Overview

WPF provides built-in support for data validation through the IDataErrorInfo interface. This interface allows you to define validation rules for bound properties and display error messages to users in real-time.

Key Concepts

• Implement IDataErrorInfo on your data model class
• Set ValidatesOnDataErrors=True in binding expressions
• Use UpdateSourceTrigger=PropertyChanged for immediate validation feedback
• Style validation errors using Validation.Errors attached property
• Display error content via tooltips

Implementation

XAML

<Window x:Class="ValidationDemo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfobuild/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfobuild/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:ValidationDemo"
        mc:Ignorable="d"
        Title="Data Validation Demo" Height="400" Width="600">
    <Window.Resources>
        <local:Person x:Key="personData"></local:Person>
        
        <Style TargetType="TextBox">
            <Setter Property="Background" Value="#f5f5f5"></Setter>
            <Setter Property="Foreground" Value="#d32f2f"></Setter>
            <Setter Property="BorderBrush" Value="#bdbdbd"></Setter>
            <Setter Property="ToolTip" Value="{Binding RelativeSource={RelativeSource Self}, Path=(Validation.Errors)[0].ErrorContent}"></Setter>
            <Style.Triggers>
                <Trigger Property="Validation.HasError" Value="True">
                    <Setter Property="BorderBrush" Value="Red"></Setter>
                    <Setter Property="BorderThickness" Value="2"></Setter>
                </Trigger>
            </Style.Triggers>
        </Style>
    </Window.Resources>
    
    <StackPanel Margin="20">
        <StackPanel.DataContext>
            <Binding Source="{StaticResource personData}"></Binding>
        </StackPanel.DataContext>
        
        <WrapPanel Margin="0,10">
            <TextBlock Text="Username: " VerticalAlignment="Center" Width="100"></TextBlock>
            <TextBox Name="UserName" 
                     Text="{Binding Path=UserName, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" 
                     Width="250"></TextBox>
        </WrapPanel>
        
        <WrapPanel Margin="0,10">
            <TextBlock Text="Age: " VerticalAlignment="Center" Width="100"></TextBlock>
            <TextBox Name="UserAge" 
                     Text="{Binding Path=UserAge, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" 
                     Width="250"></TextBox>
        </WrapPanel>
        
        <WrapPanel Margin="0,10">
            <TextBlock Text="Email: " VerticalAlignment="Center" Width="100"></TextBlock>
            <TextBox Name="Email" 
                     Text="{Binding Path=Email, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}" 
                     Width="250"></TextBox>
        </WrapPanel>
    </StackPanel>
</Window>

Code-Behind

using System.ComponentModel;
using System.Windows;

namespace ValidationDemo
{
    public partial class MainWindow : Window
    {
        private readonly Person _person;
        
        public MainWindow()
        {
            InitializeComponent();
            _person = new Person();
            this.DataContext = _person;
        }
    }
    
    public class Person : IDataErrorInfo
    {
        public Person()
        {
            UserName = "John";
            UserAge = 25;
            Email = "john@example.com";
        }
        
        public string this[string columnName]
        {
            get
            {
                string errorMessage = string.Empty;
                
                switch (columnName)
                {
                    case nameof(UserName):
                        if (string.IsNullOrWhiteSpace(UserName))
                        {
                            errorMessage = "Username cannot be empty";
                        }
                        else if (UserName.Length < 3 || UserName.Length > 20)
                        {
                            errorMessage = "Username must be between 3 and 20 characters";
                        }
                        break;
                        
                    case nameof(UserAge):
                        if (UserAge < 1 || UserAge > 150)
                        {
                            errorMessage = "Please enter a valid age (1-150)";
                        }
                        break;
                        
                    case nameof(Email):
                        if (string.IsNullOrWhiteSpace(Email))
                        {
                            errorMessage = "Email cannot be empty";
                        }
                        else if (!Email.Contains("@") || !Email.Contains("."))
                        {
                            errorMessage = "Please enter a valid email address";
                        }
                        break;
                }
                
                return errorMessage;
            }
        }
        
        public string UserName { get; set; }
        public int UserAge { get; set; }
        public string Email { get; set; }
        
        public string Error => null;
    }
}

How It Works

1. The Person class implements IDataErrorInfo, which requires an indexer that returns validation error messages for specific properties
2. When a bound property changes, WPF automatically calls the indexer with the propetry name
3. If the indexer returns a non-null string, WPF treats it as a validation error
4. The binding expression includes ValidatesOnDataErrors=True to enable this validation mechanism
5. The UpdateSourceTrigger=PropertyChanged setting ensures validation occurs on every keystroke
6. The tooltip binding displays the error message when hovering over invalid input fields