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Why would my child controls be uninitialized at the time of event attachment?

I have a page and a user control — we'll call them Detail.aspx and Selector.ascx.
Let's say the page shows the details of individual records in a database. The user control basically consists of a DropDownList control and some associated HTML. The DropDownList displays a list of other records to switch to at any time.
When the DropDownList fires its SelectedIndexChanged event, I'd like the parent page, Detail.aspx in this case, to handle it. After all, he'll need to know what was selected so that he can appropriately change the URL and the details shown, etc.
To do that, I've done what I usually do, which is also what the top answer says to do in this StackOverflow question:
public event EventHandler DropDownSelectedIndexChanged
{
add
{
MyDropDownList.SelectedIndexChanged += value;
}
remove
{
MyDropDownList.SelectedIndexChanged -= value;
}
}
The above code appears in the Selector.ascx.cs codebehind file.
As a result, on Detail.aspx, I can use it like so:
<cc1:RecordSelector ID="RecordSelector1" runat="server"
OnDropDownSelectedIndexChanged="RecordSelector1_DropDownSelectedIndexChanged" />
So far nothing fancy or surprising.
Here is my problem:
This causes a NullReferenceException when the browser hits Detail.aspx.
Debugging the problem shows that when the page is first hit, the public event I've shown above tries to add the event, but MyDropDownList is null, thus throwing the exception. From what I can tell, the events are added (or attempted to be added) before the Selector user control's Load event fires and thus also before the DropDownList's Load event fires.
Curiously, if I omit the OnDropDownSelectedIndexChanged attribute from Detail.aspx and instead put the following in the Page_Load event in Detail.aspx.cs:
protected void Page_Load(object sender, EventArgs e)
{
RecordSelector1.DropDownSelectedIndexChanged += new EventHandler(RecordSelector1_DropDownSelectedIndexChanged);
}
It works exactly as expected. The events are attached and handled just fine. No problems.
But this means several bad things:
I have to remember not to use the designer to add said event onto my user control
I have to remember not to add the event via attributes when working in source view
Worst of all, as the control's author I need to make sure everybody else using my control knows 1 and 2
So what am I doing wrong? Every example I've seen thus far shows similar usage of exposing child controls' events through a user control.

The reason this works:
protected void Page_Load(object sender, EventArgs e)
{
RecordSelector1.DropDownSelectedIndexChanged
+= new EventHandler(RecordSelector1_DropDownSelectedIndexChanged);
}
and this does not:
<cc1:RecordSelector ID="RecordSelector1" runat="server"
OnDropDownSelectedIndexChanged="RecordSelector1_DropDownSelectedIndexChanged" />
is because the first one adds the handler after the control has been initialized (via the page's Init). The second example gets parsed much earlier and as such the page is attempting to add the handler before the control has initialized.
Due to the nature of the page's life cycle I think you may have to live with adding the event handler in the code-behind. There will be no way to add the handler before the control is initialized because that control will always be null prior to initialization.

Programmatically adding a user control in ASP.NET

I have a user control that needs to load a child control when a button is clicked.
The trouble is that it has to request the control from another class.
So in the button click event, I call the function to get me my control, and add it to the page, like this:
UserControl ctrl = ExampleDataProvider.GetControl(some params...);
myDetailPane.Controls.Add(ctrl);
The GetControl method looks like:
public static UserControl GetControl(some params...)
{
ExampleDetailPane ctrl = new ExampleDetailPane();
ctrl.Value = "12";
ctrl.Comment = string.Empty;
return ctrl;
}
This isn't working due to the page's lifecycle - the Page_Load of the child control gets fired and its controls are null.
I kind-of know that my approach is wrong and why, but don't know the best way to go about fixing it! Could anyone help?

Dynamic controls must be re-created on every postback, this Article is a good link about how to persist dynamic controls and their state.

If you want to access your control in PostBack or you want to bind Event, you have to create them in CreateChildControls() method.
private UserControl _uc = null;
/// <summary>
/// Creates all controls.
/// </summary>
/// <remarks>All the controls must be created in this method for the event handler</remarks>
protected override void CreateChildControls()
{
_uc = new UserControl ();
this.Controls.Add(_uc);
base.CreateChildControls();
}

Create your control in Page_Init. Then make it visible on your Button_Click event.
(CTRL+C/CTRL+V from some other question I answered last week):
Everything that has to be maintained between page cycles should be declared in Page_Init, not Page_Load.
All the initialization, like adding event handlers, and adding controls should be added during initialization, as the state is saved between page cycles. Handling with the content of controls and the viewstate, should be done in Load.
Check also http://msdn.microsoft.com/en-us/library/ms178472.aspx.
Init
Raised after all controls have been initialized and any skin
settings have been applied. Use this
event to read or initialize control
properties.
.
Load
The Page calls the OnLoad event method
on the Page, then recursively does the
same for each child control, which
does the same for each of its child
controls until the page and all
controls are loaded.
Use the OnLoad event method to set
properties in controls and establish
database connections.

Two possible approaches:
Have the control in the page but not loaded with anything until they click their button. Then you can populate the values in the control. This has the benefit of being within the page life cycle. Note, you can always use the "display: none" style setting for the that your user control is in. Then, as part of the OnClick for the button, you can reveal the div making your control visible.
You could pop up another window, though obviously this has the potential for being blocked by popup blockers.

combobox in C# not getting populated

I have a windows forms app in C#. Platform is vS 2005.
Following is the piece of code:
namespace HostApp
{
public partial class Form1 : Form
{
private void comboBox2_SelectedIndexChanged(object sender, EventArgs e)
{
comboBox2.Items.Add("Apples");
comboBox2.Items.Add("Oranges");
comboBox2.Items.Add("Grapefruits");
}
}
}
I run the app but the fruit names do not show up in the drop down of comboBox2. I am sure I am missing some line of code to "populate" the drop down with the entered values.
Any help would be much appreciated.
Thanks,
Viren

You add the items in the handler for the SelectedIndexChanged event. You need to move the code to InitializeComponent or another appropriate place.

Please check the following things:
You have added AutoPostBack="true" in the combo-box so that the selectedChange event is fired and post back happens.
Make sure you have nothung in ur Page load which refreshed the combo box. You can use IsPostBack to acheive loading of the values.

Your items are being added when the selected item is changed, but as there are no existing items this will never happen. Move those lines to the constructor for Form1 and it'll work.

The code you provided will only add items to comboBox2 when the selection changes in the control that is hooked up to comboBox2_SelectedIndexChanged.
There are two concepts at play here: Control Initialization/Databinding, and event handling.
The code you have written essentially says "If somebody selects something new in the combo box, add these 3 options to the combo box". That would happen every time the selected index changes in the combo box. This, of course, assumes you have even hooked up this event handler to the combo box to begin with. This is event handling.
What you are probably trying to do is initialize the control. This happens when you load the page and want to setup the initial options available in your page controls. Using the Init or Load event is probably where you want to setup the choices in your control. This is also when you would initialize your event handlers to say "When something happens, do this".

Move the code to the Page_Load event ...
The SelectedIndexChanged only fires when the ComboBox index has changed AND AutoPostBack = True.
EDIT: Sorry, it's a Form, I was thinking web ... move to Form_Load

For people having difficulties with autopostback and viewstate, beware of the page_load event.
If have been getting on this page alot when trying to google, so that's the reason i'll post it here.
If you fill your dropdownlist (or any other control) in the page_load method, be sure to write an extra control is there is a postback (triggered when changing value of a dropdownlist).
If you don't make that control, your controls will be refilled.
That mistake took me a while to figure out.
So what i'm saying is
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
//fill your controls here
}
}

DropDownList.SelectedValue changes (as a child control in a FormView) aren't sticking

Okay, I have a FormView with a couple of child controls in an InsertItemTemplate. One of them is a DropDownList, called DdlAssigned. I reference it in the Page's OnLoad method like so:
protected void Page_Load(object sender, EventArgs e)
{
((DropDownList)FrmAdd.FindControl("DdlAssigned")).SelectedValue =
((Guid)Membership.GetUser().ProviderUserKey).ToString();
}
Basically I'm just setting the default value of the DropDownList to the user currently logged in.
Anyway, when the page finishes loading the SelectedValue change isn't reflected on the page. I stepped through OnLoad and I can see the change reflected in my Watch list, but when all is said and done nothing's different on the page.

I figured it out. I'm still missing exactly why it doesn't work just on FormLoad, but performing the change in the FormView's DataBound event does the trick.
protected void FrmAdd_DataBound(object sender, EventArgs e)
{
// This is the same code as before, but done in the FormView's DataBound event.
((DropDownList)FrmAdd.Row.FindControl("DdlAssigned")).SelectedValue =
((Guid)Membership.GetUser().ProviderUserKey).ToString();
}
So, I guess the general rule of thumb is that if you are having problems making changes to controls when working with databinding, try to make them immediately after it has been bound.

I had a problem with dropdownlists and making the first value say something like, "Please select a value..." but without making it an actual selectable item, nor show up on the dropdownlist. I was binding the ddl in the page_load and I have to make sure that I set the text of the dropdownlist, AFTER it's been bound with data. You've accomplished the same thing by adding it to your databound section.

ASP.NET - Add Event Handler to LinkButton inside of Repeater in a RenderContent call

I've got a Sharepoint WebPart which loads a custom User Control. The user control contains a Repeater which in turn contains several LinkButtons.
In the RenderContent call in the Webpart I've got some code to add event handlers:
ArrayList nextPages = new ArrayList();
//populate nextPages ....
AfterPageRepeater.DataSource = nextPages;
AfterPageRepeater.DataBind();
foreach (Control oRepeaterControl in AfterPageRepeater.Controls)
{
if (oRepeaterControl is RepeaterItem)
{
if (oRepeaterControl.HasControls())
{
foreach (Control oControl in oRepeaterControl.Controls)
{
if (oControl is LinkButton)
{
((LinkButton)oControl).Click += new EventHandler(PageNavigateButton_Click);
}
}
}
}
}
The function PageNavigateButton_Click is never called however. I can see it being added as an event handler in the debugger however.
Any ideas? I'm stumped how to do this.

By the time RenderContent() is called, all the registered event handlers have been called by the framework. You need to add the event handlers in an earlier method, like OnLoad():
protected override void OnLoad(EventArge e)
{ base.OnLoad(e);
EnsureChildControls();
var linkButtons = from c in AfterPageRepeater.Controls
.OfType<RepeaterItem>()
where c.HasControls()
select c into ris
from lb in ris.OfType<LinkButton>()
select lb;
foreach(var linkButton in linkButtons)
{ linkButton.Click += PageNavigateButton_Click
}
}

Have you tried assigning the CommandName and CommandArgument properties to each button as you iterate through? The Repeater control supports the ItemCommand event, which is an event that will be raised when a control with the CommandName property is hit.
From there it is easy enough to process because the CommandName and CommandArgument values are passed into the event and are readily accessible.

You need to make sure that the link button is re-added to the control tree and/or that the event is rewired up to the control before the event fires.
Article # 4guysfromrolla

I've never done a SharePoint WebPart, so I don't know if this will apply. But if it were a plain-old apsx page, I'd say that by the time it's rendering, it's too late. Try adding the event handlers in the control's Init or PreInit events.
Edit: Wait, I think Dilli-O might be right. See the Adding Button Controls to a Repeater section at the end of http://www.ondotnet.com/pub/a/dotnet/2003/03/03/repeater.html. It's in VB.NET, but you can easily do the same thing in C#.

As others have pointed out, you're adding the event handler too late in the page life cycle. For SharePoint WebParts you'd typically want to override the class' OnInit/CreateChildControls methods to handle the activity.

YOu need your webpart to implement the INamingContainer marker interface, it is used by the framework to allow postbacks to return to the correct control...
Also the controls in your webpart all need to have an ID.