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

```