WPF Set a Dependencyproperty value using a DataTrigger [duplicate] - wpf

Question

I implemented a user control with a dependency property that looks like this:
public partial class MyUC : UserControl, INotifyPropertyChanged
{
public static readonly DependencyProperty MyBackgroundProperty =
DependencyProperty.Register("MyBackground", typeof(Brush), typeof(MyUC),
new FrameworkPropertyMetadata(Brushes.White,
FrameworkPropertyMetadataOptions.AffectsRender));
public Brush MyBackground
{
get { return (Brush)GetValue(MyBackgroundProperty); }
set { SetValue(MyBackgroundProperty, value); }
}
//...
}
and try to set this property in XAML as follows:
<UserControl x:Class="Custom.MyUC"
x:Name="myUCName"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:Custom"
mc:Ignorable="d"
TabIndex="0" KeyboardNavigation.TabNavigation="Local"
HorizontalContentAlignment="Left" VerticalContentAlignment="Top"
MouseLeftButtonDown="OnMouseLeftButtonDown">
<UserControl.Style>
<Style TargetType="local:MyUC">
<Setter Property="MyBackground" Value="Black"/>
</Style>
</UserControl.Style>
<Border BorderThickness="0">
//...
</Border>
</UserControl>
It compiles but when I run the app I get the following exception:
Set property 'System.Windows.Setter.Property' threw an exception.'
Line number '..' and line position '..'."
How can I solve this?

Answer 1

The problem arises because you're trying to apply a style with TargetType="MyUC" to an element of type UserControl.
The solution is to apply the style from outside of the control. So for example when you use the control in another window:
<Window.Resources>
<Style TargetType="local:MyUC">
<Setter Property="MyBackground" Value="Red" />
</Style>
</Window.Resources>
<Grid>
<local:MyUC />
</Grid>
As a test I added this code to the user control:
public partial class MyUC
{
public MyUC()
{
InitializeComponent();
}
public static readonly DependencyProperty MyBackgroundProperty =
DependencyProperty.Register("MyBackground", typeof(Brush), typeof(MyUC),
new PropertyMetadata(Brushes.White, PropertyChangedCallback));
private static void PropertyChangedCallback(DependencyObject dependencyObject,
DependencyPropertyChangedEventArgs dependencyPropertyChangedEventArgs)
{
((MyUC)dependencyObject).MyBackgroundPropertyChanged(
(Brush)dependencyPropertyChangedEventArgs.NewValue);
}
private void MyBackgroundPropertyChanged(Brush newValue)
{
Background = newValue;
}
public Brush MyBackground
{
get { return (Brush)GetValue(MyBackgroundProperty); }
set { SetValue(MyBackgroundProperty, value); }
}
}
Which then results in the control having a red background.