Validation ErrorTemplate not showing on data error

This is a bit of a WPF whodunnit! My validation temples it not appearing when expected

The Problem

Basically, I am using IDataErrorInfo on my view model to provide feedback on data entry errors. I can see this being called and working as expected, but the validation error template for the bound control is never displayed by WPF!

I have tried both using the built-in default textbox red border error template, a style error template, and a hard coded template. None of these are displayed in a data error situation.

Heres my xaml binding code:

<TextBox x:Name="txtCaseNumber"  
Text="{Binding Path=CaseNumber, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}"/>

Wierdly, the error gets into the Validation.Errors collection. The follow code results in my IDataErrorInfo error message getting displayed next to the bound control. This leads me to think the problem is on the xaml side, rather the viewmodel side.

<ContentPresenter 
Content="{Binding ElementName=txtCaseNumber, Path=(Validation.Errors).CurrentItem}" />

Additionally, System.Windows.Controls.Validation.GetHasError(txtCaseNumber) returns true.

I am hosting WPF in winforms if that makes a difference. I can't find any references in my code or styles to Validation or ErrorTemplate so I'm pretty sure this isn't a styling own goal.

Cracked it.

The ErrorTemplate attached property uses the adorner layer to show error templates on data validation failures reported via IDataErrorInfo. The Adorner layer is not always available for all controls in all situations. Your control or one of its parents needs to explicitly provide one.

The default Window control template contains an AdornerDecorator which enables the adorner layer for its children. This why my other window showed my error templates just fine, because this window used the default control template

The window which didn't show my error template had used a style which provided a ControlTemplate for my window. This control template did not provide an <AdornerDecorator>, hence no adorner layer to show my error template.

The fix was as simple as wrapping my window control template with <AdornerDecorator>.