl in persistControls)
        {
            DropDownList list = new DropDownList();
            list.ID = "list" + count;
            list.SelectedIndexChanged += new EventHandler(list_SelectedIndexChanged);
            list.AutoPostBack = true;
            list.Items.Add(new ListItem("", "0"));
            list.Items.Add(new ListItem("Title", "1"));

            dynamicControl.ID = "panel" + count;

            Button btnRemove = new Button();
            btnRemove.Click += new EventHandler(btnDelete_Click);
            btnRemove.Text = "Remove";
            btnRemove.CommandArgument = count.ToString();             

            myPlaceholder.Controls.Add(dynamicControl);
            myPlaceholder.Controls.Add(btnRemove);
            count++;
        }
    }
}

protected void btnAdd_Click(object sender, EventArgs e)
{
    try
    {
        DropDownList list = new DropDownList();
        list.SelectedIndexChanged += new EventHandler(list_SelectedIndexChanged);
        list.AutoPostBack = true;
        list.Items.Add(new ListItem("", "0"));
        list.Items.Add(new ListItem("Title", "1"));

        Panel panelContainer = new Panel();
        panelContainer.ID = "panel" + persistControls.Count;

        panelContainer.Controls.Add(list);

        Button btnRemove = new Button();
        btnRemove.Click += new EventHandler(btnDelete_Click);
        btnRemove.Text = "Remove";
        btnRemove.CommandArgument = persistControls.Count.ToString();
        myPlaceholder.Controls.Add(panelContainer); // Pushes the Panel to the page.
        persistControls.Add(panelContainer);// Adds our Panel to the Control list

        myPlaceholder.Controls.Add(btnRemove); // Pushes our Button to the page.
        Session["persistControls"] = persistControls; // put it in the session
    }
    catch
    {
        throw;
    }
}

protected void btnDelete_Click(object sender, EventArgs e)
{
    try
    {
        int deleteThisOne = int.Parse(((Button)sender).CommandArgument);
        persistControls.Remove(persistControls[deleteThisOne]);
        Session["persistControls"] = persistControls;          
        Response.Redirect(Request.Url.ToString());
    }
    catch
    {
        throw;
    }
}

protected void list_SelectedIndexChanged(object sender, EventArgs e)
{
    throw new NotImplementedException();
}

//aspx File Snippets
<asp:Button ID="btnAdd" runat="server" Text="Add Control" onclick="btnAdd_Click" />
<asp:Button ID="btnClear" runat="server" Text="Reset" onclick="btnClear_Click"/>
<asp:PlaceHolder ID="myPlaceholder" runat="server"></asp:PlaceHolder>

Answer 1

Maintaining controls like this is a real pain. You could try an alternative approach:

1. Create a (small) class that encapsulates the data you want to display in each Panel.
2. Maintain a List of these objects in Session.
3. Use an <asp:Repeater> to display the items by data-binding it to your list.
4. In the repeater’s ItemTemplate, you can write out your <asp:Panel> that can contain a <asp:DropDownList>. Use the ItemCommand property of the drop-down list to bind it to a server-side event handler. Similarly you can put an <asp:Button> in the template and bind the ItemCommand property of this to another server-side event handler.
5. When you click Add, you’ll postback to the server, add to your list, save it in session, re-bind the repeater.
6. When you click a Remove, you’ll postback to the server, remove the item from your list, save it in session, re-bind the repeater.

This way lets ASP.Net handle all the control creation for you. However, if you really want to do all that yourself, then this Infinities Loop blog post is required reading.