Ajax – Default radio button not triggering an UpdateControl postback


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">
   <asp:DropDownList ID="dropShippingSize" runat="server" CssClass="dropdownMandatory"></asp:DropDownList>
   <asp:AsyncPostBackTrigger ControlID="radBook" />
   <asp:AsyncPostBackTrigger ControlID="radCD" />
   <asp:AsyncPostBackTrigger ControlID="radDVD" />

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


End Sub

Best Solution

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 :)

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