Order In Chaos Of Control Initialization Steps - wpf

Question

I would like to know when is actually what happening inside initalization process of controls when I start a WPF application?
When are DP initalized? When Binding? When does DataContext get set? Is DataContext avaialbe in constructor of a control? Is there any kind of order?
I realized I ran into a trap that once I set a value on getter/setter of a DP inside constructor of a control the DP value gets updated but immediately also the values gets rolled back to default value which was null.
So my guess is that contructors get initalized first and then dependency properties.
Can somebody help me out with this?
Edit: Just for Rachel. The dp receives the value 234 and immedialty rolls back to null. I think its because constructor gets called first and then subsequently the initalizing of dps happens which sets dp back to null because null is default value. Am i thinking wrong about this? What is the order of initalization steps of a control or dependency object.
class MySuperDuperCoolClass : ContentControl
{
public MySuperDuperCoolClass()
{
InitalizeComponents();
this.MySuperDuperProperty = "234";
}
public string MySuperDuperProperty
{
get { return (string)GetValue(MySuperDuperPropertyProperty);}
set { SetValue(MySuperDuperPropertyProperty, value);}
}
public static DependencyProperty MySuperDuperPropertyProperty =
DependencyProperty.Register("MySuperDuperProperty", typeof(string), typeof(MySuperDuperCoolClass),
new PropertyMetadata(null));
}

Answer 1

I find the DispatcherPriority Enum useful for recalling the exact event order:
Send
Normal - Constructors run here
DataBind
Render
Loaded
Background
ContextIdle
ApplicationIdle
SystemIdle
Inactive
Invalid
Input
As you can see, Constructors get run first, followed by data bindings.
DependencyProperties get initialized when the object gets created, just like any other property, so that would occur prior to the constructor being run so the property exists in the constructor.
Setting the DataContext property or other DependencyProperties works just like any other property you are setting. If you set them with a binding, they'll get evaluated after the constructor. If you set them in the XAML, they'll get set in the Constructor. If you set them in the Loaded event, they'll get set after everything has been constructed, bound, and rendered.
You also might find this SO answer useful:
Sequence of events when a Window is created and shown
As requested, here is the sequence of major events in WPF when a
window is created and shown:
Constructors and getters/setters are called as objects are created, including PropertyChangedCallback, ValidationCallback, etc on the
objects being updated and any objects that inherit from them
As each element gets added to a visual or logical tree its Intialized event is fired, which causes Styles and Triggers to be
found applied in addition to any element-specific initialization you
may define [note: Initialized event not fired for leaves in a logical
tree if there is no PresentationSource (eg Window) at its root]
The window and all non-collapsed Visuals on it are Measured, which causes an ApplyTemplate at each Control, which causes additional
object tree construction including more constructors and
getters/setters
The window and all non-collapsed Visuals on it are Arranged
The window and its descendants (both logical and visual) receive a Loaded event
Any data bindings that failed when they were first set are retried
The window and its descendants are given an opportunity to render their content visually
Steps 1-2 are done when the Window is created, whether or not it is
shown. The other steps generally don't happen until a Window is
shown, but they can happen earlier if triggered manually.
Edit based on code added to question
Your DependencyProperty.Register method looks funny to me. The signature of the method doesn't match any of the overloads for that method, and you're using what appears to be a custom UIProperty class to set the default value instead of the normal PropertyMetadata.
I can confirm that if your code runs as expected with a normal DependencyProperty.Register signature, so the likely cause of your problem is either somewhere within your custom code, or its with how you are using/setting the property.
The code I used for a quick sample test is this:
public partial class UserControl1 : ContentControl
{
public UserControl1()
{
InitializeComponent();
this.TestDependencyProperty = "234";
}
public string TestDependencyProperty
{
get { return (string)GetValue(TestDependencyPropertyProperty); }
set { SetValue(TestDependencyPropertyProperty, value); }
}
public static DependencyProperty TestDependencyPropertyProperty =
DependencyProperty.Register("TestDependencyProperty", typeof(string), typeof(UserControl1),
new PropertyMetadata(null));
}
and the XAML is
<ContentControl x:Class="WpfApplication1.UserControl1"
x:Name="TestPanel" ...>
<Label Content="{Binding ElementName=TestPanel, Path=TestDependencyProperty}"/>
</ContentControl>

Answer 2

In WPF you are setting default values for DP with PropertyMetaData not via constructor.
public partial class UserControl1 : ContentControl
{
public UserControl1()
{
InitializeComponent();
}
public string TestDependencyProperty
{
get { return (string)GetValue(TestDependencyPropertyProperty); }
set { SetValue(TestDependencyPropertyProperty, value); }
}
public static DependencyProperty TestDependencyPropertyProperty =
DependencyProperty.Register("TestDependencyProperty", typeof(string), typeof(UserControl1),
new PropertyMetadata("234"));
}