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Event that fires when a control gets added onto another control

Is there an event that gets fired when a user control gets added onto a Form?
I need this to get the size of the Parent control of the user control.
But when the user control gets initialized etc, the Parent = null.
Because the user control hasn't been added yet onto the Form. So i can't get the Parent control (which is the Form) at this point.
So i'll have to do the re-sizing of the user control afterwards.

There is a ParentChanged event that all Controls inherit. In the event handler method you can inspect the Parent property, which will be set to the new parent at that point.

You can try using the Control.ParentChanged event. This will fire whenever the parent of the control is changed, so you can check if the parent is the form and then continue with how you want to react.
Alternatively, you can use the Control.ControlAdded event.

You have the ControlAdded event on the form, wich is initialized in your InitializeComponent(). Most of the time this event is duely placed after the adding of components. If you place it before the adding of UserControls to the form you will fire events each time you add a component.
I would recommend adding the usercontrol you need such specific control over in your codebehind and not move the eventhandler.
If you do that you can add a parent to your usercontrol which might give you the information you need.
var textBox = new TextBox {Parent = this};

Loading ASP.NET user control at run time

I have an ASP.NET user control with a button, and I want to add it to the page when the user clicks a button from another user control. I have created an event handler to the first user control to handle it from the page and add the second user control to a page. Everything is working normally, but the button on the second user control doesn't respond to the event.
I place the second control on RadAjaxPanel
Note: When I add the second user control at design time its working fine.

All dynamically created controls should be added by the end of Page_Init (though sometimes you can get away with them added by the end of Page_Load).
If you're only adding them based on a button click event then you've done this in the event handers which fire AFTER Page_Init and Page_Load in the lifecycle - This is why your events don't fire and why it works fine when you add at design time.
This is because when a button is clicked on the second user control - the whole page lifecycle starts again. The page goes through Page_Load and Page_Init first and your control doesn't get loaded here. So, when the page lifecycle handles the "handle postback events" part, the control no longer actually exists, so the event doesn't fire.
Conversely, when you add at design time, the control exists in Page_Init and Page_Load so is able to handle the postback events from the user control because it already exists in the control tree - if this makes sense.
You need to think how you can restructure so they're added by the time Page_Load has finished at the very latest or it won't work. Without code samples or more detail it's hard to suggest exactly how you might do this. One possibility would be to set it visible instead of loading it outright - but if the control does some 'heavy lifting' on load like database hits or API calls then this might not be suitable for you.
I did something similar. What I did was to load some controls dynamically based on a selection from a DropDownList. If you have a method which loads the control for you, let's call it LoadControls(), then you can do something like this:
DropDownList_Click {
ViewState("LoadControls") = true;
LoadControls()
}
By setting the ViewState variable, you can then indicate Page_Load to load the controls on future postbacks:
Page_Load {
if (ViewState("LoadControls") == "true")
{
LoadControls();
}
}
This has the effect of then loading the control on-the-fly when the event first happens, and then at future times in the lifecycle.

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.

Dynamically add multiple instances of the same user control type when button is clicked

Is it possible to add instances of the same user control when an "add" button is clicked and maintain ViewState?
The user interface here is similar to the Gmail file-attachment process, where the user can click "attach another file" and another file upload box appears.
My page is surrounded by an UpdatePanel. I am able to get 1 control to load, but the button's click event fires after the Placeholder_Init method. I tried storing an integer in the ViewState that kept track of the number of user controls that should be rendered, but the Init method is also fired before the ViewState is restored.
Thanks!

Adding multiple controls dynamically is easy in ASP.NET. Let's say you have a panel named Panel declared in your ASPX file and you have a custom control called MyControl.
In your Page_Load function (or indeed pretty much anywhere), add something like the following:
for (int i = 0; i < NumberOfAttachments; i++) {
Panel.Controls.Add(new MyControl());
}
This works for UpdatePanels, too, but you'll need to call the .Update() function to get it to update on the client side if you don't have it to update on child postback.

Updating a dynamic control

I've got a custom web part with Accordion panes from the AJAX Control Toolkit as children that are used to render a site map hierarchy. Each pane includes a div with text input and 3 LinkButtons used to edit the sitemap data: "Add Child", "Update", and "Delete".
Currently, all these controls are created in the overridden CreateChildControls method.
When the "Add Child" LinkButton is clicked, the event handler is fired, and a new node is added to the sitemap. When the postback completes, the control should re-render with the new, empty node in the hierarchy, but it doesn't. After a new GET request, the new node appears. After reading for a while, I thought my problem was that I was creating my child controls too early in the process because CreateChildControls is called before Control events are fired, so I moved that bit to the OnPreRender method. But now the Control events don't fire because I'm hooking them up too late in the Page lifecycle (see here: Custom Control Events Not Firing).
My question is this: how do I ensure that the custom control renders the results of its child control event handlers?
As an aside, does it matter that I'm doing this in a web part rather than a custom server control (e.g., is the lifecycle different)?

You could call EnsureChildControls in OnInit of the page. This will ensure that child controls will be recreated before any events are handled.