How to change font size of validation result in WPF C#

Question

I have ValidationRule by using System.Windows.Controls and that rule return some custom message. But the font size is too small and how can I change the font size?

Screenshot...Textbox content and error content of validation result

Here is example code of WPF Window App...

public class NotEmptyValidationRule : ValidationRule
{
    public override ValidationResult Validate(object value, CultureInfo cultureInfo)
    {
        if(value != null)
            return string.IsNullOrWhiteSpace((value ?? "").ToString())
                ? new ValidationResult(false, "Field is required")
                : ValidationResult.ValidResult;
        return ValidationResult.ValidResult;
    }
}

Here is XAML... (Note: Changing Fontsize at TextBox tag didn't work)

<Page...

<TextBox 
        x:Name="txtName" 
        Grid.Row="1" 
        Grid.Column="1" 
        HorizontalAlignment="Left" 
        VerticalAlignment="Top" 
        Width="210" 
        Margin="5,0,0,0"
        FontSize="14"
        Style="{StaticResource MaterialDesignTextBox}">
    <TextBox.Text>
        <Binding Path="RCName" UpdateSourceTrigger="PropertyChanged">
           <Binding.ValidationRules>
                <local2:NotEmptyValidationRule ValidatesOnTargetUpdated="True"/>
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

</Page>

Answer 1

For your requirement, you need to modify the Validation.ErrorTemplate. Since you are using MeterialDesign, you can either write your modified style for TextBox, based on existing style or only write modified ErrorTemplate and apply it directly to TextBox using Validation.ErrorTemplate property.

    <Grid xmlns:controlzEx="clr-namespace:ControlzEx" xmlns:wpf="clr-namespace:MaterialDesignThemes.Wpf">
    <Grid.Resources>
        <BooleanToVisibilityConverter x:Key="BooleanToVisibilityConverter" />
        <!--  New Validation ErrorTemplate  -->
        <ControlTemplate x:Key="ModifiedErrorTemplate">
            <ControlTemplate.Resources>
                <DataTemplate DataType="{x:Type ValidationError}">
                    <TextBlock MaxWidth="{Binding ElementName=Placeholder,
                                                  Path=ActualWidth}"
                               Margin="2"
                               HorizontalAlignment="Left"
                               FontSize="12"
                               Foreground="{DynamicResource ValidationErrorBrush}"
                               Text="{Binding ErrorContent}"
                               TextWrapping="Wrap"
                               UseLayoutRounding="false" />
                </DataTemplate>
            </ControlTemplate.Resources>
            <StackPanel>
                <AdornedElementPlaceholder Name="Placeholder" />
                <Border Name="DefaultErrorViewer"
                        Background="{DynamicResource MaterialDesignPaper}"
                        Visibility="Collapsed">
                    <TextBlock MaxWidth="{Binding ElementName=Placeholder,
                                                  Path=ActualWidth}"
                               Margin="0 2"
                               HorizontalAlignment="Left"
                               FontSize="10"
                               Foreground="{DynamicResource ValidationErrorBrush}"
                               Text="{Binding CurrentItem.ErrorContent}"
                               TextWrapping="Wrap"
                               UseLayoutRounding="false" />
                </Border>
                <controlzEx:PopupEx x:Name="ValidationPopup"
                                    AllowsTransparency="True"
                                    IsOpen="False"
                                    Placement="Bottom"
                                    PlacementTarget="{Binding ElementName=Placeholder,
                                                              Mode=OneWay}">
                    <Border Background="{DynamicResource MaterialDesignPaper}">
                        <TextBlock Margin="0 2"
                                   HorizontalAlignment="Left"
                                   FontSize="10"
                                   Foreground="{DynamicResource ValidationErrorBrush}"
                                   Text="{Binding CurrentItem.ErrorContent}"
                                   TextWrapping="Wrap"
                                   UseLayoutRounding="false" />
                    </Border>
                </controlzEx:PopupEx>
            </StackPanel>
            <ControlTemplate.Triggers>
                <MultiDataTrigger>
                    <MultiDataTrigger.Conditions>
                        <Condition Binding="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.OnlyShowOnFocus)}" Value="False" />
                        <Condition Binding="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.UsePopup)}" Value="True" />
                        <Condition Binding="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.Suppress)}" Value="False" />
                    </MultiDataTrigger.Conditions>
                    <MultiDataTrigger.Setters>
                        <Setter TargetName="ValidationPopup" Property="IsOpen" Value="True" />
                    </MultiDataTrigger.Setters>
                </MultiDataTrigger>

                <MultiDataTrigger>
                    <MultiDataTrigger.Conditions>
                        <Condition Binding="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.OnlyShowOnFocus)}" Value="False" />
                        <Condition Binding="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.UsePopup)}" Value="False" />
                        <Condition Binding="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.Suppress)}" Value="False" />
                    </MultiDataTrigger.Conditions>
                    <MultiDataTrigger.Setters>
                        <Setter TargetName="DefaultErrorViewer" Property="Visibility" Value="Visible" />
                    </MultiDataTrigger.Setters>
                </MultiDataTrigger>

                <MultiDataTrigger>
                    <MultiDataTrigger.Conditions>
                        <Condition Binding="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.OnlyShowOnFocus)}" Value="True" />
                        <Condition Binding="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.UsePopup)}" Value="True" />
                    </MultiDataTrigger.Conditions>
                    <MultiDataTrigger.Setters>
                        <Setter TargetName="ValidationPopup" Property="IsOpen" Value="{Binding ElementName=Placeholder, Path=AdornedElement.IsKeyboardFocusWithin, Mode=OneWay}" />
                    </MultiDataTrigger.Setters>
                </MultiDataTrigger>

                <MultiDataTrigger>
                    <MultiDataTrigger.Conditions>
                        <Condition Binding="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.OnlyShowOnFocus)}" Value="True" />
                        <Condition Binding="{Binding ElementName=Placeholder, Path=AdornedElement.(wpf:ValidationAssist.UsePopup)}" Value="False" />
                    </MultiDataTrigger.Conditions>
                    <MultiDataTrigger.Setters>
                        <Setter TargetName="DefaultErrorViewer" Property="Visibility" Value="{Binding ElementName=Placeholder, Path=AdornedElement.IsKeyboardFocusWithin, Mode=OneWay, Converter={StaticResource BooleanToVisibilityConverter}}" />
                    </MultiDataTrigger.Setters>
                </MultiDataTrigger>
            </ControlTemplate.Triggers>
        </ControlTemplate>
        <!--  Derived TextBox Style  -->
        <Style x:Key="ExtendedMaterialDesignTextBox"
               BasedOn="{StaticResource MaterialDesignTextBox}"
               TargetType="TextBox">
            <Setter Property="Validation.ErrorTemplate" Value="{StaticResource ModifiedErrorTemplate}" />
        </Style>
    </Grid.Resources>
    <TextBox x:Name="txtName"
             Grid.Row="1"
             Grid.Column="1"
             Width="210" Style="{StaticResource ExtendedMaterialDesignTextBox}"
             Margin="5,0,0,0"
             HorizontalAlignment="Left"
             VerticalAlignment="Top"
             FontSize="14">
        <TextBox.Text>
            <Binding NotifyOnValidationError="True"
                     Path="RCName"
                     UpdateSourceTrigger="PropertyChanged">
                <Binding.ValidationRules>
                    <local:NotEmptyValidationRule ValidatesOnTargetUpdated="True" />
                </Binding.ValidationRules>
            </Binding>
        </TextBox.Text>
    </TextBox>
</Grid>

the style for TextBox ErrorTemplate has been copied from here