Default radio button not triggering an UpdateControl postback - asp.net

Question

I have three radio buttons on a form - A, B, C. Each of these selections populates a dropdown list with data specific to the option. When the form loads, I set option A to be checked (as the default).
When I select buttons B or C, the AsyncPostBack triggers fine and the dropdown is populated. BUT, subsequently selecting A from either B or C does not trigger the event.
I suspect that because A was checked when the form loaded, the browser is not seeing any "change" to raise the event.
So what can be done to enable the default A button recognise it is being changed from B or C in order to raise the postback?
I have tried both setting the checked state of button A in code on inital loading of the page only (ie IsPostBack is False) and alternatively setting the checked attribute of the radiobutton in the html, with the same results. If I don't default the radio button the functionality works as expected, except I don't have the radio button and dropdown list defaulted when the page first loads.
The html...
<asp:RadioButton ID="radBook" runat="server" AutoPostBack="true" GroupName="grpArticleType" Text="Book" />
<asp:RadioButton ID="radCD" runat="server" AutoPostBack="true" GroupName="grpArticleType" Text="CD" />
<asp:RadioButton ID="radDVD" runat="server" AutoPostBack="true" GroupName="grpArticleType" Text="DVD" />
<asp:UpdatePanel ID="pnlTasks" runat="server" UpdateMode="Conditional" RenderMode="Inline">
<ContentTemplate>
<asp:DropDownList ID="dropShippingSize" runat="server" CssClass="dropdownMandatory"></asp:DropDownList>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="radBook" />
<asp:AsyncPostBackTrigger ControlID="radCD" />
<asp:AsyncPostBackTrigger ControlID="radDVD" />
</Triggers>
</asp:UpdatePanel>
The code behind...
Sub Page_Load
If Not Me.IsPostBack Then
radBook.Checked = True
End If
End Sub
Private Sub rad_CheckedChanged(ByVal sender As Object, ByVal e As System.EventArgs) _
Handles radBook.CheckedChanged, radCD.CheckedChanged, radDVD.CheckedChanged
zLoadShippingSizeDropdown()
End Sub

Answer 1

I had the same problem and looked for an answer for hours. This seems to have nothing to do with ViewState or anything similar, but with some kind of incompatibility of using a pre-checked RadioButton as trigger for an Async PostBack.
The work around I found is amazingly easy and worked right away; instead of using the checked=true on the mark-up or myRadioButton.Checked on the server side, I did the following:
Not setting the attribute on Mark-up and add this on the Page_Load event:
if (!IsPostBack)
{
MyRadioButton.InputAttributes["checked"] = "true";
...
}
I hope this helps and saves some people hours of hair pulling :)

Answer 2

I'm guessing you need to need to check if the page is a postback in your load event:
protected void Form_Load(object sender, EventArgs e)
{
if (!Page.IsPostback)
{
// Set radiobutton A...
}
}

Answer 3

We had the same problem and it seems you will have to set the other "checked" property for radio buttons to "false".
So please add the lines
radCD.Checked = False
radDVD.Checked = False

Answer 4

Are you by chance handling viewstate in your code behind as well? If so then you need to handle the AJAX version of it as viewstate can often be lost on AJAX style pages. Try putting your buttons inside the update panel and see if you get the same behaviour if the panel has it's update mode set to conditional. Don't worry about the postback triggers if you do that.
The asynch triggers are only for items inside an update panel. any item outside of a panel will doa full postback by design.
<asp:UpdatePanel ID="pnlTasks" runat="server" UpdateMode="Conditional" RenderMode="Inline">
<ContentTemplate>
<asp:RadioButton ID="radBook" runat="server" AutoPostBack="true" GroupName="grpArticleType" Text="Book" />
<asp:RadioButton ID="radCD" runat="server" AutoPostBack="true" GroupName="grpArticleType" Text="CD" /><asp:RadioButton ID="radDVD" runat="server" AutoPostBack="true" GroupName="grpArticleType" Text="DVD" />
<asp:DropDownList ID="dropShippingSize" runat="server" CssClass="dropdownMandatory">
</asp:DropDownList>
</ContentTemplate>
</asp:UpdatePanel>

Answer 5

WOW, I would have never thought that could be the bug. Saved many hours of frustration.
Thanks Juan going through the carppy Microsoft issue and found a solution for the rest.