Have you ever wanted to have a TextBox that requires data or specifically formatted data and you want to enforce this validation and display this to the user.

Well, I figured this out, with a great deal of pain, but hey, it works.

You can download this project in it’s entirety here:

WpfTextBoxValidation.zip

I started with a new WPF Project in Visual Studio. Here is a general overview of the steps I took to make this example project happen.

MainWindow.xaml

1. Here, create a simple form to fill out using TextBlock and TextBox pairs in a Grid.
2. Configure the TextBox elements to use binding.
3. Configure the TextBox elements to use the appropriate Validation. (See the validation examples we will create below.)
4. Below the form you created in a StackPanel, add some TextBlock elements to display the Validation’s ErrorConent.
5. Bind each TextBlock element to the TextBox in the form they pertain to.
6. Bind the Visibility of each TextBlock to the Errors. (We will use the converter below to return Visible if there is ErrorContent and Collapsed if not.)

<Window x:Class="WpfTextBoxValidation.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfTextBoxValidation"
        Title="MainWindow" Height="350" Width="525">
    <Grid Name="MainGrid">
        <Grid.Resources>
            <Style TargetType="TextBox">
                <Setter Property="MaxWidth" Value="200" />
            </Style>
        </Grid.Resources>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition Width="Auto" MinWidth="200" />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="*" />
        </Grid.RowDefinitions>

        <!-- Labels -->
        <TextBlock Text="FirstName" />
        <TextBlock Text="LastName" Grid.Row="1" />
        <TextBlock Text="Age" Grid.Row="2" />
        <TextBlock Text="Phone" Grid.Row="3" />

        <!-- TextBlocks -->
        <TextBox Name="TextBoxFirstName" Grid.Column="1">
            <TextBox.Text>
                <Binding Path="FirstName" UpdateSourceTrigger="PropertyChanged" >
                    <Binding.ValidationRules>
                        <local:TextBoxNotEmptyValidationRule x:Name="FirstNameValidation" ValidatesOnTargetUpdated="True" 
                                                             Message="You must enter a first name."/>
                    </Binding.ValidationRules>
                </Binding>
            </TextBox.Text>
        </TextBox>
        <TextBox Name="TextBoxLastName" Grid.Row="1" Grid.Column="1">
            <TextBox.Text>
                <Binding Path="LastName" UpdateSourceTrigger="PropertyChanged" >
                    <Binding.ValidationRules>
                        <local:TextBoxNotEmptyValidationRule x:Name="LastNameValidation" ValidatesOnTargetUpdated="True" 
                                                             Message="You must enter a last name."/>
                    </Binding.ValidationRules>
                </Binding>
            </TextBox.Text>
        </TextBox>
        <TextBox Name="TextBoxAge" Grid.Row="2" Grid.Column="1">
            <TextBox.Text>
                <Binding Path="Age" UpdateSourceTrigger="PropertyChanged" >
                    <Binding.ValidationRules>
                        <local:OverThirteenValidationRule x:Name="AgeValidation" ValidatesOnTargetUpdated="True"/>
                    </Binding.ValidationRules>
                </Binding>
            </TextBox.Text>
        </TextBox>
        <TextBox Name="TextBoxPhone" Grid.Row="3" Grid.Column="1">
            <TextBox.Text>
                <Binding Path="Phone" UpdateSourceTrigger="PropertyChanged" >
                    <Binding.ValidationRules>
                        <local:TextBoxNotEmptyValidationRule x:Name="PhoneValidation" ValidatesOnTargetUpdated="True" 
                                                             Message="You must enter a phone number."/>
                    </Binding.ValidationRules>
                </Binding>
            </TextBox.Text>
        </TextBox>

        <!-- Validation List -->
        <StackPanel Grid.Row="4" Grid.ColumnSpan="2">
            <StackPanel.Resources>
                <Style TargetType="TextBlock">
                    <Setter Property="Foreground" Value="Red" />
                </Style>
                <local:ErrorCollectionToVisibility x:Key="ToVisibility" />
            </StackPanel.Resources>
            <TextBlock Visibility="{Binding ElementName=TextBoxFirstName, Path=(Validation.Errors), Converter={StaticResource ToVisibility}}">
                <TextBlock.Text>
                    <MultiBinding StringFormat="FirstName - {0}">
                        <Binding ElementName="TextBoxFirstName" Path="(Validation.Errors)[0].ErrorContent"/>
                    </MultiBinding>
                </TextBlock.Text>
            </TextBlock>
            <TextBlock Visibility="{Binding ElementName=TextBoxLastName, Path=(Validation.Errors), Converter={StaticResource ToVisibility}}">>
                <TextBlock.Text>
                    <MultiBinding StringFormat="LastName - {0}">
                        <Binding ElementName="TextBoxLastName" Path="(Validation.Errors)[0].ErrorContent"/>
                    </MultiBinding>
                </TextBlock.Text>
            </TextBlock>
            <TextBlock Visibility="{Binding ElementName=TextBoxAge, Path=(Validation.Errors), Converter={StaticResource ToVisibility}}">>
                <TextBlock.Text>
                    <MultiBinding StringFormat="Age - {0}">
                        <Binding ElementName="TextBoxAge" Path="(Validation.Errors)[0].ErrorContent"/>
                    </MultiBinding>
                </TextBlock.Text>
            </TextBlock>
            <TextBlock Visibility="{Binding ElementName=TextBoxPhone, Path=(Validation.Errors), Converter={StaticResource ToVisibility}}">>
                <TextBlock.Text>
                    <MultiBinding StringFormat="Phone - {0}">
                        <Binding ElementName="TextBoxPhone" Path="(Validation.Errors)[0].ErrorContent"/>
                    </MultiBinding>
                </TextBlock.Text>
            </TextBlock>
            <!--Text="{Binding ElementName=DirectoryBox, Path=(Validation.Errors)[0].ErrorContent}"-->
        </StackPanel>
    </Grid>
</Window>

MainWindow.xaml.cs

To this file we add a Person class and then create and instance of it and set it the instance as the DataContext.

using System;
using System.Windows;
using System.ComponentModel;

namespace WpfTextBoxValidation
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            Person p = new Person();
            MainGrid.DataContext = p;
        }
    }

    public class Person
    {
        public String FirstName { get; set; }
        public String LastName { get; set; }
        public int Age { get; set; }
        public String Phone { get; set; }
    }
}

ErrorCollectionToVisibility.cs

This class is used to convert the Validation.Error collection to a Visibility.

There is a property Validation.HasError but for some reason it is not available to bind to. If it were, I would bind to it and use the built-in BooleanToVisibility converter. But since, I can’t, I used this ErrorCollectionToVisibility converter which simply returns Visible if there is at least one item in the collection or Collapse if the Collection is null or empty.

using System;
using System.Collections.ObjectModel;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;

namespace WpfTextBoxValidation
{
    class ErrorCollectionToVisibility : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            ReadOnlyCollection<ValidationError> collection = value as ReadOnlyCollection<ValidationError>;
            if (collection != null && collection.Count > 0)
                return Visibility.Visible;
            else
                return Visibility.Collapsed;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            return new object();
        }
    }
}

OverThirteenValidationRule.cs

Here we check if the value is greater than 13 and if not, we return the false ValidationResult.

using System;
using System.Windows.Controls;

namespace WpfTextBoxValidation
{
    public class OverThirteenValidationRule : ValidationRule
    {
        public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
        {
            if (value != null)
            {
                int age = 0;
                try
                {
                    age = Convert.ToInt32(value);
                }
                catch
                {
                    return new ValidationResult(false, "You must be older than 13!");
                }

                if (age > 13)
                    return ValidationResult.ValidResult;

            }
            return new ValidationResult(false, "You must be older than 13!");
        }
    }
}

TextBoxNotEmptyValidationRule.cs

This validation just makes sure there is at least one character in a TextBlock.

using System;
using System.Windows.Controls;

namespace WpfTextBoxValidation
{
    public class TextBoxNotEmptyValidationRule : ValidationRule
    {
        public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
        {
            string str = value as string;
            if (str != null)
            {
                if (str.Length > 0)
                    return ValidationResult.ValidResult;
            }
            return new ValidationResult(false, Message);
        }

        public String Message { get; set; }
    }
}

You have now learned to bind to Validation.ErrorContent.