【WPF】ExceptionValidationRule不能出现在Validation.Errors Collection

ublic string FirstName
        {
            get { return _firstName;}
            set
            {
                if(String.IsNullOrEmpty(value))
                    throw new ApplicationException("FirstName cannot be null or empty!");
                _firstName = value; 

                OnPropertyChanged("FirstName");
            }
        }

<Style x:Key="TextBoxStyle" TargetType="TextBox">

            <Setter Property="Validation.ErrorTemplate">
                <Setter.Value>
                    <ControlTemplate>
                        <DockPanel LastChildFill="True">
                            <TextBlock DockPanel.Dock="Right"
                        Foreground="Orange"
                        FontSize="12pt"
                        Text="{Binding ElementName=MyAdorner, Path=AdornedElement.(Validation.Errors)[0].ErrorContent}">
                            </TextBlock>
                            <Border BorderBrush="Green" BorderThickness="1">
                                <AdornedElementPlaceholder Name="MyAdorner" />
                            </Border>
                        </DockPanel>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>

        </Style>

<TextBox Name="txtFirstName" Style="{StaticResource TextBoxStyle}" Grid.Column="1"  Grid.Row="0" Height="20" Width="100" Margin="10">
                <TextBox.Text>
                    <Binding Path="FirstName">
                        <Binding.ValidationRules>
                            <ExceptionValidationRule />
                        </Binding.ValidationRules>
                    </Binding>
                </TextBox.Text>

            </TextBox>