SelectedIndexChanged事件未在子控件中触发

Question

I have a asp.net control that references a user control(popup) when a button is pressed. When the button is pressed I find the drop down list in the popup user control and I set the selected value of it.

  protected void btnMyBUtton_Click(object sender, EventArgs args)
    {
        pnlUserControl_ModalPopupExtender.Show();

        if (ddParent.SelectedIndex > 0)
        {
            DropDownList dd = ucMyControl.FindControlRecursive("ddChild") as DropDownList;

            dd.SelectedValue = ddParent.SelectedValue;

        }
    }

In the popup user control I'm expecting the SelectedIndexChanged event to fire. However, it isn't. Any ideas why this would be the case?

Markup for my child control(only relevant code) is below:

<asp:FormView ID="myForm" runat="server" DataSourceID="dsMyDS" DefaultMode="Insert">
<EditItemTemplate>
    <asp:Panel runat="server">
        <table>

            <tr>
                <td>
                    <asp:Label Text="Name" runat="server" /></td>
                <td>
                    <asp:DropDownList ID="ddChild" runat="server" DataSourceID="ddDS" DataMember="" DataValueField="Id" DataTextField="Name" SelectedValue='<%# Bind("Id") %>' AutoPostBack="true" OnSelectedIndexChanged="ddChild_SelectedIndexChanged" OnDataBound="ddChild_DataBound" />
                </td>
            </tr>

        </table>
        <table>
            <tr>
                <td>
                    <br />
                    <asp:Button ID="btInsertOk" runat="server" Text="Add" CommandName="Insert" />
                    <asp:Button ID="btInsertCancel" runat="server" Text="Cancel" />
                </td>
            </tr>
        </table>
    </asp:Panel>
</EditItemTemplate>

Thanks in advance.

Answer 1

It seems the event doesn't fire when changing the selected value in the code-behind. It only fires when the user changes the value by selecting a new value in the drop-down.

What you could do, is move your code out of the event into a separate method, and call that method from the event. Your event will still fire when the user makes a change via the UI.

protected void ddChild_SelectedIndexChanged(object sender, EventArgs e)
{
    ImportantStuff();
}

public void ImportantStuff()
{
    // everything that was in the SelectedIndexChanged event
}

Then in your button click event, store the old SelectedIndex value, make the change, and compare it to the new SelectedIndex value. If they're different, then the selected index has changed, and you can call the same method being called by the SelectedIndexChanged event above:

protected void btnMyBUtton_Click(object sender, EventArgs args)
{
    pnlUserControl_ModalPopupExtender.Show();

    if (ddParent.SelectedIndex > 0)
    {
        DropDownList dd = ucMyControl.FindControlRecursive("ddChild") as DropDownList;

        var oldIndex = dd.SelectedIndex;

        dd.SelectedValue = ddParent.SelectedValue;

        var newIndex = dd.SelectedIndex;

        if (oldIndex != newIndex)  // selected index changed
            ImportantStuff();
    }
}

As Sam noted, the ImportantStuff() method should be public in the UserControl, so you can access it from the Page the UserControl is in.

Better yet, assuming the "important stuff" doesn't need to access other controls in the UserControl, you could move that code into a separate class, and then call it from both places without requiring a Page to call public methods in the UserControl (your code stays a little bit cleaner, and less tangled up).

Answer 2

SelectedIndexChanged event is bit confusing. According to MSDN the definition for this method is

Occurs when the selection from the list control changes between posts to the server.

So, this event fires in the next postback in this case (since there's no AutoPostback ) even if you don't select anything from the DropDownList. This is because the selected index has changed between the postbacks. Not sure whether this is a bug. I did little bit of testing and this is true. Correct me if I'm wrong. I learnt something new today :)

Solution

The solution to the problem is what @GrantWinney suggested. Extract the common functionality in the SelectedIndexChanged event into a public event and call that method just after the following line.

dd.SelectedValue = ddParent.SelectedValue;

If your ddChild control was in the same method you could just invoke it's SelectedIndexChanged event by calling it as follows

ddChild_SelectedIndexChanged(sender, args);

Answer 3

I assume you are showing your user control in ajax popup. You might try popping up your usercontrol like this:

<asp:Panel ID="pseudopopup" runat="server" visible="false">
 <table style="position: fixed; z-index: 1; left: 0px; top: 0px"   border="0"  width="100%" height="100%">
    <tr>
        <td valign="top"  align="center"  >
        <div style="margin:top:60px; width:600px" class="shadow"
        <ascx:WebUserControl ID="mycontrol" runat="server" />   
        </div> 
        </td>        
    </tr>
    </table> 
 </asp:Panel>

In code behind:

protected void btnMyBUtton_Click(object sender, EventArgs args)
pseudopopup.Visible = true;