Why do dynamic controls disappear on postback and not raise events?

One common error that many have run into when first starting out with creating dynamic page controls is an incorrect assumption that those dynamic controls will survive page postbacks.

For example - a common scenario is to:

1.
Declare a button (Button1) in your page markup
2.
Add an event handler for Button1 and in that handler, create a new dynamic button (Button2)
3.
Wire up and event handler for Button2
4.
Page loads an you see Button1
5.
Click on Button1 and the page posts-back Button1's Click event fires and you now see Button1 and Button2
6.
Click on Button2 and...the page posts-back, Button2 disappears and no event for Button2 is raised.

What happened to Button2?

Since Button2 is not part of your pages declared markup, the framework has no way to know if it should be recreated or not on a postback. You'll need to keep track of the fact that you have
created dynamic control(s) and you will need to add the necessary code to recreate those dynamic control(s) yourself on all subsequent page postbacks. In order for your dynamic control(s)
to work correctly, you'll need to get them all recreated by the Page_Load event at the latest.

The following example demonstrates how to toggle between 2 different sets of dynamic controls and how to handle events raised from those controls. In this example, I've chosen ViewState as the place where i will store the information needed to know which dynamic controls need to be recreated on a postback.

First - here's the basic page markup:

<form id="form1" runat="server">
<div>
<hr />
<asp:Button ID="cmdAlphabet" runat="server" Text="Load Alphabet" />
<asp:Button ID="cmdNumbers" runat="server" Text="Load Numbers" />
<asp:Label ID="lblViewStateValue" runat="server" Text="" EnableViewState="false"></asp:Label>
<hr />
<asplaceHolder ID="PlaceHolder1" runat="server"></asplaceHolder>
<hr />
<asp:Label ID="lblClickResult" runat="server" Text="" EnableViewState="false"></asp:Label>
</div>
</form>

And here's the associated codebehind:

Partial Class DynamicControls
Inherits System.Web.UI.Page

Const ALPHABET_SELECTION As String = "ALPHABET"
Const NUMBER_SELECTION As String = "NUMBERS"
Const VIEWSTATEKEY_DYNCONTROL As String = "DynamicControlSelection"

'store property value in viewstate so that it will survive postbacks
Private Property DynamicControlSelection() As String
Get
Dim result As String = ViewState.Item(VIEWSTATEKEY_DYNCONTROL)
If result Is Nothing Then
'doing things like this lets us access this property without
'worrying about this property returning null/Nothing
Return String.Empty
Else
Return result
End If
End Get
Set(ByVal value As String)
ViewState.Item(VIEWSTATEKEY_DYNCONTROL) = value
End Set
End Property

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) _
Handles Me.Load
'running this code on every page_load - even when it's a postback
'check our page property (that we stored in viewstate) to see
'if we need to load a specific set of dynamic controls
Select Case Me.DynamicControlSelection
Case ALPHABET_SELECTION
CreateDynamicAlphabetLinks()

Case NUMBER_SELECTION
CreateDynamicNumberButtons()

Case Else
'no dynamic controls need to be loaded...yet
End Select

End Sub

Private Sub onClick(ByVal sender As Object, ByVal e As EventArgs)
'all of the dynamic linkbuttons/buttons will trigger this event handler
'since we used both linkbuttons and regular buttons for our dynamic controls,
'we will cast the sender control to an interface that is common to both
'of those button controls - the IButtonControl interface
Dim btn As IButtonControl = DirectCast(sender, IButtonControl)
Me.lblClickResult.Text = _
String.Format("You clicked - CommandName: {0} CommandArgument: {1}", _
btn.CommandName, btn.CommandArgument)
End Sub

Protected Sub cmdAlphabet_Click(ByVal sender As Object, ByVal e As System.EventArgs) _
Handles cmdAlphabet.Click
'user is selecting to show the dynamic Alphabet buttons
Me.CreateDynamicAlphabetLinks()
End Sub

Protected Sub cmdNumbers_Click(ByVal sender As Object, ByVal e As System.EventArgs) _
Handles cmdNumbers.Click
'user is selecting to show the dynamic Number buttons
Me.CreateDynamicNumberButtons()
End Sub

Private Sub CreateDynamicAlphabetLinks()

'clear the placeholder first - in case something else was dynamically loaded
Me.PlaceHolder1.Controls.Clear()

'dynamically create a series of linkbuttons
For keycode As Integer = 65 To 90 'one for each letter in the alphabet
Dim lnk As New LinkButton

'assign the ID ourself to make sure it is consistent
'if you let the framework assign it, the dynamic control may
'not behave correctly
lnk.ID = "alpha_" & keycode.ToString
lnk.Text = Chr(keycode)

'we'll add a CommandName and a CommandArgument
'so we can determine what was clicked when the event is raised
lnk.CommandName = "ALPHABET"
lnk.CommandArgument = Chr(keycode)

'have them all use the same event handler
AddHandler lnk.Click, AddressOf onClick

'add these dynamic controls to our strategically place placeholder control
'the position of the placeholder determines
'where on the page the dynamic controls will appear
Me.PlaceHolder1.Controls.Add(lnk)
Me.PlaceHolder1.Controls.Add(New LiteralControl(" ")) 'space them out
Next

'VERY IMPORTANT -> remember that we created these controls for the next postback
Me.DynamicControlSelection = ALPHABET_SELECTION

End Sub

Private Sub CreateDynamicNumberButtons()

'clear the placeholder first - in case something else was dynamically loaded
Me.PlaceHolder1.Controls.Clear()

'dynamically create a series of button controls
For number As Integer = 0 To 25
Dim btn As New Button
'assign the ID ourself to make sure it is consistent
'if you let the framework assign it, the dynamic control may
'not behave correctly
btn.ID = "number_" & number.ToString
btn.Text = number.ToString

'we'll add a CommandName and a CommandArgument
'so we can determine what was clicked when the event is raised
btn.CommandName = "NUMBER"
btn.CommandArgument = number.ToString

'have them all use the same event handler
AddHandler btn.Click, AddressOf onClick

'add these dynamic controls to our strategically place placeholder control
'the position of the placeholder determines
'where on the page the dynamic controls will appear
Me.PlaceHolder1.Controls.Add(btn)
Me.PlaceHolder1.Controls.Add(New LiteralControl(" ")) 'space them out
Next

'VERY IMPORTANT -> remember that we created these controls for the next postback
Me.DynamicControlSelection = NUMBER_SELECTION

End Sub

Protected Sub Page_PreRender(ByVal sender As Object, ByVal e As System.EventArgs) _
Handles Me.PreRender
Me.lblViewStateValue.Text = Me.DynamicControlSelection
End Sub
End Class