WPF Control Style and Templates

Hi,

The basic structure of my app is:
BaseControls - generally inherit from existing MS Control and add functionality (e.g. MyTextBox)
AppControls - inherit from BaseControls and implement specific functionality (e.g. AccountNumber, AccountName)
Windows - which incorporate AppControls to create a business function.

BaseControls are WPF CustomControls with no xaml
AppControls are CustomControls with only framework xaml
Styles / Templates are in a resource dictionary at the Window level (in App.xaml) and reference the BaseControl

So for example i wish all my text boxes to be displayed with a blue border if the property IsMandatory has been set.

I create a BaseControl called MyTextBox inherited from TextBox and i add a DependencyProperty Is Mandatory

I create two controls inherited from MyTextBox called AccountNumber and AccountName and i add additional validation to each.

I place AccountNumber and AccountName on a form and set the IsMandatory Propery of each to true

In App,xaml i create a style which has a style trigger that sets the border to blue if the IsMandatory Property is set.

Now to the question - If I set the TargetType to AccountNumber then it works correctly - if I set the TargetType to MyTextBox (i.e. the type it is inherited from) then it does not work - the default TextBox is displayed.

As there are over 300 fields in this app i don't want a style for each, i just want a style for the base class

As a suplementary is this related to DefaultStyleKeyProperty.OverrideMetadata whos operation i have not completely understood - i've tried various settings in both MyTextBox and AccountNumber and either it stays the same or is not displayed at all.

Many thanks

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 <Style TargetType="oactl:AccountNumber">
            <Style.Triggers>
                <Trigger Property="IsMandatory" Value="true">
                    <Setter Property="BorderBrush" Value ="Blue"></Setter>
                    <Setter Property="ToolTip" Value="This information is required"/>
                </Trigger>
             </Style.Triggers>
        </Style>