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Data validation

This page explains how to use Avalonia to validate data input by users.

Data annotations validation plugin​

Avalonia's data annotations validation support allows you to validate any Validation-Attributes associated with the Properties of your ViewModel.

You can validate built-in validation attributes, CustomValidationAttribute, or your own attributes derived from ValidationAttribute.

Enable DataAnnotationsValidationPlugin​

Since Avalonia v12, the data annotations validation plugin is disabled by default. To enable the plugin:

  1. Go to the Program.cs file of your project.
  2. Within the AppBuilder, add this line:
.WithDataAnnotationsValidation()

If you are migrating an earlier project that manually specified the removal of the data annotations validation plugin with BindingPlugins.DataValidators.Remove(plugin); or similar, you can now delete it. It is no longer needed.

Example: The property EMail is required and must be a valid e-mail-address​

note

RaiseAndSetIfChanged is a ReactiveUI method. This example requires ReactiveUI to work.

[Required]
[EmailAddress]
public string? EMail
{
    get { return _EMail; }
    set { this.RaiseAndSetIfChanged(ref _EMail, value); }
}

Customize the appearance of the validation message​

The DataValidationErrors class is used to display validation error messages. It can be placed as a control inside the ControlTemplate of any control that supports data validation, such as a TextBox.

Set a Style in your .axaml file to customize the appearance of the error message.

Example: Custom data validation error message​

<Style Selector="DataValidationErrors">
  <Setter Property="Template">
    <ControlTemplate>
      <DockPanel LastChildFill="True">
        <ContentControl DockPanel.Dock="Right"
                        ContentTemplate="{TemplateBinding ErrorTemplate}"
                        DataContext="{TemplateBinding Owner}"
                        Content="{Binding (DataValidationErrors.Errors)}"
                        IsVisible="{Binding (DataValidationErrors.HasErrors)}"/>
        <ContentPresenter Name="PART_ContentPresenter"
                          Background="{TemplateBinding Background}"
                          BorderBrush="{TemplateBinding BorderBrush}"
                          BorderThickness="{TemplateBinding BorderThickness}"
                          CornerRadius="{TemplateBinding CornerRadius}"
                          ContentTemplate="{TemplateBinding ContentTemplate}"
                          Content="{TemplateBinding Content}"
                          Padding="{TemplateBinding Padding}"/>
      </DockPanel>
    </ControlTemplate>
  </Setter>
  <Setter Property="ErrorTemplate">
    <DataTemplate x:DataType="{x:Type x:Object}">
      <Canvas Width="14" Height="14" Margin="4 0 1 0" 
              Background="Transparent">
        <Canvas.Styles>
          <Style Selector="ToolTip">
            <Setter Property="Background" Value="LightRed"/>
            <Setter Property="BorderBrush" Value="Red"/>
          </Style>
        </Canvas.Styles>
        <ToolTip.Tip>
          <ItemsControl ItemsSource="{Binding}"/>
        </ToolTip.Tip>
        <Path 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="Red" 
              StrokeThickness="2"/>
      </Canvas>
    </DataTemplate>
  </Setter>
</Style>

See also​

On this page