We Keep Coding

XDG0008 error occuring while implementing DependencyProperty into TemplateBinding [duplicate]

This question already has answers here:
Template Binding with Attached Properties
(2 answers)
Closed 1 year ago.
To easily change the template-specific brushes of a button without directly changing the template, I decided to make a DependencyProperty that will bind to a template-specific brush. That way, I can change this brush just as easy as changing any other regular property. However, after implementing this DependencyProperty, I encountered an error: "Name "ExtensionClass" does not exist in namespace "clr-namespace:extensions"." What causes this error?
XAML:
<ResourceDictionary xmlns:ext="clr-namespace:Extensions"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:Themes="clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Aero2"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
<ControlTemplate x:Key="ButtonBaseControlTemplate1" TargetType="{x:Type ButtonBase}">
<ControlTemplate.Triggers>
<Trigger Property="IsMouseOver" Value="True">
<Setter Property="Background" TargetName="border" Value="{TemplateBinding Property=ext:ExtensionsClass.MouseOverBackground}"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</ResourceDictionary>
C#:
namespace Extensions {
public class ExtensionsClass {
public static readonly DependencyProperty MouseOverBackgroundProperty = DependencyProperty.Register("MouseOverBackground", typeof(Brush), typeof(Button));
public static void SetMouseOverBackground(UIElement element, Brush value) {
element.SetValue(MouseOverBackgroundProperty, value);
}
public static Brush GetMouseOverBackground(UIElement element) {
return (Brush)element.GetValue(MouseOverBackgroundProperty);
}
}
}

In addition to the problem with the Binding, which is covered in the answer to the duplicate question, you also have to be aware that you are declaring an attached property, which has to be registered with the RegisterAttached method.
Besides that, in both the Register and the RegisterAttached methods, the third argument has to be the type that declares the property, not the type of element where you intend to set the property, i.e. typeof(ExtensionsClass) here.
public static class ExtensionsClass
{
public static readonly DependencyProperty MouseOverBackgroundProperty =
DependencyProperty.RegisterAttached(
"MouseOverBackground",
typeof(Brush),
typeof(ExtensionsClass),
null);
public static void SetMouseOverBackground(UIElement element, Brush value)
{
element.SetValue(MouseOverBackgroundProperty, value);
}
public static Brush GetMouseOverBackground(UIElement element)
{
return (Brush)element.GetValue(MouseOverBackgroundProperty);
}
}
You bind to an attached property by means of a Binding Path with parentheses:
<Setter
Property="Background"
TargetName="border"
Value="{Binding Path=(ext:ExtensionsClass.MouseOverBackground),
RelativeSource={RelativeSource TemplatedParent}}"/>

How to change the roundness of a button in a generic way

I am currently working on making my WPF application a little bit more generic.
Up to this point, for each button I wanted to create, I used a different style to modify roundness (and it creates a lot of useless code).
Using the following code I've managed to create a variable I can change from the XAML file, but I cannot link it to the roundness itself.
Could anyone please tell me what I am doing wrong? I already have checked on so many forums but no one seems to have the answer other than "don't do it in a generical way".
I can precise that everything is compiling and the style is otherwise correctly applied to the button (there is no xaml linking problem).
The style I am using:
<Style x:Key="AwakeButton" TargetType="{x:Type customcontrols:AwakeButton}" BasedOn="{StaticResource {x:Type Button}}"
xmlns:extensions="Awake.Services.Properties:Extensions">
<Setter Property="customcontrols:AwakeButton.BorderRoundness" Value="4.0"/>
<Style.Resources>
<Style TargetType="Border">
<Setter Property="CornerRadius" Value="{Binding Path=BorderRoundness}" />
<!--<Setter Property="CornerRadius" Value="10" />-->
</Style>
</Style.Resources>
</Style>
The overload of the button I created to do so:
public class AwakeButton : Button
{
public AwakeButton()
{
}
public static DependencyProperty BorderRoundnessProperty =
DependencyProperty.RegisterAttached("BorderRoundness", typeof(double), typeof(AwakeButton));
public static void SetBorderRoundness(UIElement element, double value)
{
element.SetValue(BorderRoundnessProperty, value);
}
public static double GetBorderRoundness(UIElement element)
{
return (double)element.GetValue(BorderRoundnessProperty);
}
}
How I am using it in the page:
<customcontrols:AwakeButton Style="{StaticResource AwakeButton}" Margin="142,115,0,0" Width="136" Height="167" BorderRoundness="5">

You have to bind the BorderRoundness to the parent AwakeButton, otherwise it is resolved using the current DataContext, which does not contain this property. Furthermore, if you derive from Button, you do not have to make the dependency property attached, you could just register a normal one using the Register(...) method. Also make DPs static and readonly.
<Setter Property="CornerRadius" Value="{Binding BorderRoundness, RelativeSource={RelativeSource AncestorType={x:Type local:AwakeButton}}}" />
If you do not change anything special about the button, you could also create attached properties instead of a dedicated sub type just for exposing a BorderRoundness property.
public static class ButtonProperties
{
public static readonly DependencyProperty BorderRoundnessProperty =
DependencyProperty.RegisterAttached("BorderRoundness", typeof(double), typeof(ButtonProperties));
public static void SetBorderRoundness(UIElement element, double value)
{
element.SetValue(BorderRoundnessProperty, value);
}
public static double GetBorderRoundness(UIElement element)
{
return (double)element.GetValue(BorderRoundnessProperty);
}
}
You can refer to the BorderRoundness using attached property binding syntax (parentheses).
<Style x:Key="AwakeButton" TargetType="{x:Type Button}" BasedOn="{StaticResource {x:Type Button}}">
<Setter Property="local:ButtonProperties.BorderRoundness" Value="4.0"/>
<Style.Resources>
<Style TargetType="Border">
<Setter Property="CornerRadius" Value="{Binding (local:ButtonProperties.BorderRoundness), RelativeSource={RelativeSource AncestorType={x:Type Button}}}" />
</Style>
</Style.Resources>
</Style>
You use regular button now with the newly created attached border roundness property.
<Button Grid.Row="0" Style="{StaticResource AwakeButton}" Margin="142,115,0,0" Width="136" Height="167" local:ButtonProperties.BorderRoundness="5"/>

The roundness is applied as CornerRadius to the Border of the Button. The Border is defined in the ControlTemplate of the Button. The ControlTemplate defines the appearance of a control.
In other words, you need to delegate the property values to the related elements in the ControlTemplate.
To delegate the values to the ControlTemplate, you have to override this template and bind the templated parents properties to the template elements:
In your AwakeButton define the BorderRoundness property as simple DependencyProperty (not attached) and the override the default style definition, so that the AwakeButton will use its own default Style. This way the Button is reusable withgout having to redefine the Style each time you weant to use it, which is especially important wjen you publish your project as library:
AwakeButton.cs
public class AwakeButton : Button
{
public static readonly DependencyProperty BorderRoundnessProperty = DependencyProperty.Register(
"BorderRoundness",
typeof(Thickness),
typeof(AwakeButton),
new PropertyMetadata(default(Thickness)));
public Thickness DestinationPath
{
get => (Thickness) GetValue(AwakeButton.BorderRoundnessProperty);
set => SetValue(AwakeButton.BorderRoundnessProperty, value);
}
static AwakeButton()
{
DefaultStyleKeyProperty.OverrideMetadata(typeof(AwakeButton), new FrameworkPropertyMetadata(typeof(AwakeButton)));
}
}
Generic.xaml.cs
This file is located in the Themes folder and contains all default styles. WPF will automatically check this file for a default style and apply it if no other Style override was found.
<Style TargetType="AwakeButton">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="AwakeButton">
<Border BorderBrush={TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}"
BorderBrush="{TemplateBinding BorderBrush}"
CornerRadius="{TemplateBinding BorderRoundness}">
<ContentPresenter />
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Stayle>
Example
<Grid>
<AwakeButton BorderRoundness="8" />
</Grid>
But if you want to make it really general, using an attached property, you have to make a attached behavior. The following code works with every DependencyObject that contains a Border as child in its visual tree:
class Element : DependencyObject
{
#region CornerRoundness attached property
public static readonly DependencyProperty CornerRoundnessProperty = DependencyProperty.RegisterAttached(
"CornerRoundness",
typeof(CornerRadius),
typeof(Element),
new PropertyMetadata(default(CornerRadius), Element.OnCornerRoundnessChanged));
public static void SetCornerRoundness(DependencyObject attachingElement, CornerRadius value) =>
attachingElement.SetValue(Element.CornerRoundnessProperty, value);
public static CornerRadius GetCornerRoundness(DependencyObject attachingElement) =>
(CornerRadius) attachingElement.GetValue(Element.CornerRoundnessProperty);
#endregion CornerRoundness attached property
private static void OnCornerRoundnessChanged(DependencyObject attachingElement, DependencyPropertyChangedEventArgs e)
{
if (Element.TryFindVisualChildElement(attachingElement, out Border elementBorder))
{
elementBorder.CornerRadius = (CornerRadius) e.NewValue;
}
}
public static bool TryFindVisualChildElement<TChild>(DependencyObject parent, out TChild resultElement)
where TChild : DependencyObject
{
resultElement = null;
if (parent is Popup popup)
{
parent = popup.Child;
if (parent == null)
{
return false;
}
}
for (var childIndex = 0; childIndex < VisualTreeHelper.GetChildrenCount(parent); childIndex++)
{
DependencyObject childElement = VisualTreeHelper.GetChild(parent, childIndex);
if (childElement is TChild child)
{
resultElement = child;
return true;
}
if (Element.TryFindVisualChildElement(childElement, out resultElement))
{
return true;
}
}
return false;
}
}
Example
<StackPanel>
<Button Element.CornerRoundness="8" />
<ToggleButton Element.CornerRoundness="8" />
</StackPanel>

WPF: Parent which would display a specific child depending on input value

As far as I understand, usual Visibility={Binding SomeValue, Converter={…}} still keeps node in visual and logical tree even if it’s invisible. But what if I want to remove it completely and at the same time keep syntax light?
Right now, I’ve made a class called Switch allowing me to do stuff like that:
<Switch Value="{Binding Status}">
<TextBlock Switch.When="{x:Static Status.NotFound}" Text="Not found" />
<Button Switch.When="{x:Static Status.ConnectionError}" Text="Connection error. Try again?" />
<Grid Switch.When="{x:Static Status.Loaded}">…</Grid>
</Switch>
Here is a source code of that Switch thing.
I like how it looks and works, but sometimes some errors occur. For instance, while removing visual child from its previous location, rarely, but System.InvalidOperationException: Cannot modify the logical children for this node at this time because a tree walk is in progress. might happen. Cases like this are solvable, but the whole thing with them makes me think I’m doing something very wrong. What could it be? Maybe the whole idea is just not compatible with WPF at all? Or maybe I’m just missing something (like that thing that I have to override IEnumerator LogicalChildren { get; } to make it work properly)?

I think the answer is probably that you're trying to reparent the child controls the right way. Bad idea! They're just arbitrary content; don't treat them as controls. The actual reparenting in my code is done by hidden ContentPresenter magic in the template. All our control class code does is just sling them around like potatoes.
Here's a working version of the control that shouldn't give you any backtalk. But note that I had trouble comparing boxed enum values to each other. I'm interested in how you solved that problem.
Switch.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Markup;
namespace SwitchTestProject
{
[ContentProperty("Items")]
public class Switch : Control
{
public Switch()
{
Items = new List<DependencyObject>();
}
static Switch()
{
DefaultStyleKeyProperty.OverrideMetadata(typeof(Switch), new FrameworkPropertyMetadata(typeof(Switch)));
}
public override void OnApplyTemplate()
{
base.OnApplyTemplate();
OnValueChanged(null);
}
#region Switch.When Attached Property
public static Object GetWhen(DependencyObject obj)
{
return (Object)obj.GetValue(WhenProperty);
}
public static void SetWhen(DependencyObject obj, Object value)
{
obj.SetValue(WhenProperty, value);
}
public static readonly DependencyProperty WhenProperty =
DependencyProperty.RegisterAttached("When", typeof(Object), typeof(Switch),
new PropertyMetadata(null));
#endregion Switch.When Attached Property
#region Content Property
public Object Content
{
get { return (Object)GetValue(ContentProperty); }
protected set { SetValue(ContentPropertyKey, value); }
}
internal static readonly DependencyPropertyKey ContentPropertyKey =
DependencyProperty.RegisterReadOnly(nameof(Content), typeof(Object), typeof(Switch),
new PropertyMetadata(null));
public static readonly DependencyProperty ContentProperty = ContentPropertyKey.DependencyProperty;
#endregion Content Property
#region Value Property
public Object Value
{
get { return (Object)GetValue(ValueProperty); }
set { SetValue(ValueProperty, value); }
}
public static readonly DependencyProperty ValueProperty =
DependencyProperty.Register(nameof(Value), typeof(Object), typeof(Switch),
new FrameworkPropertyMetadata(null, Value_PropertyChanged));
protected static void Value_PropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
(d as Switch).OnValueChanged(e.OldValue);
}
private void OnValueChanged(object oldValue)
{
if (Value is IComparable)
{
// Boxed value types have to be a special case.
// Unless I jumped to an unwarranted conclusion about == not working.
var icompval = Value as IComparable;
foreach (var item in Items)
{
var icompwhen = GetWhen(item) as IComparable;
if (icompwhen != null && icompval.CompareTo(icompwhen) == 0)
{
Content = item;
return;
}
}
}
else
{
Content = Items.FirstOrDefault(item => GetWhen(item) == Value);
}
}
#endregion Value Property
#region Items Property
public List<DependencyObject> Items
{
get { return (List<DependencyObject>)GetValue(ItemsProperty); }
protected set { SetValue(ItemsPropertyKey, value); }
}
internal static readonly DependencyPropertyKey ItemsPropertyKey =
DependencyProperty.RegisterReadOnly(nameof(Items), typeof(List<DependencyObject>), typeof(Switch),
new PropertyMetadata(null));
public static readonly DependencyProperty ItemsProperty = ItemsPropertyKey.DependencyProperty;
#endregion Items Property
}
}
App.xaml or Themes\Generic.xaml
You could do a lot more with styling the parent here.
<Style TargetType="local:Switch">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="local:Switch">
<ContentPresenter
/>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Usage:
<local:Switch
Value="{Binding Status}"
>
<TextBlock
local:Switch.When="{x:Static local:Status.NotFound}"
>This is a test</TextBlock>
<TextBlock
local:Switch.When="{x:Static local:Status.ConnectionError}"
>There was an error in the connection</TextBlock>
</local:Switch>
Pure XAML alternative
The bug in your Switch control is probably fixable, but this will work reliably without any nonsense (other than all the verbosity).
<ContentControl>
<ContentControl.Style>
<Style TargetType="ContentControl">
<Style.Triggers>
<DataTrigger Binding="{Binding Status}" Value="NotFound">
<Setter Property="ContentTemplate">
<Setter.Value>
<DataTemplate>
<TextBlock Text="Not found" />
</DataTemplate>
</Setter.Value>
</Setter>
</DataTrigger>
<DataTrigger Binding="{Binding Status}" Value="ConnectionError">
<Setter Property="ContentTemplate">
<Setter.Value>
<DataTemplate>
<Button Content="Connection error. Try again?" />
</DataTemplate>
</Setter.Value>
</Setter>
</DataTrigger>
<DataTrigger Binding="{Binding Status}" Value="Loaded">
<Setter Property="ContentTemplate">
<Setter.Value>
<DataTemplate>
<Ellipse
Height="32"
Width="32"
Fill="DeepSkyBlue"
/>
</DataTemplate>
</Setter.Value>
</Setter>
</DataTrigger>
</Style.Triggers>
</Style>
</ContentControl.Style>
</ContentControl>
We set the content using DataTemplates rather than setting the Content property directly because if we use the latter method, only one instance of each child control will ever exist, so we wouldn't be able to factor the style out as a resource and reuse it.
And because templating is the canonical way to create new controls in XAML.

Update Normal Property in Dependency property/AttachedProperty,

I am trying to bind a normal property of AvalonDock,
xmlns:xcad="http://schemas.xceed.com/wpf/xaml/avalondock"
<xcad:LayoutAnchorable Title="Folder" CanHide="{Binding IsHideExplorerView}">
<Views:ExplorerView DataContext="{Binding ExplorerViewModel}"/>
</xcad:LayoutAnchorable>
Here CanHide is a Normal property, if trying to bind will throw the exception like
A 'Binding' can only be set on a DependencyProperty of a DependencyObject.
My question is, Is it possible any way to make a normal property to override DependencyProperty to make it Bindable.
Edit
Added a class which inherit LayoutAnchorable but PropertyChangedCallback of DependencyProperty Never calls.
public class ExtendedAnchorableItem : LayoutAnchorable
{
public static readonly DependencyProperty IsCanHideProperty =
DependencyProperty.RegisterAttached("IsCanHide", typeof(bool), typeof(ExtendedAnchorableItem),
new FrameworkPropertyMetadata((bool)false,
new PropertyChangedCallback(OnCanHideChanged)));
public bool IsCanHide
{
get { return (bool)GetValue(IsCanHideProperty); }
set { SetValue(IsCanHideProperty, value);
this.IsVisible = value; // No effect.
}
}
private static void OnCanHideChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
((ExtendedAnchorableItem)d).Hide();
}
}
XAML
<xcad:LayoutAnchorablePane>
<Utility:ExtendedAnchorableItem IsCanHide="{Binding IsHideExplorer}">
<Views:ExplorerView DataContext="{Binding ExplorerViewModel}"/>
</Utility:ExtendedAnchorableItem>
</xcad:LayoutAnchorablePane>
Similarly i have tried creating an AttachedProperty which can hook it to LayoutAnchorable but PropertyChangedCallback Never get called click here for a new question i have posted.
Any Help guys ?

I did and example previously in my case i need to create new button with 2 images one when the button is available and the other one when it's disabled, to do that first i created new user control named "MyButton" my xaml was like this
<Button ToolTip="{Binding ButtonLabel,RelativeSource={RelativeSource AncestorType=UserControl,Mode=FindAncestor},UpdateSourceTrigger=PropertyChanged}"
Command="{Binding ButtonCommand,RelativeSource={RelativeSource AncestorType=UserControl,Mode=FindAncestor},UpdateSourceTrigger=PropertyChanged}"
Cursor="Hand" VerticalAlignment="Center" >
<Button.Template>
<ControlTemplate>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="45"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Image Name="ButtonImage" IsEnabled="{Binding Path=IsEnabled,RelativeSource={RelativeSource AncestorType=Button,Mode=FindAncestor}}" >
<Image.Style>
<Style TargetType="{x:Type Image}">
<Style.Triggers>
<Trigger Property="IsEnabled" Value="True">
<Setter Property="Source" Value="{Binding ActiveImage,RelativeSource={RelativeSource AncestorType=UserControl,Mode=FindAncestor},UpdateSourceTrigger=PropertyChanged}"/>
</Trigger>
<Trigger Property="IsEnabled" Value="False">
<Setter Property="Source" Value="{Binding DeactiveImage,RelativeSource={RelativeSource AncestorType=UserControl,Mode=FindAncestor},UpdateSourceTrigger=PropertyChanged}"/>
</Trigger>
</Style.Triggers>
</Style>
</Image.Style>
</Image>
<Label Name="LabelContent" Content="{Binding ButtonLabel,RelativeSource={RelativeSource AncestorType=UserControl,Mode=FindAncestor},UpdateSourceTrigger=PropertyChanged}"
Grid.Column="1" IsEnabled="{Binding Path=IsEnabled,RelativeSource={RelativeSource AncestorType=Button,Mode=FindAncestor}}" VerticalContentAlignment="Center" />
</Grid>
</ControlTemplate>
</Button.Template>
</Button>
then i added dependency Properties for ActiveImage and DeactiveImage using this code
public static DependencyProperty activeImage =
DependencyProperty.Register("ActiveImage", typeof(type of this property like "string"), typeof(type of the custom control that you need like "MyButton"), new FrameworkPropertyMetadata("", FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));
public string ActiveImage
{
get { return (string)GetValue(activeImage); }
set { SetValue(activeImage, value); }
}
then i used this new control in my project
<custom:MyButton ButtonCommand="{Binding DecreaseImagesCount}" ButtonLabel="ZoomIn" ActiveImage="/Images/ActiveImages/ZoomIn.png" DeactiveImage="/Images/GrayImages/ZoomIn.png"
Grid.Column="2" Margin="3,4" />
notice that i can do binding the path for Button Image now

If it is enough for you to just set that property from your view model then you could use an attached behavior.
Just create a new class and add an attached property like this (I did not really test this, since I actually do not have AvalonDock at hand, but you should get the idea):
public class YourBehavior
{
public static readonly DependencyProperty YourCanHideProperty = DependencyProperty.RegisterAttached(
"YourCanHide",
typeof(bool),
typeof(LayoutAnchorable),
new PropertyMetadata(YourCanHidePropertyChanged));
private static void YourCanHidePropertyChanged(
DependencyObject dependencyObject,
DependencyPropertyChangedEventArgs e)
{
LayoutAnchorable control = dependencyObject as LayoutAnchorable;
if (control != null)
{
control.CanHide = e.NewValue as bool;
}
}
public static bool GetYourCanHideProperty(LayoutAnchorablewindow)
{
return window.GetValue(YourProperty) as bool?;
}
public static void SetYourCanHideProperty(LayoutAnchorable control, bool value)
{
window.SetValue(YourProperty, value);
}
}
Now you should be able to use that behavior like this:
<xcad:LayoutAnchorable Title="Folder" namespacealias:YourBehavior.YourCanHideProperty="{Binding IsHideExplorerView}"/>
If you want to have it working in both directions just check out the attached Blend behaviors.

Yes, you can do it.. you need to implement INotifypropertyChanged interface and raise a ProprtyChanged Event inside the property setter. After changing the property to a DependencyProperty, you will get the notification mechanism, so the property change is propagated to the target (in this case xcad) .
you can find lot of examples implementing the INotifyPropertyChanged..

XAML How to own StaticResource in derived UserControl class

I have the following problem. I have a class which derives from UserControl, here is the code:
public partial class MyUC : UserControl
{
[...]
public bool IsFlying { get { return true; } }
[...]
}
I want to use a style, which is created for the class MyUC, below is the style code. It is located in App.Xaml :
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:dc="clr-namespace:MyNamespace"
<Application.Resources>
<Style x:Key="mystyle" TargetType="dc:MyUC ">
<Style.Triggers>
<Trigger Property="IsFlying" Value="true">
<Setter Property = "Background" Value="Blue"/>
</Trigger>
</Style.Triggers>
</Style>
</Application.Resources>
As you can see I want to use a property which I declared in MyUC.
The problem is that when I am trying to add a style to my control, an error occurres.
<UserControl x:Class="MyNamespace.MyUC"
[...]
Style="{StaticResource mystyle}">
<UserControl.Resources>
</UserControl.Resources>
</UserControl>
The error is: 'MyUC' TargetType does not match type of element 'UserControl'.
As far as I understand, the compiler do not recognize class MyUC to be derived from UserControl. How to fix it?
Thanks in advance!

Error might be at design time only, it should work fine at runtime. Run your app and see if it works for you.
Moreover your trigger won't work for normal CLR property, you need to make it a Dependency Property -
public bool IsFlying
{
get { return (bool)GetValue(IsFlyingProperty); }
set { SetValue(IsFlyingProperty, value); }
}
public static readonly DependencyProperty IsFlyingProperty =
DependencyProperty.Register("IsFlying", typeof(bool),
typeof(SampleUserControl), new UIPropertyMetadata(true));
Also, you can remove the x:Key="mystyle" from your style declaration. It will automatically gets applied to your UserControl.
That way you won't have to explicitly set style on your UserControl. This line won't be required then - Style="{StaticResource mystyle}"