Usercontrol style trigger not working - wpf

Question

I am pretty sure this question has been asked from time to time since I found another question with about the same contents, here. But when I try to figure using all those pages, I'm just stuck...
This is what I'm trying to do. I created a usercontrol to select a file and show the path to the file in a textbox. Just like in HTML (input type=file). That all works great and as expected. But when I try to change the colour of the textbox using a trigger (FilePathIsValid), it just does not work. The dependency properties all work fine, as said above. But the style just is not assigned to the textbox.
Here is my XAML - can anybody tell me what I'm doing wrong? (Code Behind here, if needed)
<UserControl x:Class="Project.Controls.SelectFileBox"
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:Controls="clr-namespace:Project.Controls"
x:Name="ThisUserControl"
mc:Ignorable="d" d:DesignHeight="30" d:DesignWidth="300">
<UserControl.Resources>
<!-- This does not work... -->
<Style TargetType="{x:Type Controls:SelectFileBox}">
<Style.Triggers>
<Trigger Property="FilePathIsValid" Value="false">
<Setter Property="TextBoxBorderColor" Value="red"/>
</Trigger>
</Style.Triggers>
</Style>
<!-- Neither does this: FilePathIsValid can't be found -->
<Style TargetType="{x:Type TextBox}">
<Style.Triggers>
<Trigger Property="FilePathIsValid" Value="false">
<Setter Property="BorderBrush" Value="red"/>
</Trigger>
</Style.Triggers>
</Style>
</UserControl.Resources>
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="100"/>
</Grid.ColumnDefinitions>
<TextBox Grid.Column="0" Margin="3" IsEnabled="{Binding ElementName=ThisUserControl,Path=TextBoxIsEnabled}" Text="{Binding ElementName=ThisUserControl,Path=FilePath}" BorderThickness="{Binding ElementName=ThisUserControl, Path=TextBoxBorderThickness}"/>
<Button Grid.Column="1" Margin="0,3,3,3" Content="{Binding ElementName=ThisUserControl, Path=ButtonText}" Click="SelectFileClick"/>
</Grid>

Answer 1

Ok, I understood the problem after looking at the codebehind.
All you need to do is add a binding to the BorderBrush property on the TextBox like below to get the right BorderBrush applied.
BorderBrush="{Binding ElementName=ThisUserControl, Path=TextBoxBorderColor}"
Full XAML for TextBox element:
<TextBox Grid.Column="0" Margin="3"
IsEnabled="{Binding ElementName=ThisUserControl,Path=TextBoxIsEnabled}"
Text="{Binding ElementName=ThisUserControl,Path=FilePath}"
BorderThickness="{Binding ElementName=ThisUserControl, Path=TextBoxBorderThickness}"
BorderBrush="{Binding ElementName=ThisUserControl, Path=TextBoxBorderColor}"/>
I have tried and tested, below is the screenshot
Alternative Way:
You could have a style for TextBox and have data triggers to change the borderbrush:
<Style TargetType="TextBox">
<Style.Triggers>
<DataTrigger Binding="{Binding ElementName=ThisUserControl,
Path=FilePathIsValid}" Value="False">
<Setter Property="BorderBrush" Value="Red" />
</DataTrigger>
</Style.Triggers>
</Style>
With this approach, you don't need style for SelectFileBox and binding for BorderBrush property on TextBox.

Answer 2

If i understand correctly you want to set the textBoxbroder as red if the path is invalid then change your code as below
<Style TargetType="{x:Type TextBox}">
<Style.Triggers>
<Trigger Property="FilePathIsValid" Value="false">
<Setter Property="BorderBrush" Value="red"/>
</Trigger>
</Style.Triggers>
</Style>