Visual Studio .NET - dynamic dropdownlist control

Hi 

    in my code for aspx i create a dropdown list dynamically and add it to a place holder control ,but upon postback i am unable to find the control or find the selected value in the dropbox so that i can create another dropbox with some other values.Here is the code 
 how do i solve this issue ? i tried adding the control again after postback

Please help 


---------------------------------------------------------
Dim dd1 As DropDownList = New DropDownList
Dim dd2 As DropDownList = New DropDownList

    Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        If Me.IsPostBack = True Then
           ' PlaceHolder1.Controls.Add(dd1)
           ' Dim dd2 As DropDownList = CType(Me.Placeholder1.FindControl("dd1"), DropDownList)
            If dd1.SelectedIndex = 2 Then
                dd2.Items.Add("None")
                dd2.Items.Add("B1")
                dd2.Items.Add("B2")
                PlaceHolder1.Controls.Add(dd2)
            End If
        Else
            dd1.Items.Add("None")
            dd1.Items.Add("A1")
            dd1.Items.Add("A2")
            dd1.AutoPostBack = True
            dd1.EnableViewState = False
            PlaceHolder1.Controls.Add(dd1)

        End If
    End Sub
-----------------------------------------------------------------------

MUST be re-created on a postback. Repeat after me: "Dynamically created controls...."

I have tried recreating the controls that I had dynamically created prior to postback, and I can access the controls, but their values are all null.  I've seen this answer (that you have to "recreate" the controls after postback), but I'm doing this, and the controls still show as not having anything in them.

What I have:
Upon page load, I create several textbox objects and add them to a System.Web.UI.WebControls.Table object.  Here's some sample code:

Protected WithEvents dynInsertTbl As System.Web.UI.WebControls.Table
Dim bCell As System.Web.UI.WebControls.TableCell
Dim tmp As New System.Web.UI.WebControls.TextBox
Dim boxRow As New System.Web.UI.WebControls.TableRow

tmp.ID = colHeaders(i)
tmp.Width = System.Web.UI.WebControls.Unit.Pixel(45)

bCell = New System.Web.UI.WebControls.TableCell
bCell.Controls.Add(tmp)
boxRow.Cells.Add(bCell)

dynInsertTbl.Rows.Add(boxRow)

Upon post back, I call a function that does exactly the above, and then print out the textboxes:

Response.Write(CType(dynInsertTbl.Rows(1).Cells(i).Controls(0), TextBox).Text().Trim() + "<br />")

Note: there are several textboxes, hence why I use "cells(i)" above.

When I access the textbox upon postback, it doesn't show what I typed into the textbox prior to postback?

I don't really understand why recreating the textboxes allows you to access the dynamically generated textboxes.  Doesn't recreating the textbox also set it's value to null?

you have to restore the values since after they are re-created, they are "Brand new'.
You can do this by overriding or saving to ViewState, or by storing values in Session.

But that's the point I'm trying to make....I have no access to the values because they are gone.  I'm saying, when I try to access the textboxes, they show a null value.

How do I access the value that was typed into textboxes?

Dynamically added controls have no object reference variable in the codebehind class. 
They appear only in the control collection of the containing control, i.e. the Page.Controls collection. 
When the page is posted back to the server as a result of user interaction a new instance of the codebehind class is instantiated, and all the variables of the class are set with values from  ViewState. 

This means that the objects we are accessing from the codebehind class actually are new ones that got their predecessors' values via ViewState.

So, the controls that were dynamically created are no longer there and consequently the values returned from these controls have no place to go. They are lost in the viewstate. 

In order to catch these values the dynamically generated controls needs to be re-generated at Page_Load. The important thing is to assign the same ID to each control as it previously had.  The ViewState uses this ID property of the Control object to reinstate the values. 

Example:

private void Page_Load(object sender, System.EventArgs e)
    {
        if(!Page.IsPostBack)      
            this.NumberOfControls = 0;                
        else
	 // your method to add dynamic controls programmatically
           this.createControls(); 
    }
   
    private void createControls()
    {
        int count = this.NumberOfControls;

        for(int i = 0; i < count; i++)
        {
            TextBox txb = new TextBox();
            txb.ID = "ControlID_" + i.ToString();
            //Add the Controls to the container of your choice
            Page.Controls.Add(txb);
        }
    }

    // example of dynamic addition of controls
    // note the use of the ViewState variable
    private void addSomeControl()
    {
        TextBox txb = new TextBox();
        txb.ID = "ControlID_" + NumberOfControls.ToString();

        Page.Controls.Add(txb);
        this.NumberOfControls++;
    }

-- Hope this clarifies things for you.

I understand the theory you're presenting, and I've tried a simpler experiment than I was attempting previously.

I have a function called "InjectInsertTable()" that instantiates a textbox and adds it to a panel object.

sub InjectInsertTable()
  Dim tmp As New System.Web.UI.WebControls.TextBox
  tmp.ID = "blake"
  tmp.Visible = True
  InsertRecordPnl.Controls.Add(tmp)
  InsertRecordPnl.Visible = True
end sub

"InsertRecordPnl" is a panel that I have created in the Visual Studio designer.  I'm placing a textbox on the panel, dynamically, and the textbox shows when the page loads.

Next, I enter something in the textbox, and click a button that posts the form back.  Here is the code for the button handler:

private sub AddRecordBtn_click(...)
  InjectInsertTable() ' <-- recreate the textbox
  Dim meal As TextBox = InsertRecordPnl.FindControl("blake")
  Response.Write("::" + meal.Text.Trim() + "::")
end sub

It prints nothing.

I have created the textbox once with the InjectInsertTable function, and upon postback, I call the SAME function again, thereby recreating it.  Then I print, and I still get nothing.

I am sorry, I am a nube...: /

Ok, I wasn't listening.  I was trying to recreate the textboxes in my button event handler, which, apparently is not where the recreation should occur.  I put my "recreation" code in the postback part of the page_load event, and I am indeed able to get the values that were in the textboxes from the button handler function.  Ah, 10 hours of growing pains....  Thanks for the help and excellent explanation Pete.