WPF: Validation made easy with IDataErrorInfo

    <Style TargetType="{x:Type TextBox}">
                <Setter Property="VerticalAlignment" Value="Center" />
                <Setter Property="Margin" Value="0,2,40,2" />
                <Setter Property="Validation.ErrorTemplate">
                    <Setter.Value>
                        <ControlTemplate>
                            <DockPanel LastChildFill="true">
                                <Border Width="20"
                                        Height="20"
                                        Margin="5,0,0,0"
                                        Background="Red"
                                        CornerRadius="10"
                                        DockPanel.Dock="right"
                                        ToolTip="{Binding ElementName=customAdorner,
                                                          Path=AdornedElement.(Validation.Errors)[0].ErrorContent}">
                                    <TextBlock HorizontalAlignment="center"
                                               VerticalAlignment="center"
                                               FontWeight="Bold"
                                               Foreground="white"
                                               Text="!" />
                                </Border>
                                <AdornedElementPlaceholder Name="customAdorner" VerticalAlignment="Center">
                                    <Border BorderBrush="red" BorderThickness="1" />
                                </AdornedElementPlaceholder>
                            </DockPanel>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>