How to remove event handler that is assigned through style - wpf

Question

I've created a WPF window. In that I've created style for a textbox which have size information as well as some eventsetters; Then, I've created some textboxes assigning the above style.
Now for one case I need to remove events of that textbox. But I cant do that.
Even though I do the following the event is not detached. It still there.
txt9.PreviewLostKeyboardFocus -= txt9_PreviewLostKeyboardFocus;
This occurs only when the event is attached in the style itself.
If it were within the TextBox control the event detaches fine.
My Code:
XAML:
<Window.Resources>
<ResourceDictionary>
<Style x:Key="txtStyle11" TargetType="TextBox">
<Setter Property="Width" Value="150"/>
<Setter Property="Height" Value="35"/>
<EventSetter Event="PreviewLostKeyboardFocus" Handler="txt9_PreviewLostKeyboardFocus"/>
</Style>
</ResourceDictionary>
</Window.Resources>
<Grid MinWidth="50">
<TextBox Style="{StaticResource txtStyle11}" x:Name="txt9"/>
</Grid>
CS:
public MainWindow()
{
txt9.PreviewLostKeyboardFocus -= txt9_PreviewLostKeyboardFocus;
}

Answer 1

Hi we cannot Unsubscribe events whcih are subscribed in EventSetter, because
internally EventSetter class becomes immutable and so the properties of the
EventSetter object cannot be modified. Below is the sample which I tried to
illustrate the same. This sample throws exception when we try to change the
property of setterbase class
public partial class MainWindow : Window
{
KeyboardFocusChangedEventHandler mydelegate, emptydelegate;
EventSetter eventSetter;
public MainWindow()
{
InitializeComponent();
emptydelegate = delegate(object sender, KeyboardFocusChangedEventArgs e)
{
};
mydelegate = delegate(object sender, KeyboardFocusChangedEventArgs e)
{
MessageBox.Show("My Own Event is still Unsubscribed");
};
eventSetter = new EventSetter();
eventSetter.Event = TextBox.PreviewLostKeyboardFocusEvent;
eventSetter.Handler = mydelegate;
Style myStyle = new System.Windows.Style(txt9.GetType());
myStyle.Setters.Add(eventSetter);
txt9.Style = myStyle;
}
private void Button_Click(object sender, RoutedEventArgs e)
{
eventSetter.Handler= emptydelegate;
}
}