How do you bind a viewmodel which implements IDataErrorInfo to a UserControl and propagate the validation errors?

I've written a UserControl that exposes a few dependency properties to modify the layout of the control (i.e. think of it as a generic type-safe editor control - so my data types are dates (edited through a date picker), enumerations (edited through a combobox) and a numeric (edited through a textbox). I've also exposed the value of the 3 editor controls as a dependency property so that it can be databound.

The controls within the usercontrol all bind to the exposed dependency properties to get their values (with appropriate converters where necessary).

This control forms a tiny piece of a larger UI which binds to a viewmodel - the value to be edited, the datatype flag for the custom control and a list of possible valid values are all bound to an object in the viewmodel.

My problem is this: I've implemented IDataErrorInfo on the viewmodel and set the controls' bindings within the custom usercontrol to have ValidatesOnDataErrors=True, NotifyOnValidationError=True but the validation is not displaying.

This is the XAML for my usercontrol (there is no other code-behind logic beyond the dependency property declarations):

<UserControl.Resources>
    <!-- Converters -->
    <local:ValidationBooleanToImageConverter x:Key="ValidationBooleanToImageConverter"/>
    <local:ValidationErrorToStringConverter x:Key="ValidationErrorToStringConverter"/>
    <local:DataTypeToVisibilityConverter DataType="Date" x:Key="DateDataTypeToVisibilityConverter"/>
    <local:DataTypeToVisibilityConverter DataType="Numeric" x:Key="NumericDataTypeToVisibilityConverter"/>
    <local:DataTypeToVisibilityConverter DataType="Enumeration" x:Key="EnumerationDataTypeToVisibilityConverter"/&g开发者_开发技巧t;
    <local:DateToStringConverter x:Key="DateToStringConverter"/>
    <!-- Styles -->
    <Style TargetType="{x:Type TextBox}">
        <Style.Triggers>
            <Trigger Property="Validation.HasError" Value="True">
                <Setter Property="ToolTip"
                        Value="{Binding RelativeSource={RelativeSource Self}, Path=(Validation.Errors).CurrentItem, Converter={StaticResource ResourceKey=ValidationErrorToStringConverter}}"/>
            </Trigger>
        </Style.Triggers>
    </Style>
</UserControl.Resources>
<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition/>
        <ColumnDefinition Width="16"/>
    </Grid.ColumnDefinitions>
    <toolkit:DatePicker Height="25" Margin="0,0,5,0" Visibility="{Binding Path=DataType, ElementName=UserControl, Converter={StaticResource ResourceKey=DateDataTypeToVisibilityConverter}}" SelectedDate="{Binding Path=Value, ElementName=UserControl, Converter={StaticResource ResourceKey=DateToStringConverter}}"/>
    <ComboBox Height="25" Margin="0,0,5,0" Visibility="{Binding Path=DataType, ElementName=UserControl, Converter={StaticResource ResourceKey=EnumerationDataTypeToVisibilityConverter}}" SelectedItem="{Binding Path=Value, ElementName=UserControl, Mode=TwoWay}" ItemsSource="{Binding Path=Items, ElementName=UserControl}" />
    <TextBox x:Name="_txtValue" Height="25" Margin="0,0,5,0" Visibility="{Binding Path=DataType, ElementName=UserControl, Converter={StaticResource ResourceKey=NumericDataTypeToVisibilityConverter}}" Text="{Binding Path=Value, ElementName=UserControl, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True, NotifyOnValidationError=True}"/>
    <Image Grid.Column="2" Grid.Row="0" Source="{Binding ElementName=_txtValue, Path=(Validation.HasError), Converter={StaticResource ResourceKey=ValidationBooleanToImageConverter} }" ToolTip="{Binding ElementName=_txtValue, Path=ToolTip}" Width="16" Height="16"/>
</Grid>

...and the usercontrol reference from within the larger View is...

<control:ValueEditorControl DataType="{Binding Path=ContextualSelectedTagDataType}" Items="{Binding Path=ContextualSelectedTagItems}" Value="{Binding Path=ContextualSelectedTagDataObjectValue, Mode=TwoWay}" Height="25" VerticalAlignment="Top"/>

Can anyone point me in the right direction?

Does your VM implement INotifyPropertyChanged? Even if you implement IDataErrorInfo if WPF isn't notified of changes to the VM then it won't bind to those changes.

That being said I would change your ToolTip setter to this:

<Setter Property="ToolTip"
            Value="{Binding RelativeSource={RelativeSource Self}, 
                   Path=(Validation.Errors)[0].ErrorContent}"/>

If you want the entire Style I would recommend this:

<Style TargetType="{x:Type TextBox}">
    <Setter Property="Validation.ErrorTemplate">
        <Setter.Value>
            <ControlTemplate>
                <AdornedElementPlaceholder Name="controlWithError" />
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <Style.Triggers>
        <Trigger Property="Validation.HasError" Value="true">
            <Setter Property="Background" Value="LightPink"/>
            <Setter Property="ToolTip"
            Value="{Binding RelativeSource={RelativeSource Self}, 
                   Path=(Validation.Errors)[0].ErrorContent}"/>
        </Trigger>
    </Style.Triggers>
</Style>