WPF error template not showing

Question

I get no binding errors and this code works at another place. I haven't found out yet what I do now differently to the code where it works and it's not that much code.

In UserControl.Resource:

<Style TargetType="TextBox">
  <Setter Property="BorderBrush" Value="DarkBlue"/>
  <Setter Property="BorderThickness" Value="1"/>
  <Setter Property="Margin" Value="0,1,0,1"/>
  <Setter Property="Validation.ErrorTemplate">
    <Setter.Value>
      <ControlTemplate>
        <StackPanel Orientation="Horizontal">
          <AdornedElementPlaceholder/>
          <Grid Margin="2,0,0,0">
            <Ellipse Width="20" Height="20" Fill="Red"/>
            <TextBlock Foreground="White" Text="X" FontWeight="Bold"
                       HorizontalAlignment="Center" VerticalAlignment="Center"/>
          </Grid>
        </StackPanel>
      </ControlTemplate>
    </Setter.Value>
  </Setter>
  <Style.Triggers>
    <Trigger Property="Validation.HasError" Value="True">
      <Setter Property="ToolTip"
              Value="{Binding RelativeSource={RelativeSource Self},
                  Path=(Validation.Errors)[0].ErrorContent}"/>
    </Trigger>
  </Style.Triggers>
</Style>

Below in Xaml too:

<TextBlock Height="23" HorizontalAlignment="Left" Margin="22,90,0,0"
           Text="Keywords" VerticalAlignment="Top"/>
<TextBox Height="23" HorizontalAlignment="Left" Margin="22,108,0,0"
         VerticalAlignment="Top" Width="244">
  <Binding Path="Tags" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged">
    <Binding.ValidationRules>
      <DataErrorValidationRule ValidatesOnTargetUpdated="False"/>
    </Binding.ValidationRules>
  </Binding>
</TextBox>

The button SAVE in my ViewModel is only activated when the Model.Tags property is longer 10 chars input from the user. The button activation/disable works fine when I enter 10,11 and then back 8 chars. All the property changes are fired.

Model:

namespace TBM.Model
{
    public class Document : EntityBase , IDataErrorInfo
    {
        public int Id { get; set; }
        public string DocumentName { get; set; }
        public string Tags { get; set; }
        public byte[] DocumentData { get; set; }
        public int PeriodId { get; set; }

        string IDataErrorInfo.Error { get { return null; } }

        string IDataErrorInfo.this[string propertyName]
        {
            get { return this.GetValidationError(propertyName); }
        }

        public bool IsValid
        {
            get
            {
                foreach (string property in ValidatedProperties)
                    if (GetValidationError(property) != null)
                        return false;

                return true;
            }
        }

        static readonly string[] ValidatedProperties = { "Tags", };

        private string GetValidationError(string propertyName)
        {
            if (Array.IndexOf(ValidatedProperties, propertyName) < 0)
                return null;

            string error = null;

            switch (propertyName)
            {               
                case "Tags": error = this.IsTagsEmpty(Tags); break;

                default:
                    Debug.Fail("Unexpected property being validated on Document: " + propertyName);
                    break;
            }
            return error;
        }  

        private string IsTagsEmpty(string value)
        {
            if (value != null && value.Trim().Length >= 10)
                return null;
            else
               return "The keywords must have at least 10 chars!";            
        }
    }
}

ViewModel:

 public RelayCommand SaveDocumentCommand
 {
     get { return _saveDocumentCommand ?? (_saveDocumentCommand =
         new RelayCommand(() => SaveDocument(),() => CanSaveDocument())); }
     }

     private bool CanSaveDocument()
     {
         return _document.IsValid;
     }
//...

What does not work is the ErrorTemplate with the red Ellipse is not showing at all?

UPDATE: Exactly the below code works in a TEST project. But in my productive project It does not find the Resource??? Why the heck this?

<TextBlock Height="23" HorizontalAlignment="Left" Margin="22,89,0,0"
           Text="Keywords" VerticalAlignment="Top"/>
  <TextBox Style="{StaticResource bla}" Height="23" HorizontalAlignment="Left"
           Margin="22,109,0,0" VerticalAlignment="Top" Width="244">
    <Binding Path="Tags" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged">
      <Binding.ValidationRules>
        <DataErrorValidationRule ValidatesOnTargetUpdated="False"
                                 ValidationStep="UpdatedValue"/>
      </Binding.ValidationRules>
    </Binding>
  </TextBox>

<UserControl.Resources>
  <Style x:Name="bla" TargetType="TextBox">
    <Setter Property="BorderBrush" Value="DarkBlue"/>
    <Setter Property="BorderThickness" Value="1"/>
    <Setter Property="Margin" Value="0,1,0,1"/>
    <Setter Property="Validation.ErrorTemplate">
      <Setter.Value>
        <ControlTemplate>
          <StackPanel Orientation="Horizontal">                          
            <AdornedElementPlaceholder/>
            <Grid Margin="2,0,0,0">
              <Ellipse Width="20" Height="20" Fill="Red"/>
              <TextBlock Foreground="White" Text="X" FontWeight="Bold"
                         HorizontalAlignment="Center" VerticalAlignment="Center"/>
            </Grid>
          </StackPanel>
        </ControlTemplate>
      </Setter.Value>
    </Setter>
    <Style.Triggers>
      <Trigger Property="Validation.HasError" Value="True">
        <Setter Property="ToolTip"
                Value="{Binding RelativeSource={RelativeSource Self},
                    Path=(Validation.Errors)[0].ErrorContent}"/>
      </Trigger>
    </Style.Triggers>
  </Style>
</UserControl.Resources>

Answer 1

I had similar problem. Fight it for hours just to realize that something was wrong with adorner layer.

What I did is put my with controls inside . And that was it. For some reason this decorator layer sometimes is gone. This is certainly true for TabControl (but in my case it was some other reason).

So it should look like this

<AdornerDecorator>
  <Grid>
    <TextBox .../>
  </Grid>
</AdornerDecorator>

Hope this helps!