I can't seem to find anything related to the problem I'm encountering.
GOAL
I'm trying to get the value of a custom data attribute from the button (Web Control) that I created in my code-behind when the button is clicked. I'm referencing MSDN-Attribute.GetCustomAttribute Method but I'm not sure if this is appropriate for what I'm trying to do.
My current click sub is below but does not function.
Default.ascx
<asp:UpdatePanel ID="itemWrapper" UpdateMode="Conditional" runat="server">
<ContentTemplate>
<asp:PlaceHolder ID="itemsPlaceholder" runat="server" />
</ContentTemplate>
</asp:UpdatePanel>
Default.ascx.vb (Code-behind)
Protected Sub Items_Load()
Dim dynamicID = "13"
Dim itemBtn = New Button()
itemBtn.Attributes.Add("data-custom-id", dynamicID) '<- Custom attribute I want to get
itemBtn.UseSubmitBehavior = False
itemBtn.Text = "select"
itemBtn.ID = "Button_" & dynamicID
AddHandler itemBtn.Click, AddressOf ItemSelectBtn_Click
itemsPlaceholder.Controls.Add(itemBtn)
End Sub
Public Sub ItemSelectBtn_Click(ByVal sender As Object, ByVal e As EventArgs)
Dim clickedBtn = DirectCast(sender, Button),
clickedBtnID As String = clickedBtn.Attributes("data-custom-id")
Console.WriteLine("this button's ID is " & clickedBtnID)
End Sub
HTML OUTPUT
<div id="Content_C001_itemWrapper">
<input type="button"
name="ctl00$Content$C001$1"
value="select"
onclick="target = '_blank';__doPostBack('ctl00$Content$C001$1','')"
data-custom-id="13">
</div>
UPDATE
This was the simplest answer. Not sure why this wasn't working but as #zgood suggested to get the attribute. VB.NET uses Attributes("attribute-name") and not Attributes["attribute-name"] like in C#. Just an FYI.
Thanks for the help!
Related
I am writing a web page in ASP.Net. Presently, I have an asp:Table that I am using as a sort of "log" for processing output. The idea is that the user selects several files and clicks a button, and each file is "processed" with the log showing what is happening. Processing occurs asynchronously.
Here is the relevant processing segment:
Protected Sub DoAsyncWork()
Dim count = 0
For Each row As GridViewRow In gvList.Rows
count = count + 1
If CType(row.FindControl("cbImport"), System.Web.UI.WebControls.CheckBox).Checked Then
push_to_log("")
push_to_log("Updating Active Projects +" + HttpUtility.HtmlDecode(row.Cells(1).Text).ToString.Substring(0, 30) + "...")
Dim xp(3) As Object
xp(0) = HttpUtility.HtmlDecode(row.Cells(0).Text)
xp(1) = HttpUtility.HtmlDecode(row.Cells(1).Text)
xp(2) = HttpUtility.HtmlDecode(row.Cells(2).Text)
xp(3) = 0
'oDC.UpdateData("Import_P3e_Project ", xp)
If (xp(3) <> 0) Then
push_to_log("Success: " + xp(3).ToString + " have been updated")
Else
push_to_log("Failure: " + xp(3).ToString + " activities updated")
End If
End If
Next
push_to_log("")
push_to_log("Import Complete!")
End Sub
This is how I am calling the process worker function:
Protected Sub button_Import(sender As Object, e As EventArgs) Handles btnImport.Click
Dim t As New Thread(New ThreadStart(AddressOf DoAsyncWork))
t.Priority = Threading.ThreadPriority.Normal
t.Start()
push_to_log("Start Import")
End Sub
The way I am appending things to the log is by dynamically creating rows and cells, then adding them to my table. Here is the relevant subroutine:
Protected Sub push_to_log(ByVal str As String)
Dim newRow As TableRow = New TableRow
Dim newCell As TableCell = New TableCell
logArrayList.Add(str)
Me.ViewState.Add("arrayListInViewState", logArrayList)
newCell.Text = str
newCell.Style("Color") = "White"
newCell.ID = "cell" + (logArrayList.Count - 1).ToString
newRow.ID = "row" + (logArrayList.Count - 1).ToString
newRow.Cells.Add(newCell)
logTable.Rows.Add(newRow)
HiddenButton_Click(HiddenButton, New EventArgs())
'UpdateLogPanel.Update()
'UpdateLogPanel.Focus()
End Sub
I've got the log persisting correctly by using the ViewState to store my arraylist of data and recreating the log on postbacks. The relevant markup for my log looks like this:
<asp:UpdatePanel ID="UpdateLogPanel" UpdateMode="Conditional" runat="server">
<Triggers>
<asp:AsyncPostBackTrigger ControlID="HiddenButton" />
</Triggers>
<ContentTemplate>
<div ID="Div1" class="DefinitionPanel" style="text-align:left;height:200px;overflow:hidden;" runat="server">
<span style="display:inline-block; width:100px;"></span>
<div class="scrollingtable">
<div>
<div id="viewContainer">
<asp:table id="logTable" runat="server" enableviewstate="false">
</asp:table>
</div>
</div>
</div>
</div>
<asp:Button ID="HiddenButton" runat="server" style="display:none;" />
</ContentTemplate>
</asp:UpdatePanel>
I am trying to make my asp:Table update every time a message is posted to it. I thought that enabling partial postbacks and using an UpdatePanel would be the correct solution, but my log still does not output anything until the entire process has completed.
After I add a message to my asp:Table/log, I tried calling
UpdateLogPanel.Update()
which didn't seem to make a difference. Finally I tried adding the asp:AsyncPostBackTrigger and hidden button with the hope that it would fix things but it doesn't seem to. Here is what the hiddenButton event looks like:
Protected Sub HiddenButton_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles HiddenButton.Click
UpdateLogPanel.Visible = True
End Sub
Any guidance on how to make my log re-render itself when I add a message to it would be highly regarded.
I chose to implement using pooling instead of an asynchronous method.
I still use an asp:UpdatePanel. Instead of a table inside of the UpdatePanel, I am now using a gridview, and a timer. When the timer fires, I rebind the gridview, thereby showing any new contents
The key components to making this work:
Square things away with the parent update panel and master page:
Private Sub Page_Init(sender As Object, e As System.EventArgs) Handles Me.Init
'set reference to master site page
mstr = CType(Master, Site)
'setup partial rendering so Log can update asynchronously
scriptManager = CType(mstr.FindControl("ScriptManager1"), ScriptManager)
scriptManager.EnablePartialRendering = True
scriptManager.AsyncPostBackTimeout = 28800
CType(mstr.FindControl("UpdatePanel1"), UpdatePanel).UpdateMode = UpdatePanelUpdateMode.Conditional
CType(mstr.FindControl("UpdatePanel1"), UpdatePanel).ChildrenAsTriggers = False
End Sub
The mark-up for the UpdatePanel looks like
<asp:UpdatePanel ID="UpdateLogPanel" UpdateMode="Conditional"
RenderMode="Inline" ChildrenAsTriggers="false" runat="server">
<ContentTemplate>
<%--The Gridview and other Hidden Fields--%>
<asp:Timer ID="myTimer" OnTick="timer_tick" runat="server" Interval="1000" Enabled="false"/>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="myTimer" EventName="Tick" />
</Triggers>
</asp:UpdatePanel>
Triggering the logging mechanism is done simply by setting
myTimer.Enabled = True
The timer_tick event looks like
Public Sub timer_tick(ByVal sender As Object, ByVal e As EventArgs)
generate_log()
//other logging logic (increment counters, "timeout" mechanism)
UpdateLogPanel.Update()
End Sub
I'm attempting to use Controls.AddAt(), but it apparently breaks controls at later indexes:
Here's my minimal example:
Aspx put in a form:
<asp:DropDownList runat="server" ID="ddl" />
<asp:Button Text="text" runat="server" OnClick="Unnamed2_Click" />
Code Behind:
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If Not IsPostBack Then
ddl.Items.Add("Click the button")
'Controls.Add(New HyperLink) 'Works fine, but is put at end of collection.
'Controls.AddAt(2 ,New HyperLink) 'Is also safe but I wanted the control first
Controls.AddAt(0, New HyperLink) 'ddl loses it's item after postback
End If
End Sub
On the first postback of the page after calling AddAt, the DropDownList loses it's item. It doesn't matter what kind of control I add even HTMLControls. Viewstate is not disabled.
How do I dynamically add controls without breaking others?
If you used a PlaceHolder to add your HyperLink into, it would not mess up the rest of the page:
<asp:PlaceHolder ID="PlaceHolder1" runat="server"></asp:PlaceHolder>
<asp:DropDownList ID="ddl" EnableViewState="true" runat="server" />
<asp:Button ID="bn1" Text="text" OnClick="Unnamed2_Click" runat="server" />
With code like
Protected Sub Unnamed2_Click(sender As Object, e As EventArgs) Handles bn1.Click
Dim newItem = "Click the button" & DateTime.Now.ToString("HH:mm:ss")
ddl.Items.Add(newItem)
ddl.SelectedIndex = ddl.Items.Count - 1
PlaceHolder1.Controls.Add(New HyperLink With {.ID = "hyp", .Text = "Hyperlink here"})
End Sub
And always give your asp:Controls an ID if they take one.
hello i am trying to pass parameters from asp LinkButton to a sub routine and it doesn't work
this is the linkbutton
<asp:LinkButton runat="server" ID="linkId" OnClick="doSomething(10)">ID
</asp:LinkButton>
and this is my sub
Public Sub doSomething(ByVal mon As Integer)
MsgBox("this is the number : " & mon)
End Sub
can some one tell me how can we make this work in VB.net
Try with CommandArgument property
<asp:LinkButton runat="server" ID="linkId" OnClick="doSomething"
CommandArgument="10">ID</asp:LinkButton>
and
Public Sub doSomething(sender As Object, e As EventArgs)
Dim btn As LinkButton = DirectCast(sender, LinkButton )
Dim msg As String = "this is the number : " & Int32.Parse(btn.CommandArgument)
End Sub
You cannot pass anything directly to an event handler. But you can use LinkButton's CommandArgument property:
<asp:LinkButton runat="server"
ID="linkId"
OnClick="LinkButton_Click"
CommandArgument="10">ID
</asp:LinkButton>
in your codebehind (with the correct signature for this event):
Protected Sub LinkButton_Click(sender As Object, e As EventArgs)
Dim lb = DirectCast(sender, LinkButton)
Dim ID As String = lb.CommandArgument;
End Sub
Note that you should not use a MessageBox in an ASP.NET environment since it would be shown on the server and not in the browser.
You don't have to use the DirectCast method. It may make more sense for your sub to look like this:
Public Sub doSomething(sender As Object, e As CommandEventArgs)
Dim msg As String = "this is the number : " & e.CommandArgument
End Sub
You can also store a second value in the CommandName property of the control and use it the same way.
edit: With a slight change to your control, use OnCommand instead of OnClick:
<asp:LinkButton runat="server" ID="linkId" OnCommand="doSomething"
CommandArgument="10" Text="ID" />
I'm trying to make a tabbed menu for the asp.net website I'm working on. One of the pre-requisite is obviously to color the current tab differently, so the user can know on which tab is currently is.
To do this, I made this method in my code-behind:
Protected Function GetCssClass(ByVal ctrl As LinkButton) As String
If ctrl.ID = currentLink Then
Return "current"
Else
Return String.Empty
End If
End Function
and I call the method like this in my aspx page:
<asp:LinkButton ID="LinkButton1" runat="server" PostBackUrl="/Default.aspx" CssClass="<%#GetCssClass(LinkButton1) %>" >Home</asp:LinkButton>
<asp:LinkButton ID="LinkButton2" runat="server" PostBackUrl="/Directory/page1.aspx" CssClass="<%#GetCssClass(LinkButton2) %>" >Page1</asp:LinkButton>
But the method is never hit... As I understand, the method should be called each time the LinkButton is drawn...
Does someone have an idea why?
Thanks in advance !
Edit: Just as a precision, all this code is in the masterpage.
Edit2: Here are the changes I made according to Quagland's suggestion.
In the aspx masterpage:
<asp:HiddenField ID="currentLink" runat="server" />
<asp:LinkButton ID="LinkButton1" runat="server" PostBackUrl="/Default.aspx" OnClick="LinkButton_Click" OnPreRender="LinkButton_PreRender" >Home</asp:LinkButton>
<asp:LinkButton ID="LinkButton2" runat="server" PostBackUrl="/OtherDirectory/Page1.aspx" OnClick="LinkButton_Click" OnPreRender="LinkButton_PreRender" >Page1</asp:LinkButton>
<asp:LinkButton ID="LinkButton3" runat="server" PostBackUrl="/OtherDirectory/Page2.aspx" OnClick="LinkButton_Click" OnPreRender="LinkButton_PreRender" >Page2</asp:LinkButton>
<asp:LinkButton ID="LinkButton4" runat="server" PostBackUrl="/OtherDirectory/Page3.aspx" OnClick="LinkButton_Click" OnPreRender="LinkButton_PreRender" >Page3</asp:LinkButton>
And in the code behind:
Protected Sub LinkButton_Click(ByVal sender As Object, ByVal e As EventArgs) Handles LinkButton1.Click, LinkButton2.Click, LinkButton3.Click, LinkButton4.Click, LinkButton5.Click, LinkButton6.Click, LinkButton7.Click, LinkButton8.Click
Dim lnk As LinkButton = CType(sender, LinkButton)
currentLink.Value = lnk.ID
End Sub
Protected Function GetCssClass(ByVal ctrl As LinkButton) As String
If ctrl.ID = currentLink.Value Then
Return "current"
Else
Return String.Empty
End If
End Function
Protected Sub LinkButton_PreRender(ByVal sender As Object, ByVal e As EventArgs) Handles LinkButton1.PreRender, LinkButton2.PreRender, LinkButton3.PreRender, LinkButton4.PreRender, LinkButton5.PreRender, LinkButton6.PreRender, LinkButton7.PreRender, LinkButton8.PreRender
Dim lnk As LinkButton = CType(sender, LinkButton)
lnk.CssClass = GetCssClass(lnk)
End Sub
The problem is now that the click event is not always fired. On first click, nothing happens, but on second click on a tab, the click event is correctly triggered. Any clue ?
Edit3: Could it be that the value stored in the hidden field is reset each time the masterpage is reloaded (I mean, each link points to a couple masterpage + content page) ?
You need to use <%= ... %> (i.e. replace # with =).
The hash form is used with data binding, you want to just create output.
For .NET 4 prefer <%: ... %> (use a colon) to automatically do HTML encoding.
I have quickly tested a solution that might work for you. I'm just not sure where your 'currentlink' variable comes from. I have implemented it as a hidden field here.
In Masterpage.aspx:
<asp:HiddenField ID="currentLink" runat="server" />
<asp:LinkButton ID="LinkButton1" runat="server">Home</asp:LinkButton>
<asp:LinkButton ID="LinkButton2" runat="server">Page1</asp:LinkButton>
In code behind:
use your original GetCssClass function add this:
Protected Sub LinkButton_PreRender(ByVal sender As Object, ByVal e As System.EventArgs)
Handles LinkButton1.PreRender, LinkButton2.PreRender
Dim lnk As LinkButton = CType(sender, LinkButton)
lnk.CssClass = GetCssClass(lnk)
End Sub
I have put it in PreRender because i was using the LinkButton click events to set the hidden field value (and Click happens after Load but before PreRender):
Protected Sub LinkButton_Click(ByVal sender As Object, ByVal e As System.EventArgs) _
Handles LinkButton1.Click, LinkButton2.Click
Dim lnk As LinkButton = CType(sender, LinkButton)
currentLink.Value = lnk.ID
End Sub
also, if there are numerous Linkbuttons, you may want to declare like:
<asp:LinkButton ID="LinkButton1" runat="server" OnClick="LinkButton_Click" OnPreRender="LinkButton_PreRender">Home</asp:LinkButton>
EDIT:
Here is another solution that will work with cross page postbacks. Not much code but you'll need to put some on every page. Anyway:
Masterpage.aspx:
<asp:LinkButton ID="LinkButton1" runat="server" PostBackUrl="Default.aspx">Home</asp:LinkButton>
<asp:LinkButton ID="LinkButton2" runat="server" PostBackUrl="Page1.aspx">Page1</asp:LinkButton>
<asp:LinkButton ID="LinkButton3" runat="server" PostBackUrl="Page2.aspx">Page2</asp:LinkButton>
<asp:LinkButton ID="LinkButton4" runat="server" PostBackUrl="Page3.aspx">Page3</asp:LinkButton>
Masterpage.aspx.vb:
Public Sub SetCssClass(ByVal ctrl As String)
CType(FindControl(ctrl), LinkButton).CssClass = "current"
End Sub
All content pages:
*.aspx:
add this directive to create a strongly typed reference to the masterpage
<%# MasterType VirtualPath="~/MasterPage.master" %>
*.aspx.vb:
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
Master.SetCssClass("LinkButton1") '' << put the name of the relevant link button here
End Sub
Set the CssClass in the code-behind. This will state your intent more clearly than calling DataBind on your linkbuttons if you use databinding syntax. Since you're doing it for multiple controls maybe stick them in a method called ApplyCssClasses.
I'm not sure why <%= %> isn't working but personally I don't like to mix code and markup, I like to keep them completely separate.
I would suggest setting the CSS class in code perhaps on the page load event.
lnkPopulate.CssClass = "current"
<%# ... %> will work if you're OK with doing LinkButton1.DataBind() in your code-behind at the appropriate time.
I'm trying to figure out the best way to achieve this:
I need to show a drop down list when the page loads, the default selected value is nothing, or an empty element (such as "-"). When the user selects one value from the list, another drop down list is added below the first one, and so on.
My problem with this is how you make the page to remember the drop down lists created through postbacks and the values they have selected? Do I have to use some kind of array to store them or something?
Thank you very much.
EDIT: I did this example for dynamically add drop down lists and suscribe them to an event handler but the event won't fire.
EDIT 2: Changed the code because I labelled for VB.Net and posted something in C#. I update the code with some trivial improvements, but still won't fire the event :(
Private myDdlArray As New List(Of DropDownList)
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
If (Session("ddl") Is Nothing) Then
Session("ddl") = myDdlArray
End If
If (Session("ddlcount") Is Nothing) Then
Session("ddlcount") = 0
End If
myDdlArray = CType(Session("ddl"), List(Of DropDownList))
Dim myDdl As New DropDownList
myDdl = New DropDownList
For Each myDdl In myDdlArray
panel.Controls.Add(myDdl)
panel.Controls.Add(New LiteralControl("<br/>"))
Next
Session("ddl") = myDdlArray
End Sub
Protected Sub btn_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btn.Click
myDdlArray = CType(Session("ddl"), List(Of DropDownList))
Dim ddlCount As Integer = CInt(Session("ddlcount")) + 1
Dim newDdl As New DropDownList
newDdl.ID = String.Format("ddlNPKMaterial{0}", ddlCount)
newDdl.AutoPostBack = True
AddHandler newDdl.SelectedIndexChanged, AddressOf cbo_SelectedIndexChanged
newDdl.Items.Add("Uno")
newDdl.Items.Add("Dos")
newDdl.Items.Add("Tres")
myDdlArray.Add(newDdl)
panel.Controls.Clear()
Dim myDdl As New DropDownList
myDdl = New DropDownList
For Each myDdl In myDdlArray
panel.Controls.Add(myDdl)
panel.Controls.Add(New LiteralControl("<br/>"))
Next
Session("ddl") = myDdlArray
Session("ddlcount") = ddlCount + 1
End Sub
Protected Sub btnReset_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnReset.Click
Session("ddl") = Nothing
Session("ddlcount") = Nothing
End Sub
Protected Sub btnShow_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnShow.Click
' Insert brain here
End Sub
Public Sub cbo_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs)
Response.Write(CType(sender, DropDownList).ID)
End Sub
Protected Sub Page_PreRender(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.PreRender
If (Session("ddl") Is Nothing) Then
panel.Controls.Clear()
End If
End Sub
in the aspx I have:
<form id="form1" runat="server">
<div>
<asp:Button ID="btn" runat="server" Text="Add" />
<asp:Button runat="server" ID="btnShow" Text="Tell me" />
<asp:Button runat="server" ID="btnReset" Text="Reset" />
<br />
<asp:Panel runat="server" ID="panel">
<asp:GridView runat="server" ID="grd">
<Columns>
<asp:BoundField HeaderText="Id" DataField="Id" />
</Columns>
</asp:GridView>
</asp:Panel>
</div>
</form>
I'll use OnLoad to recreate the controls, but if you're not storing the # of controls in ViewState (you could put it into Session, a HiddenField, Cookie, etc.) then you can do it in OnInit. Either way should still fire the postback event.
<asp:PlaceHolder id="phDdl" runat="server">
<asp:DropDwonList id="ddl" runat="server"
OnSelectedIndexChanged="ddl_SelectedIndexChanged" />
</asp:PlaceHolder>
<script runat="server">
int DdlIndex {
get {
object o = ViewState["_ddlIndex"];
if (o == null) {
ViewState["_ddlIndex"] = 0;
}
return (int)ViewState["_ddlIndex"];
}
set {
ViewState["_ddlIndex"] = value;
}
}
protected override void OnLoad(EventArgs e) {
base.OnLoad(e);
for (int i = 0; i < DdlIndex; i++) {
AddDropDownList(i);
}
}
void ddl_SelectedIndexChanged(object sender, EventArgs e) {
var ddl = (DropDownList)sender;
if (ddl.SelectedIndex > 0) {
AddDropDownList(DdlIndex++);
}
}
void AddDropDownList(int i) {
var ddl = new DropDownList();
ddl.Id = string.Format("ddl{0}", i);
ddl.SelectedIndexChanged += ddl_SelectedIndexChanged;
// add items
phDdls.Add(ddl);
}
</script>
You can simply have both dropdown lists exist in your asp code with only one visible on first page load. So something like...
<asp:DropDownList ID="mainDDL" Visible="true" runat="server">
<asp:ListItem Text="Elements here" Value="0" />
<asp:ListItem Text="More elements" Value="1" />
</asp:DropDownList>
<asp:DropDownList ID="dynamicDDL" Visible="false" runat="server">
<asp:ListItem Text="This is an element of the dynamic DDL" Value="3" />
</asp:DropDownList>
And then when, say the "More Elements" item selected, switch the dynamicDDL's visibility to true.
Then on each postback, on the Page_Load event, check what the value of mainDDL is. If it is 0, set dynamicDDL to have visible=true
Edit:
Okay, I took a stab at this. There is some headway in this, however, and maybe it will lead us to some clues.
To start off, we DO need an array to store this. We will need a static array of DDLs and a static integer to count our elements. These can be defined as...
Private Shared ddl_arr As DropDownList() = New DropDownList(100) {} 'max, 100 ddls.
Private Shared ddl_count As Integer = 0
Now, we'll need a panel to store our DDLs in. This is simple asp scripting, such as...
<asp:Panel ID="parentPanel" runat="server">
<asp:DropDownList ID="mainDDL" AutoPostBack="true" Visible="true" runat="server">
<asp:ListItem Text="Elements here" Value="0" />
<asp:ListItem Text="More elements" Value="1" />
</asp:DropDownList>
</asp:Panel>
So now, on our page load we will want to load any of our dropdowns that we have saved so far. This can be coded in a way such as..
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
Try
If TypeOf ddl_arr(0) Is DropDownList Then
For Each ddl As DropDownList In ddl_arr
add_ddl(ddl)
Next
End If
Catch ex As Exception ' this is a bad idea, but for brevity..
End Try
End Sub
Our add_ddl method will simply add our new drop down to the parentPanel.
Protected Sub add_ddl(ByVal ddl As DropDownList)
Try
parentPanel.Controls.Add(ddl)
'add any formatting you would like after each ddl here.
Catch ex As Exception
End Try
End Sub
And finally, our method when we change the ddl. This creates a brand new ddl, gives it an id (and whatever properties you may want for it), adds it to the array, increments our counter, and adds it to the page..
Protected Sub mainDDL_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles mainDDL.SelectedIndexChanged
Dim newDDL As DropDownList = New DropDownList()
newDDL.ID = "ddlID" & ddl_count 'we will need to store a new ID for each one. So, dynamically generate this.
'continue to add properties to your ddl in this fashion...
ddl_arr(ddl_count) = newDDL
ddl_count = ddl_count + 1
add_ddl(newDDL)
End Sub
This method should definitely check for the end of the array (among other things), but lets keep things simple. This code will only add a new DDL whenever you change the index of the ORIGINAL DDL. You will need to set each newly created DDL to have a method called (performing the above instructions) whenever the selected index changes for all of those newly crafted DDLs.
Hopefully this gets you in the right direction. This was way less organized than I'd hoped, sorry!
Dynamic controls are pretty tricky to create. I would start by reading this.
Create and add the dropdowns during the Page_Init method and generate the names based on the backing data that you're gathering. When getting input from them, either read their returned values off of the page using the FindControl() method or by getting the returned value of the control from the Request.Form[]. FindControl uses the id of the control, and Requst.Name uses the client id - you can get this information using the same function you use to generate the names.
Overall, ASP.NET isn't very good at working with generated controls, but by keeping the data you're working with in a model or datastructure separate from the controls that are being generated it isn't difficult to both create the controls and retrieve the results.