Again on dynamic control creation in asp.net 2.0

In a previous post I explain best practices to follow when dealing with dynamic controls in asp.net 2.0, but I missed one important point. Try to insert this snippet of code in a blank page

   Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs)
      Trace.Warn(“clicked button “ + DirectCast(sender, Button).Text)
   End Sub
 
   Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
      CreateButtons()
   End Sub
 
   Private Sub CreateButtons()
 
      If Not Me.IsPostBack Then
         Dim button As New Button
         button.Text = “Bottone che poi scompare”
         AddHandler button.Click, AddressOf Button1_Click
         Page.Form.Controls.Add(button)
      End If
      Dim button2 As New Button
      button2.Text = “Button2″
      AddHandler button2.Click, AddressOf Button1_Click
      Page.Form.Controls.Add(button2)
   End Sub

In this code we create two buttons dynamically during the get of the page, but in each subsequent post only one button gets created, because IsPostBack is true. The behavior of the page can be surprising, pressing “button2″ the first time does not fire the Click event, but subsequent clicks work as ususal. The problem is due to the fact that during the first rendering there are two buttons on the page, the first click does the first post on the page, now the code creates only one button and when the asp.net engine check the post variables to look for any event to be generated find this post variables (Sniffed with Fiddler).

__VIEWSTATE=/wEPDwUJNzgzNDMwNTMzZGQ=

Ctl23=Button2

As we can see the button is called ctl23 because it does not have an id, but when the page render in the first postback the first button does not gets rendered, so the button with text “Button2″ now has Id Ctl22, and no event can be generated. In subsequent call the name of the control does not change so no problems happen. As a proof of this theory try to create the two buttons in this way.

      Dim button2 As New Button
      button2.Text = “Button2″
      AddHandler button2.Click, AddressOf Button1_Click
      Page.Form.Controls.Add(button2)
      If Not Me.IsPostBack Then
         Dim button As New Button
         button.Text = “Bottone che poi scompare”
         AddHandler button.Click, AddressOf Button1_Click
         Page.Form.Controls.Add(button)
      End If

The only differences is that the button that gets created only in the first call now is created after the button with text “Button2″, this means that the id of the button with text “button2″ is Ctl22 at each post and all works as expected.

The obvious solution to this kind of problems is to assign an unique ID to each dynamically generated control so the id in the page will be unique and no problem will arise if controls tree change at each postback.

Alk.