An Overview of the Implementation of ValidationRules in WPF C#

In WPF, the ValidationRule class serves the purpose of validating user input within controls such as TextBox by enabling the creation of custom validation rules. A ValidationRule can be incorporated into a binding to define specific criteria that the input must satisfy, which may include restrictions such as permitting only numeric entries or adhering to a designated range of values.

Procedures for executing the ValidationRules

Step 1. To establish a Custom ValidationRule, begin by defining a class that derives from ValidationRule. Subsequently, override the Validate method to incorporate your specific validation logic.

using System.Globalization;
using System.Windows.Controls;
namespace WpfValidationExample
{
    public class DoubleValidationRule : ValidationRule
    {
        public override ValidationResult Validate(object value, CultureInfo cultureInfo)
        {
            // Attempt to interpret the input value as a double.
            if (double.TryParse(value as string, out double result))
            {
                return ValidationResult.ValidResult;
            }          
            // Return an error if the value does not constitute a valid double.
            return new ValidationResult(false, "The number format is invalid. Kindly input a valid decimal number.");
        }
    }
}

A detailed description of the above class

The DoubleValidationRule class extends the ValidationRule class, thereby creating a specialized rule for validating user input.
The validation method is redefined to incorporate particular validation logic. This method is invoked whenever the binding target, such as TextBox.Text, refreshes its source.
Upon successful parsing, the method yields ValidationResult.ValidResult, signifying that the validation has been successfully completed.

Step 2. Utilize the ValidationRule within XAML.

<Window x:Class="WpfValidationExample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfValidationExample"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">

    <Window.Resources>
        <local:DoubleValidationRule x:Key="DoubleValidationRule" />
        <ControlTemplate x:Key="ErrorTemplate">
            <StackPanel>
                <AdornedElementPlaceholder />
                <TextBlock Foreground="Red" FontSize="12" Text="{Binding [0].ErrorContent}" />
            </StackPanel>
        </ControlTemplate>
    </Window.Resources>

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition Width="Auto" />
        </Grid.ColumnDefinitions>

        <TextBlock Text="Double Value" Grid.Row="0" Grid.Column="0" FontSize="20" Margin="0,10,0,0" />
        
        <TextBox HorizontalAlignment="Center" Width="200" Height="40" Grid.Row="1" Grid.Column="1" Margin="20,10,0,0">
            <TextBox.Text>
                <Binding Path="DoubleValue" UpdateSourceTrigger="PropertyChanged" ValidatesOnDataErrors="True" NotifyOnValidationError="True" StringFormat="F2">
                    <Binding.ValidationRules>
                        <local:DoubleValidationRule />
                    </Binding.ValidationRules>
                </Binding>
            </TextBox.Text>
            <TextBox.Style>
                <Style TargetType="TextBox">
                    <Style.Triggers>
                        <!-- In the event of a validation error, display the error template. -->
                        <Trigger Property="Validation.HasError" Value="True">
                            <Setter Property="ToolTip" Value="{Binding RelativeSource={RelativeSource Self}, Path=(Validation.Errors)[0].ErrorContent}" />
                            <Setter Property="BorderBrush" Value="Red" />
                            <Setter Property="Validation.ErrorTemplate" Value="{StaticResource ErrorTemplate}" />
                        </Trigger>
                    </Style.Triggers>
                </Style>
            </TextBox.Style>
        </TextBox>
    </Grid>
</Window>

<!-- Step 3: Validation errors can be displayed by applying a Style that emphasizes controls when validation is unsuccessful. For instance, consider incorporating an error template into the TextBox. -->
<Window.Resources>
    <local:DoubleValidationRule x:Key="DoubleValidationRule" />
    <ControlTemplate x:Key="ErrorTemplate">
        <StackPanel>
            <AdornedElementPlaceholder />
            <TextBlock Foreground="Red" FontSize="12" Text="{Binding [0].ErrorContent}" />
        </StackPanel>
    </ControlTemplate>
</Window.Resources>

The above implementation is explained as follows.

Create a custom ValidationRule by extending the ValidationRule class and overriding the Validate method.
Implement the ValidationRule in XAML by including it in the ValidationRules collection of a Binding.
Utilize styles or templates to visually indicate validation errors in the user interface, such as a red border or a tooltip displaying the error message.

Step 4. The results of the aforementioned implementation.

Implementation