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How to: Implement Binding Validation

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This example shows how to use an ErrorTemplate and a style trigger to provide visual feedback to inform the user when an invalid value is entered, based on a custom validation rule.

Example

The text content of the TextBox in the following example is bound to the Age property (of type int) of a binding source object named ods. The binding is set up to use a validation rule named AgeRangeRule so that if the user enters non-numeric characters or a value that is smaller than 21 or greater than 130, a red exclamation mark appears next to the text box and a tool tip with the error message appears when the user moves the mouse over the text box.

<TextBox Name="textBox1" Width="50" FontSize="15"
         Validation.ErrorTemplate="{StaticResource validationTemplate}"
         Style="{StaticResource textBoxInError}"
         Grid.Row="1" Grid.Column="1" Margin="2">
  <TextBox.Text>
    <Binding Path="Age" Source="{StaticResource ods}"
             UpdateSourceTrigger="PropertyChanged" >
      <Binding.ValidationRules>
        <c:AgeRangeRule Min="21" Max="130"/>
      </Binding.ValidationRules>
    </Binding>
  </TextBox.Text>
</TextBox>

The following example shows the implementation of AgeRangeRule, which inherits from ValidationRule and overrides the Validate method. The Int32.Parse method is called on the value to make sure that it does not contain any invalid characters. The Validate method returns a ValidationResult that indicates if the value is valid based on whether an exception is caught during the parsing and whether the age value is outside of the lower and upper bounds.

public class AgeRangeRule : ValidationRule
{
    public int Min { get; set; }
    public int Max { get; set; }

    public AgeRangeRule()
    {
    }

    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
        int age = 0;

        try
        {
            if (((string)value).Length > 0)
                age = Int32.Parse((String)value);
        }
        catch (Exception e)
        {
            return new ValidationResult(false, $"Illegal characters or {e.Message}");
        }

        if ((age < Min) || (age > Max))
        {
            return new ValidationResult(false,
              $"Please enter an age in the range: {Min}-{Max}.");
        }
        return ValidationResult.ValidResult;
    }
}

Public Class AgeRangeRule
    Inherits ValidationRule

    ' Properties
    Public Property Max As Integer
    Public Property Min As Integer
        
    ' Methods
    Public Overrides Function Validate(value As Object, cultureInfo As CultureInfo) As ValidationResult
        Dim num1 As Integer = 0
        Try 
            If (CStr(value).Length > 0) Then
                num1 = Integer.Parse(CStr(value))
            End If
        Catch exception1 As Exception
            Return New ValidationResult(False, $"Illegal characters or {exception1.Message}")
        End Try
        If ((num1 < Min) OrElse (num1 > Max)) Then
            Return New ValidationResult(False, $"Please enter an age in the range: {Min}-{Max}.")
        End If
        Return ValidationResult.ValidResult
    End Function

End Class

The following example shows the custom ControlTemplate validationTemplate that creates a red exclamation mark to notify the user of a validation error. Control templates are used to redefine the appearance of a control.

<ControlTemplate x:Key="validationTemplate">
  <DockPanel>
    <TextBlock Foreground="Red" FontSize="20">!</TextBlock>
    <AdornedElementPlaceholder/>
  </DockPanel>
</ControlTemplate>

As shown in the following example, the ToolTip that shows the error message is created using the style named textBoxInError. If the value of HasError is true, the trigger sets the tool tip of the current TextBox to its first validation error. The RelativeSource is set to Self, referring to the current element.

<Style x:Key="textBoxInError" TargetType="{x:Type TextBox}">
  <Style.Triggers>
    <Trigger Property="Validation.HasError" Value="true">
      <Setter Property="ToolTip"
        Value="{Binding RelativeSource={x:Static RelativeSource.Self},
                        Path=(Validation.Errors)/ErrorContent}"/>
    </Trigger>
  </Style.Triggers>
</Style>

For the complete example, see Bind Validation sample.

Note that if you do not provide a custom ErrorTemplate the default error template appears to provide visual feedback to the user when there is a validation error. See "Data Validation" in Data Binding Overview for more information. Also, WPF provides a built-in validation rule that catches exceptions that are thrown during the update of the binding source property. For more information, see ExceptionValidationRule.

See also