Avalonia/src/Avalonia.Themes.Fluent/Controls/DataValidationErrors.xaml

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:sys="using:System"
        x:CompileBindings="True"
        x:DataType="DataValidationErrors">
  <Design.PreviewWith>
    <Border Padding="20">
      <StackPanel Spacing="20">
        <TextBox Text="Sample">
          <DataValidationErrors.Error>
            <sys:Exception />
          </DataValidationErrors.Error>
        </TextBox>
        <TextBox Text="Sample">
          <TextBox.Styles>
            <Style Selector="DataValidationErrors">
              <Setter Property="Template" Value="{DynamicResource TooltipDataValidationContentTemplate}" />
              <Setter Property="ErrorTemplate" Value="{DynamicResource TooltipDataValidationErrorTemplate}" />
            </Style>
          </TextBox.Styles>
          <DataValidationErrors.Error>
            <sys:Exception />
          </DataValidationErrors.Error>
        </TextBox>
      </StackPanel>
    </Border>
  </Design.PreviewWith>

  <Style Selector="DataValidationErrors">
    <Style.Resources>
      <DataTemplate x:Key="InlineDataValidationErrorTemplate">
        <ItemsControl Items="{Binding}" x:DataType="DataValidationErrors" TextBlock.Foreground="{DynamicResource SystemControlErrorTextForegroundBrush}">
          <ItemsControl.Styles>
            <Style Selector="TextBlock">
              <Setter Property="TextWrapping" Value="Wrap" />
            </Style>
          </ItemsControl.Styles>
        </ItemsControl>
      </DataTemplate>
      <ControlTemplate x:Key="InlineDataValidationContentTemplate" TargetType="DataValidationErrors">
        <DockPanel LastChildFill="True">
          <ContentControl x:Name="InlineDataValidationContentControl"
                          Margin="0,4,0,0"
                          Content="{Binding (DataValidationErrors.Errors)}"
                          ContentTemplate="{TemplateBinding ErrorTemplate}"
                          DataContext="{TemplateBinding Owner}"
                          DockPanel.Dock="Bottom"
                          IsVisible="{Binding (DataValidationErrors.HasErrors)}" />
          <ContentPresenter Name="PART_ContentPresenter"
                            Padding="{TemplateBinding Padding}"
                            Background="{TemplateBinding Background}"
                            BorderThickness="{TemplateBinding BorderThickness}"
                            CornerRadius="{TemplateBinding CornerRadius}"
                            Content="{TemplateBinding Content}"
                            ContentTemplate="{TemplateBinding ContentTemplate}" />
        </DockPanel>
      </ControlTemplate>

      <DataTemplate x:DataType="DataValidationErrors" x:Key="TooltipDataValidationErrorTemplate">
        <Panel Name="PART_InlineErrorTemplatePanel" Background="Transparent">
          <Panel.Styles>
            <Style Selector="Panel#PART_InlineErrorTemplatePanel">
              <Setter Property="Margin" Value="8,0" />
            </Style>
            <Style Selector="Panel#PART_InlineErrorTemplatePanel ToolTip">
              <Setter Property="BorderBrush" Value="{DynamicResource SystemControlErrorTextForegroundBrush}" />
            </Style>
            <Style Selector="Panel#PART_InlineErrorTemplatePanel ToolTip TextBlock">
              <Setter Property="TextWrapping" Value="Wrap" />
            </Style>
          </Panel.Styles>
          <ToolTip.Tip>
            <ItemsControl Items="{Binding}" />
          </ToolTip.Tip>
          <Path Width="14"
                Height="14"
                Data="M14,7 A7,7 0 0,0 0,7 M0,7 A7,7 0 1,0 14,7 M7,3l0,5 M7,9l0,2"
                Stroke="{DynamicResource SystemControlErrorTextForegroundBrush}"
                StrokeThickness="2" />
        </Panel>
      </DataTemplate>
      <ControlTemplate x:Key="TooltipDataValidationContentTemplate" TargetType="DataValidationErrors">
        <DockPanel LastChildFill="True">
          <ContentControl Content="{Binding (DataValidationErrors.Errors)}"
                          ContentTemplate="{TemplateBinding ErrorTemplate}"
                          DataContext="{TemplateBinding Owner}"
                          DockPanel.Dock="Right"
                          IsVisible="{Binding (DataValidationErrors.HasErrors)}" />
          <ContentPresenter Name="PART_ContentPresenter"
                            Padding="{TemplateBinding Padding}"
                            Background="{TemplateBinding Background}"
                            BorderBrush="{TemplateBinding BorderBrush}"
                            BorderThickness="{TemplateBinding BorderThickness}"
                            CornerRadius="{TemplateBinding CornerRadius}"
                            Content="{TemplateBinding Content}"
                            ContentTemplate="{TemplateBinding ContentTemplate}" />
        </DockPanel>
      </ControlTemplate>
    </Style.Resources>

    <Setter Property="Template" Value="{StaticResource InlineDataValidationContentTemplate}" />
    <Setter Property="ErrorTemplate" Value="{StaticResource InlineDataValidationErrorTemplate}" />
  </Style>
</Styles>